Glossary of terms used in physical organic chemistry (IUPAC Recommendations 2021)

A factor

Arrhenius factor

(SI unit same as rate constant: s⁻¹ for first-order reaction).

Pre-exponential factor in the Arrhenius equation for the temperature dependence of a reaction rate constant.

Note 1: According to collision theory, A is the frequency of collisions with the correct orientation for reaction.

Note 2: The common unit of A for second-order reactions is dm³ mol⁻¹ s⁻¹.

See also A value, energy of activation, entropy of activation.

See [11, 12].

rev[3]

A value

Steric substituent parameter expressing the conformational preference of an equatorial substituent relative to an axial one in a monosubstituted cyclohexane.

Note 1: This parameter equals Δ_rG° for the equatorial to axial equilibration, in kJ mol⁻¹. For example, A_CH3 is 7.28 kJ mol⁻¹, a positive value because an axial methyl group is destabilized by a steric effect.

Note 2: The values are also known as Winstein-Holness A values.

See [3, 13, 14].

abstraction

Chemical reaction or transformation, the main feature of which is the bimolecular removal of an atom (neutral or charged) from a molecular entity.

Examples:

See detachment.

[3]

acceptor parameter (A)

acceptor number (deprecated).

Unit and dimension 1.

Quantitative measure, devised by Gutmann [15] of the Lewis acidity of a solvent, based on the ³¹P chemical shift of dissolved triethylphosphine oxide (triethylphosphane oxide).

Note: The term acceptor number, designated by AN, is a misnomer and ought to be called acceptor parameter, A, because it is an experimental value [16].

rev[3]

acid

Molecular entity or chemical species capable of donating a hydron (proton) or capable of forming a bond with the electron pair of a Lewis base.

See Brønsted acid, Lewis acid, Lewis base.

See also hard acid.

[3]

acidity

(1) Of a compound:

Tendency of a Brønsted acid to act as a hydron (proton) donor or tendency of a Lewis acid to form Lewis adducts and π-adducts.

Note: Acidity can be quantified by the acidity constant, by association constants for formation of Lewis adducts and π-adducts, or by the enthalpy or Gibbs energy of deprotonation in the gas phase. In water the acidity constant K_a of acid HA is {H₃O⁺}{A^–}/{HA}.

(2) Of a medium, usually one containing Brønsted acids:

Tendency of the medium to hydronate a specific reference base.

Note 1: The acidity of a medium is quantitatively expressed by the appropriate acidity function.

Note 2: Media having an acidity greater than that of 100 % H₂SO₄ are often called superacids.

See [17].

rev[3]

acidity function

Measure of the thermodynamic hydron-donating or -accepting ability of a solvent system or a closely related thermodynamic property, such as the tendency of the lyate ion of the solvent system to form Lewis adducts.

Note 1: Acidity functions are not unique properties of the solvent system alone but depend on the solute (or family of closely related solutes) with respect to which the thermodynamic tendency is measured.

Note 2: Commonly used acidity functions are extensions of pH to concentrated acidic or basic solutions. Acidity functions are usually established over a range of compositions of such a system by UV/Vis spectrophotometric or NMR measurements of the degree of hydronation (or Lewis adduct formation) for the members of a series of structurally similar indicator bases (or acids) of different strength: the best known of these is the Hammett acidity function H_o (for primary aromatic amines as indicator bases).

For detailed information on other acidity functions, on excess acidity, on the evaluation of acidity functions, and on the limitations of the concept, see [18], [19], [20], [21].

[3]

activated complex

See activated state.

rev[3]

activated state

In theories of unimolecular reactions an energized chemical species, often characterized by the superscript ^‡, where the excitation is specific and the molecule is poised for reaction.

Note 1: Often used as a synonym for activated complex or transition state, but not restricted to transition-state theory.

Note 2: This is distinct from an energized molecule, often characterized by the superscript *, in which excitation energy is dispersed among internal degrees of freedom.

Note 3: This is not a complex according to the definition in this Glossary.

See also transition state, transition structure.

activation energy

See energy of activation.

See [11], section 2.12.

[3]

activation strain model

See distortion interaction model.

addition reaction

Chemical reaction of two or more reacting molecular entities, resulting in a product containing all atoms of all components, with formation of two chemical bonds and a net reduction in bond multiplicity in at least one of the reactants.

Note 1: The addition to an entity may occur at only one site (1,1-addition, insertion), at two adjacent sites (1,2-addition), or at two non-adjacent sites (1,3- or 1,4-addition, etc.). Examples

Note 2: This is distinguished from adduct formation, which is less specific about bonding changes.

Note 3: The reverse process is called an elimination reaction.

See also cheletropic reaction, cycloaddition, insertion.

rev[3]

additivity principle

Hypothesis that each of several structural features of a molecular entity makes an independent, transferable, and additive contribution to a property of the substance concerned.

Note 1: More specifically, it is the hypothesis that each of the several substituent groups in a parent molecule makes a separate and additive contribution to the standard Gibbs energy change or Gibbs energy of activation corresponding to a particular chemical reaction.

Note 2: The enthalpies of formation of series of compounds can be described by additivity schemes [22].

Note 3: Deviations from additivity may be remedied by including terms describing interactions between atoms or groups.

See transferability [23, 24].

rev[3]

adduct

New chemical species AB, each molecular entity of which is formed by direct combination of two (or more) separate molecular entities A and B in such a way that there is no loss of atoms from A or B.

Example: adduct formed by interaction of a Lewis acid with a Lewis base:

Note 1: Stoichiometries other than 1:1 are also possible, e.g., a bis-adduct (2:1).

Note 2: An intramolecular adduct can be formed when A and B are groups contained within the same molecular entity.

Note 3: If adduct formation is prevented by steric hindrance, frustrated Lewis pairs may result.

See also addition reaction, frustrated Lewis pair, Lewis adduct, Meisenheimer adduct, π-adduct.

rev[3]

agostic

Feature of a structure in which a hydrogen atom is bonded to both a main-group atom and a metal atom.

Example, [(1–3-η)-but-2-en-1-yl-η²-C⁴,H⁴](η⁵-cyclopentadienyl)cobalt(+1), where the agostic bond is denoted by a half arrow.

Note: The expression η-hydrido-bridged is also used to describe the bonding arrangement with a bridging hydrogen, but this usage is deprecated.

See [25], [26], [27], [28].

rev[3]

alcoholysis

Reaction with an alcohol solvent.

Examples:

See solvolysis.

rev[3]

allotropes

Different structural modifications of an element, with different bonding arrangements of the atoms.

Examples for carbon include diamond, fullerenes, graphite, and graphene.

See [28].

allylic substitution reaction

Substitution reaction on an allylic system with leaving group (nucleofuge) at position 1 and double bond between positions 2 and 3. The incoming group may become attached to atom 1, or else, the incoming group may become attached at position 3, with movement of the double bond from 2,3 to 1,2.

Example:

Note: This term can be extended to systems such as:

and also to propargylic substitution, with a triple bond between positions 2 and 3 and possible rearrangement to an allenic product.

rev[3]

alternant

Property of a conjugated system of π electrons whose atoms can be divided into two sets (marked as “starred” and “unstarred”) so that no atom of either set is directly linked to any other atom of the same set.

Examples:

Note 1: According to several approximate theories (including HMO theory), the π MOs for an alternant hydrocarbon are paired, such that for an orbital of energy α + xβ there is another of energy α − xβ. The coefficients of paired molecular orbitals at each atom are the same, but with opposite sign for the unstarred atoms, and the π electron density at each atom in a neutral alternant hydrocarbon is unity.

See [29], [30], [31].

rev[3]

ambident

ambidentate, bidentate

Characteristic of a chemical species whose molecular entities each possess two alternative, distinguishable, and strongly interacting reactive centers, to either of which a bond may be made.

Note 1: Term most commonly applied to conjugated nucleophiles, for example, an enolate ion (which may react with electrophiles at either the α-carbon atom or the oxygen) or a 4-pyridone, and also to the vicinally ambident cyanide ion and to the cyanate ion, thiocyanate ion, sulfinate ions, nitrite ion, and unsymmetrical hydrazines.

Note 2: Ambident electrophiles are exemplified by carboxylic esters RC(O)OR′, which react with nucleophiles at either the carbonyl carbon or the alkoxy carbon, and by Michael acceptors, such as enones, that can react at either the carbonyl or the β-carbon.

Note 3: Molecular entities containing two non-interacting (or feebly interacting) reactive centers, such as dianions of dicarboxylic acids, are not generally considered to be ambident or bidentate and are better described as bifunctional.

Note 4: The Latin root of the word implies two reactive centers, but the term has also been applied to chemical species with more than two reactive centers, such as an acyl thiourea, RCONHCSNHR′, with nucleophilic O, S, and N. For such species, the term polydentate (or multidentate) is more appropriate.

See [32], [33], [34], [35].

[3]

ambiphilic

Both nucleophilic and electrophilic.

Example:

See also amphiphilic, but the distinction between ambi (Latin: both) and amphi (Greek: both) and the application to hydrophilic and lipophilic or to nucleophilic and electrophilic is arbitrary.

aminoxyl

Compound having the structure

Note: The synonymous term ‘nitroxyl radical’ erroneously suggests the presence of a nitro group; its use is deprecated.

amphiphilic

Both hydrophilic and lipophilic, owing to the presence in the molecule of a large organic cation or anion and also a long hydrocarbon chain (or other combination of polar and nonpolar groups, as in nonionic surfactants)

Examples:

Note: The presence of distinct polar (hydrophilic) and nonpolar (hydrophobic) regions promotes the formation of micelles in dilute aqueous solution.

See also ambiphilic, hydrophilicity, hydrophobicity.

[3]

amphiprotic (solvent)

Feature of a self-ionizing solvent possessing characteristics of both Brønsted acid and base.

Examples: H₂O, CH₃OH.

[3]

amphoteric

Property of a chemical species that can behave as either an acid or as a base.

Examples: H₂O, HCO₃⁻ (hydrogen carbonate)

Note: This property depends upon the medium in which the species is investigated. For example HNO₃ is an acid in water but becomes a base in H₂SO₄.

rev[3]

anchimeric assistance

neighboring group participation

[3]

anionotropy

Rearrangement or tautomerization in which the migrating group moves with its electron pair.

See [36].

[3]

annelation

Alternative, but less desirable term for annulation.

Note: The term is widely used in German and French languages.

[3]

annulation

Transformation involving fusion of a new ring to a molecule via two new bonds.

Note: Some authors use the term annelation for the fusion of an additional ring to an already existing one and annulation for the formation of a ring from an acyclic precursor.

See [37, 38].

See also cyclization.

[3]

annulene

Conjugated monocyclic hydrocarbon of the general formula C_nH_n (n even) with the maximum number of noncumulative double bonds and without side chains.

Note: In systematic nomenclature, an annulene may be named [n]annulene, where n is the number of carbon atoms, e.g., [8] annulene for cycloocta-1,3,5,7-tetraene.

See [39].

See aromatic, Hückel (4n + 2) rule.

[3]

anomeric effect

Tendency of an electronegative substituent alpha to a heteroatom in a six-membered ring to prefer the axial position, as in the anomers of glucopyranose.

Note 1: The effect can be generalized to the conformational preference of an electronegative substituent X to be antiperiplanar to a lone pair of atom Y in a system R–Y–C–X with geminal substituents RY and X.

Note 2: The effect can be attributed, at least in part, to n-σ* delocalization of the lone pair on Y into the C–X σ* orbital.

See [10, 40], [41], [42], [43].

anomers

The two stereoisomers (epimers) of a cyclic sugar or glycoside that differ only in the configuration at C1 of aldoses or C2 of ketoses (the anomeric or acetal/ketal carbon).

Example:

See [10].

[3]

antarafacial, suprafacial

Two spatially different ways whereby bonding changes can occur when a part of a molecule undergoes two changes in bonding (bond-making or bond-breaking), either to a common center or to two related centers external to itself. These are designated as antarafacial if opposite faces of the molecule are involved and suprafacial if both changes occur at the same face. The concept of face is clear from the diagrams in the cases of planar (or approximately planar) frameworks with interacting π orbitals.

For examples of the use of these terms, see cycloaddition, sigmatropic rearrangement.

See also anti, σ, π.

rev[3]

anti

Stereochemical relationship of two substituents that are on opposite sides of a reference plane, in contrast to syn, which means “on the same side”.

Note 1: Two substituents attached to atoms joined by a single bond are anti if the torsion angle (dihedral angle) between the bonds to the substituents is greater than 90°, in contrast to syn if it is less than 90°.

Note 2: A further distinction is made between antiperiplanar, synperiplanar, anticlinal, and synclinal.

See [10, 44], [45], [46].

Note 3: When the terms are used in the context of chemical reactions or transformations, they designate the relative orientation of substituents in the substrate or product:

1. Addition to a carbon–carbon double bond:

1. Alkene-forming elimination:

1. Aldol reaction (where syn and anti designate the relative orientation of CH₃ and OH in the product)

Note 4: In examples (1) and (2), anti processes are always antarafacial and syn processes are suprafacial.

Note 5: In the older literature, the terms anti and syn were used to designate stereoisomers of oximes and related compounds. That usage was superseded by the terms trans and cis or E and Z.

See [10].

rev[3]

anti-Hammond effect

If a structure lying off the minimum-energy reaction path (MERP) is stabilized, the position of the transition state moves toward that structure.

See Hammond Postulate, More O’Ferrall–Jencks diagram, perpendicular effect.

rev[3]

aprotic (solvent)

non-HBD solvent (non-Hydrogen-Bond Donating solvent)

Solvent that is not capable of acting as a hydrogen-bond donor.

Note: Although this definition applies to both polar and nonpolar solvents, the distinction between HBD and non-HBD (or between protic and aprotic) is relevant only for polar solvents.

See HBD solvent.

rev[3]

aquation

Incorporation of one or more integral molecules of water into a species, with or without displacement of one or more other atoms or groups.

Example: The incorporation of water into the inner ligand sphere of an inorganic complex.

See [47].

See also hydration.

[3]

aromatic (adj.), aromaticity (n.)

1. Having a chemistry typified by benzene (traditionally).

2. Feature of a cyclically conjugated molecular entity whose electronic energy is significantly lower or whose stability is significantly greater (owing to delocalization) than that of a hypothetical localized structure (e.g., Kekulé structure).

Note 1: If the molecular entity is of higher energy or less stable than a hypothetical localized structure, the entity is said to be antiaromatic.

Note 2: A geometric parameter indicating bond-length equalization has been used as a measure of aromatic character, as expressed in the harmonic oscillator model of aromaticity [48].

Note 3: The magnitude of the magnetically induced ring current, as observed experimentally by NMR spectroscopy or by the calculated nucleus-independent chemical shift (NICS) value, is another measure of aromaticity [49].

Note 4: The terms aromatic and antiaromatic have been extended to describe the stabilization or destabilization of transition states of pericyclic reactions. The hypothetical reference structure is here less clearly defined, and use of the term is based on application of the Hückel (4n + 2) rule and on consideration of the topology of orbital overlap in the transition state, whereby a cycle with (4n + 2) electrons and a Möbius cycle with 4n electrons are aromatic. Reactions of molecules in the ground state involving antiaromatic transition states proceed much less easily than those involving aromatic transition states.

See [50], [51], [52]. See 19 articles in [53].

See also Hückel (4n + 2) rule, Möbius aromaticity.

[3]

Arrhenius equation

Empirical expression for the temperature dependence of a reaction rate constant k as

with A being the pre-exponential factor (Arrhenius A factor) and E_A being the Arrhenius energy of activation, both considered to be temperature-independent.

rev[3]

aryne

Hydrocarbon derived from an arene by formal removal of two vicinal hydrogen atoms.

Example: 1,2-didehydrobenzene (benzyne)

Note 1: 1,4-Didehydrobenzene (“p-benzyne diradical”, structure above right). Despite common usage, this is not an aryne because there is no triple bond, and the usage is deprecated.

Note 2: Arynes are usually transient species.

Note 3: The analogous heterocyclic compounds are called heteroarynes or hetarynes.

See [39, 54].

rev[3]

association

Assembling of separate molecular entities into any aggregate, especially of oppositely charged free ions into ion pairs or larger and not necessarily well-defined clusters of ions held together by electrostatic attraction.

Note: The term signifies the reverse of dissociation, but is not commonly used for the formation of definite adducts by colligation or coordination.

[3]

asymmetric induction

Preferential formation in a chemical reaction of one enantiomer or diastereoisomer over the other as a result of the influence of a chiral center (stereogenic center, chiral feature) in the substrate, reagent, catalyst, or environment.

Note: The term also refers to the formation of a new chiral center or chiral feature preferentially in one configuration under such an influence.

See [10].

rev[3]

atomic charge

Net charge due to the nucleus and the average electronic distribution in a given region of space. This region is considered to correspond to an atom in a molecular entity.

Note 1: The boundary limits of an atom in a polyatomic molecular entity cannot be defined, as they are not a quantum-mechanical observable. Therefore, different conceptual schemes of dividing a molecule into individual atoms will result in different atomic charges.

Note 2: The atomic charge on an atom should not be confused with its formal charge. For example, the N in NH₄⁺ is calculated to carry a net negative charge even though its formal charge is +1, and each H is calculated to carry a net positive charge even though its formal charge is 0.

See [7].

atomic orbital

Wavefunction that depends explicitly on the spatial coordinates of only one electron around a single nucleus.

See also molecular orbital.

See [7].

[3]

atropisomers

Stereoisomers that are enantiomeric owing to hindered rotation about a single bond.

Example (2,2′-bis(diphenylphosphino)-1,1′-binaphthyl, BINAP):

See [10].

attachment

Transformation by which one molecular entity (the substrate) is converted into another by the formation of one (and only one) two-center bond between the substrate and another molecular entity and which involves no other changes in connectivity in the substrate.

Example: formation of an acyl cation by attachment of carbon monoxide to a carbenium ion (R⁺):

See also colligation.

rev[3]

autocatalytic reaction

Chemical reaction in which a product (or a reaction intermediate) also functions as a catalyst.

Note: In such a reaction, the observed rate of reaction is often found to increase with time from its initial value.

Example: acid-catalyzed bromination of acetophenone, PhCOCH₃, because the reaction generates HBr, which functions as a catalyst.

rev[3]

automerization

degenerate rearrangement

[3]

autoprotolysis

Proton transfer reaction (hydron transfer reaction) between two identical amphoteric molecules (usually of a solvent), one acting as a Brønsted acid and the other as a Brønsted base.

Example:

[3]

autoprotolysis constant

Product of the activities of the species produced as the result of autoprotolysis.

Example: The autoprotolysis constant for water, K_w (or K_AP), is equal to the product of the relative activities of the hydronium and hydroxide ions at equilibrium in pure water.

Note: Because the relative activity a(H₂O) of water at equilibrium is imperceptibly different from unity (with mole fraction as the activity scale and pure un-ionized water as the standard state), the denominator in the expression for the thermodynamic equilibrium constant K_w° for autoprotolysis has a value very close to 1.

Furthermore, owing to the low equilibrium extent of dissociation, and if infinite dilution is selected as the standard state, the activity coefficients of the hydronium and hydroxide ions in pure water are very close to unity. This leads to the relative activities of H₃O⁺ and HO⁻ being virtually identical with the numerical values of their molar concentrations, if the molar scale of activity is used and 1 mol dm⁻³ is chosen as the standard state. Thus,

where {H₃O⁺} and {HO⁻} are the reduced concentrations. The autoprotolysis constant has the unit (and dimension) 1 because each relative activity has dimension 1.

See [55], [56], [57].

rev[3]

α (alpha)

1. Designation applied to the carbon to which a functional group is attached.

2. In carbohydrate nomenclature, a stereochemical designation of the configuration at the anomeric carbon.

3. Parameter in a Brønsted relation expressing the sensitivity of the rate of protonation to acidity.

4. Parameter in Leffler’s relation expressing the sensitivity of changes in Gibbs activation energy to changes in overall Gibbs energy for an elementary reaction.

See [10].

rev[3]

α-effect

Positive deviation of an α nucleophile (one bearing an unshared pair of electrons on an atom adjacent to the nucleophilic site) from a Brønsted-type plot of lg {k_nuc} vs. pK_a. The argument in the lg function should be of dimension 1. Thus, reduced rate coefficients should be used. Here {k_nuc} = k_nuc/[k_nuc] is the reduced k_nuc.

Note 1: More generally, it is the influence on the reactivity at the site adjacent to the atom bearing a lone pair of electrons.

Note 2: The term has been extended to include the effect of any substituent on an adjacent reactive center, e.g., the α-silicon effect.

See [58], [59], [60].

See also Brønsted relation.

rev[3]

α-elimination

1,1-elimination

Transformation of the general type

where the central atom Z is commonly carbon.

See also elimination.

rev[3]

Baldwin’s rules

Set of empirical rules for closures of 3- to 7-membered rings.

Note: The favored pathways are those in which the length and nature of the linking chain enable the terminal atoms to achieve the proper geometry and orbital overlaps for reaction.

Example: Hex-5-en-1-yl radical undergoes 5-exo-trig cyclization to cyclopentylmethyl radical rather than 6-endo-trig cyclization to cyclohexyl radical.

See [61, 62].

rev[3]

base

Chemical species or molecular entity having an available pair of electrons capable of forming a bond with a hydron (proton) (see Brønsted base) or with the vacant orbital of some other species (see Lewis base).

See also hard base, superbase.

[3]

basicity

1. Tendency of a Brønsted base to act as a hydron (proton) acceptor.

Note 1: The basicity of a chemical species is normally expressed by the acidity or acid-dissociation constant of its conjugate acid (see conjugate acid–base pair).

Note 2: To avoid ambiguity, the term pK_aH should be used when expressing basicity by the acid-dissociation constant of its conjugate acid. Thus, the pK_aH of NH₃ is 9.2, while its pK_a, expressing its acidity, is 38.

1. Tendency of a Lewis base to act as a Lewis acid acceptor.

Note 3: For Lewis bases, basicity is expressed by the association constants of Lewis adducts and π-adducts or by the enthalpy of an acid/base reaction.

Note 4: Spectroscopic shifts induced by acid/base adduct formation can also be used as a measure of the strength of interaction.

See [63].

rev[3]

bathochromic shift (effect)

Shift of a spectral band to lower frequencies (longer wavelengths).

Note: This is informally referred to as a red shift and is opposite to a hypsochromic shift (“blue shift”), but these historical terms are discouraged because they apply only to visible transitions.

See [8].

[3]

Bell–Evans–Polanyi principle

Linear relation between energy of activation (E_A) and enthalpy of reaction (Δ_rH), sometimes observed within a series of closely related reactions.

See [64], [65], [66], [67].

benzyne

1,2-Didehydrobenzene (a C₆H₄aryne derived from benzene) and its derivatives formed by substitution.

Note: The terms m- and p-benzyne are occasionally used for 1,3- and 1,4-didehydrobenzene, respectively, but these are incorrect because there is no triple bond.

See [39, 54].

[3]

bidentate

Feature of a ligand with two potential binding sites.

bifunctional catalysis

Catalysis (usually of hydron transfer) by a chemical species involving a mechanism in which two functional groups are implicated in the rate-limiting step, so that the corresponding catalytic coefficient is larger than that expected for catalysis by a chemical species containing only one of these functional groups.

Example: Hydrogen carbonate is a particularly effective catalyst for the hydrolysis of 4-hydroxybutyranilide (N-phenyl-4-hydroxybutanamide) because it catalyzes the breakdown of the tetrahedral intermediate to expel aniline:

Note: The term should not be used to describe a concerted process involving the action of two different catalysts.

See [68], [69], [70], [71], [72].

rev[3]

bifurcation

Feature on a potential-energy surface whereby a minimum-energy reaction path (MERP) emanating from a saddle point (corresponding to a transition structure) splits in two and leads to alternative products without intervening minima or secondary barriers to overcome. A bifurcation arises when the curvature of the surface in a direction perpendicular to the MERP becomes zero and then negative; it implies the existence of a lower-energy transition structure with a transition vector orthogonal to the original MERP.

See [73, 74].

bimolecular

See molecularity.

[3]

rev[3]

binding site

Specific region (or atom) in a molecular entity that is capable of entering into a stabilizing interaction with another atomic or molecular entity.

Example: an active site in an enzyme that interacts with its substrate.

Note 1: Typical modes of interaction are by covalent bonding, hydrogen bonding, coordination, and ion-pair formation, as well as by dipole-dipole interactions, dispersion forces, hydrophobic interactions, and desolvation.

Note 2: Two binding sites in different molecular entities are said to be complementary if their interaction is stabilizing.

[3]

biradical

diradical

See [8].

rev[3]

blue shift

Informal expression for hypsochromic shift, but this historical term is discouraged because it applies only to visible transitions.

rev[3]

Bodenstein approximation

See steady state.

[3]

bond

Balance of attractive and repulsive forces between two atoms or groups of atoms, resulting in sufficient net stabilization to lead to the formation of an aggregate conveniently considered as an independent molecular entity.

Note: The term usually refers to the covalent bond.

See [75].

See also agostic, coordination, hydrogen bond, multi-center bond.

rev[3]

bond dissociation

See heterolysis, homolysis.

Note: In ordinary usage the term refers to homolysis. If not, it should be specified as heterolytic.

[3]

bond-dissociation energy

(derived SI unit: kJ mol⁻¹)

Energy required to break a given bond of some specific molecular entity by homolysis from its potential-energy minimum.

Note: This is the quantity that appears in the Morse potential.

See also bond-dissociation enthalpy.

rev[3]

bond-dissociation enthalpy

(derived SI unit: kJ mol⁻¹)

Standard molar enthalpy required to break a given bond of some specific molecular entity by homolysis.

Example: For CH4→CH3·+H·, the bond-dissociation enthalpy is symbolized as DH°(CH₃–H).

Note: Although DH° is commonly used, ΔdissH° is more consistent with the notation for other thermodynamic quantities.

See also bond-dissociation energy, bond energy, heterolytic bond-dissociation enthalpy.

bond energy

(derived SI unit: kJ mol⁻¹)

Enthalpy of bond dissociation at 0 K.

See bond-dissociation enthalpy.

rev[3]

bond enthalpy (mean bond enthalpy, mean bond energy)

Average value of the gas-phase bond-dissociation enthalpies (usually at a temperature of 298 K) for all bonds of the same type within the same chemical species.

Example: For methane, the mean bond enthalpy is 415.9 kJ mol⁻¹, one-fourth the enthalpy of reaction for

Note: More commonly, tabulated mean bond energies (which are really enthalpies) are values of bond enthalpies averaged over a number of selected chemical species containing that type of bond, such as 414 kJ mol⁻¹ for C–H bonds in a group of R₃CH (R = H, alkyl).

See [76].

bond order

Theoretical index of the degree of bonding between two atoms, relative to that of a normal single bond, i.e., the bond provided by one localized electron pair.

Example: In ethene, the C–C bond order is 2, and the C–H bond order is 1.

Note 1: In valence-bond theory, it is a weighted average of the bond orders between the respective atoms in the various resonance forms. In molecular-orbital theory, it is calculated from the weights of the atomic orbitals in each of the occupied molecular orbitals. For example, in valence-bond theory, the bond order between adjacent carbon atoms in benzene is 1.5; in Hückel molecular orbital theory, it is 1.67.

Note 2: Bond order is often derived from the electron distribution.

Note 3: The Pauling bond order n (as often used in the bond-energy-bond-order model) is a simple function of change in bond length d, where the value of the coefficient c is often 0.3 Å (for n > 1) or 0.6 Å (for n < 1).

rev[3]

bond-energy-bond-order model (BEBO):

Empirical procedure for estimating activation energy, involving relationships among bond length, bond-dissociation energy, and bond order.

See [12, 77].

bond-stretch isomers

Two (or more) molecules with the same spin multiplicity but with different lengths for one or more bonds.

Note: This feature arises because the potential-energy surface, which describes how the energy of the molecule depends on geometry, shows two (or more) minima that are not merely symmetry-related.

See [78], [79], [80], [81].

borderline mechanism

Mechanism intermediate between two extremes, for example, a nucleophilic substitution intermediate between S_N1 and S_N2 or intermediate between electron transfer and S_N2.

[3]

Born–Oppenheimer approximation

Representation of the complete wavefunction as a product of electronic and nuclear parts, Ψ(r, R)=ψel(r, R)⋅ψnuc(R), so that the two wavefunctions can be determined separately by solving two different Schrödinger equations.

See [7].

Bredt’s rule

Prohibition of placing a double bond with one terminus at the bridgehead atom of a polycyclic system unless the rings are large enough to accommodate the double bond without excessive strain.

Example: Bicyclo[2.2.1]hept-1-ene (A), which is capable of existence only as a transient species, although its higher homologues, bicyclo[3.3.1]non-1-ene (B) and bicyclo[4.2.1]non-1(8)-ene (C), with double bond at the bridgeheads, have been isolated.

See [82], [83], [84], [85].

For limitations, see [86].

Note: For an alternative formulation, based on the instability of a trans double bond in a small ring (fewer than 8 atoms), see [87].

rev[3]

bridged carbocation

Carbocation (real or hypothetical) in which there are two (or more) carbon atoms that could in alternative Lewis formulas be designated as carbenium centers but which is instead represented by a structure in which a group (a hydrogen atom or a hydrocarbon residue, possibly with substituents in non-involved positions) bridges these potential carbenium centers.

Note: Electron-sufficient bridged carbocations are distinguished from electron-deficient bridged carbocations. Examples of the former, where the bridging uses π electrons, are phenyl-bridged ions (for which the trivial name phenonium ion has been used), such as A. These ions are straightforwardly classified as carbenium ions. The latter type of ion necessarily involves three-center bonding because the bridging uses σ electrons. The hydrogen-bridged carbocation B contains a two-coordinate hydrogen atom, whereas structures C, D, and E (the 2-norbornyl cation) contain five-coordinate carbon atoms.

See [88].

See also carbonium ion, multi-center bond, neighboring group participation.

For the definitive X-ray structure of the norbornyl cation, see [89].

rev[3]

bridgehead (atom)

Atom that is part of two or more rings in a polycyclic molecule and that is separated from another bridgehead atom by bridges all of which contain at least one other atom.

Example: C1 and C4 in bicyclo[2.2.1]heptane, but not C4a and C8a in decahydronaphthalene.

bridging ligand

Ligand attached to two or more, usually metallic, central atoms.

See [28].

[3]

Brønsted acid

Molecular entity capable of donating a hydron (proton) to a base (i.e., a hydron donor) or the corresponding chemical species.

Examples: H₂O, H₃O⁺, CH₃COOH, H₂SO₄, HSO4−, HCl, CH₃OH, NH₃, etc.

See also conjugate acid–base pair.

[3]

Brønsted base

Molecular entity capable of accepting a hydron (proton) from a Brønsted acid (i.e., a hydron acceptor) or the corresponding chemical species.

Examples: HO⁻, H₂O, CH3CO2−, HSO4−, SO4−, Cl⁻, CH₃O⁻,NH2−, etc.

See also conjugate acid–base pair.

[3]

Brønsted relation

Either of the equations

where α, β, and C are constants for a given reaction series (α and β are called Brønsted exponents or Brønsted parameters). The arguments in the lg functions should be of dimension 1. Thus, reduced rate coefficients should be used: {k_A} = k_A/[k_A] and {k_HA} = k_HA/[k_HA], which are the reduced catalytic coefficients of reactions whose rates depend on the concentrations of acid HA or of its conjugate base A^–, {K_HA} = K_HA/[K_HA] is the reduced acid dissociation constant of HA, p is the number of equivalent acidic protons in HA, and q is the number of equivalent basic sites in A^–. The chosen values of p and q should always be specified. (The charge designations of HA and A^– are only illustrative.).

Note 1: The equations are often written without reduced variables, whereupon the slope α or β, obtained from a graph or least-squares analysis, is correct because it is the derivative of a logarithmic quantity, of dimension 1.

Note 2: The Brønsted relation is often termed the Brønsted catalysis law. Although justifiable on historical grounds, use of this name is not recommended, since Brønsted relations are known to apply to many uncatalyzed and pseudo-catalyzed reactions (such as simple proton [hydron] transfer reactions).

Note 3: The term pseudo-Brønsted relation is sometimes used for reactions that involve nucleophilic catalysis instead of acid–base catalysis. Various types of Brønsted parameters have been proposed such as β_nuc or β_lg for nucleophile or leaving group, respectively.

See also linear free-energy relation (linear Gibbs energy relation).

rev[3]

Bunnett–Olsen equations

Relations between lg([SH⁺]/[S]) + H_o and H_o + lg{[H⁺]} for base S in aqueous acid solutions, where H_o is Hammett’s acidity function and H_o + lg{[H⁺]} represents the activity function lg(γ_Sγ_H+/γ_SH+) for the nitroaniline reference bases to build H_o. ϕ is an empirical parameter that is determined by the slope of the linear correlation of lg([SH⁺]/[S]) − lg{[H⁺]} vs. H_o + lg{[H⁺]}.

Arguments in the lg functions should be of dimension 1. Thus, concentrations should be divided by the respective unit (unless they are eliminated as in the ratio of two concentrations), i.e., the reduced quantity should be used, indicated by curly brackets.

Note 1: These equations avoid using (or defining) an acidity function for each family of bases, including those for which such a definition is not possible. In many cases, ϕ (or ϕ − 1) values for base families defining an acidity function are very similar. Broadly, the value of ϕ is related to the degree of solvation of SH⁺.

Note 2: These equations are obsolete, and the Cox–Yates equation, with the equivalent parameter m* (= 1 − ϕ), is now preferred.

See [20, 90].

See also Cox–Yates equation.

rev[3]

β, β_nuc, β_lg

Parameter in a Brønsted relation expressing the sensitivity of the rate of deprotonation to basicity.

Note: β_nuc and β_lg are used to correlate nucleophilic reactivity and leaving-group ability, respectively.

β-elimination

1,2-elimination

Transformation of the general type

where the central atoms A and B are commonly, but not necessarily, carbon.

See also elimination.

cage

Aggregate of molecules, generally solvent molecules in the condensed phase, that surrounds fragments formed by thermal or photochemical dissociation.

Note: Because the cage hinders the separation of the fragments by diffusion, they may preferentially react with one another (“cage effect”) although not necessarily to re-form the precursor species.

Example:

See also geminate recombination.

[3]

cage compound

Polycyclic compound capable of encapsulating another compound.

Example (adamantane, where the central cavity is large enough to encapsulate He, Ne, or Na⁺):

Note: A compound whose cage is occupied is called an inclusion complex.

See [91, 92].

rev[3]

canonical form

resonance form

rev[3]

captodative effect

Combined action of an electron-withdrawing (“captor”) substituent and an electron-releasing (“dative”) substituent, both attached to a radical center, on the stability of a carbon-centered radical.

Example:

See [93], [94], [95].

rev[3]

carbanion

Generic name for an anion containing an even number of electrons and having an unshared pair of electrons on a carbon atom which satisfies the octet rule and bears a negative formal charge in its Lewis structure or in at least one of its resonance forms.

Examples:

See also radical ion.

See [39, 96].

rev[3]

carbene

Generic name for the species H₂C: (“methylidene”) and substitution derivatives thereof, containing an electrically neutral bivalent carbon atom with two nonbonding electrons.

Note 1: The nonbonding electrons may have antiparallel spins (singlet state) or parallel spins (triplet state).

Note 2: Use of the alternative name “methylene” as a generic term is not recommended.

See [39].

See also diradical.

[3]

carbenium ion

Generic name for a carbocation whose electronic structure can be adequately described by two-electron-two-center bonds.

Note 1: The name implies a hydronated carbene or a substitution derivative thereof.

Note 2: The term is a replacement for the previously used term carbonium ion, which now specifies a carbocation with penta- or higher-coordinate carbons. The names provide a useful distinction between tricoordinate and pentacoordinate carbons.

Note 3: To avoid ambiguity, the name should be avoided as the root for the nomenclature of carbocations. For example, the term “ethylcarbenium ion” might refer to either CH₃CH₂⁺ (ethyl cation, ethylium) or CH₃CH₂CH₂⁺ (propyl cation, propylium).

See [39, 97].

rev[3]

carbenoid

Carbene-like chemical species but with properties and reactivity differing from the free carbene, arising from additional substituents bonded to the carbene carbon.

Example: R₁R₂C(Cl)M (M = metal)

rev[3]

carbocation

Positive ion containing an even number of electrons and with a significant portion of the excess positive charge located on one or more carbon atoms.

Note 1: This is a general term embracing carbenium ions, all types of carbonium ions, vinyl cations, etc.

Note 2: Carbocations may be named by adding the word “cation” to the name of the corresponding radical [97, 98].

Note 3: Such names do not imply structure (e.g., whether three-coordinated or five-coordinated carbon atoms are present) [98].

See also bridged carbocation, radical ion.

See [39].

[3]

carbonium ion

1. Carbocation that contains at least one carbon atom with a coordination number of five or greater.

2. Carbocation whose structure cannot adequately be described by only two-electron two-center bonds.

Example: methanium (CH₅⁺).

Note 1: In most of the earlier literature, this term was used for all types of carbocations, including those that are now defined as a (tricoordinate) carbenium ion.

Note 2: To avoid ambiguity, the term should be avoided as the root for the nomenclature of carbocations. For example, the name “ethylcarbonium ion” might refer to either CH₃CH₂⁺ (ethyl cation) or CH₃CH₂CH₂⁺ (propyl cation).

See [98, 99].

rev[3]

carbyne

methylidyne

Generic name for the species HC·: and substitution derivatives thereof, such as EtOCO–C·: (2-ethoxy-2-oxoethylidyne), containing an electrically neutral univalent carbon atom with three non-bonding electrons.

Note: Use of the alternative name “methylidyne” as a generic term is not recommended.

[3]

Catalán solvent parameters

Quantitative measure of solvent polarity, based on the solvent’s hydrogen-bond-donor ability, hydrogen-bond-acceptor ability, polarizability, and dipolarity.

See [100, 101].

See also solvent parameter.

catalyst

Substance that increases the rate of a chemical reaction (owing to a change of mechanism to one having a lower Gibbs energy of activation) without changing the overall standard Gibbs energy change (or position of equilibrium).

Note 1: The catalyst is both a reactant and product of the reaction, so that there is no net change in the amount of that substance.

Note 2: At the molecular level, the catalyst is used and regenerated during each set of microscopic chemical events leading from a molecular entity of reactant to a molecular entity of product.

Note 3: The requirement that there be no net change in the amount of catalyst is sometimes relaxed, as in the base catalysis of the bromination of ketones, where base is consumed, but this is properly called pseudo-catalysis.

Note 4: Catalysis can be classified as homogeneous, in which only one phase is involved, and heterogeneous, in which the reaction occurs at or near an interface between phases.

Note 5: Catalysis brought about by one of the products of a reaction is called autocatalysis.

Note 6: The terms catalyst and catalysis should not be used when the added substance reduces the rate of reaction (see inhibition).

Note 7: The above definition is adequate for isothermal-isobaric reactions, but under other experimental conditions, the state function that is lowered by the catalyst is not the Gibbs activation energy but the quantity corresponding to those conditions (e.g., the Helmholtz energy under isothermal-isochoric conditions).

See [12].

See also autocatalytic reaction, bifunctional catalysis, catalytic coefficient, electron-transfer catalysis, general acid catalysis, general base catalysis, intramolecular catalysis, micellar catalysis, Michaelis–Menten kinetics, phase-transfer catalysis, pseudo-catalysis, rate of reaction, specific catalysis.

rev[3]

catalytic antibody

abzyme

monoclonal antibody with enzymatic activity

Note 1: A catalytic antibody acts by binding its antigen and catalyzing a chemical reaction that converts the antigen into desired products. Despite the existence of natural catalytic antibodies, most of them were specifically designed to catalyze desired chemical reactions.

Note 2: Catalytic antibodies are produced through immunization against a transition-state analogue for the reaction of interest. The resulting antibodies bind strongly and specifically the transition-state analogue, so that they become catalysts for the desired reaction.

Note 3: The concept of catalytic antibodies and the strategy for obtaining them were advanced by W. P. Jencks [102]. The first catalytic antibodies were finally produced in 1986 [103, 104].

catalytic coefficient

If the rate of reaction v is expressible in the form

where A, B, … are reactants and C_i represents one of a set of catalysts, then the proportionality factor k_i is the catalytic coefficient of the particular catalyst C_i.

Note: Normally, the partial order of reaction (n_i) with respect to a catalyst will be unity, so that k_i is an (a + b +⋯+ 1)^th-order rate coefficient.

[3]

catalytic cycle

Sequence of reaction steps in the form of a loop. One step is binding of a reactant to the active catalyst (sometimes formed from a precatalyst), and another step is the release of product and regeneration of the catalyst.

Example: A + B → C, catalyzed by X formed from precatalyst X′.

cation-π interaction

Noncovalent attractive force between a positive ion (metal cation, protonated Brønsted base, etc.) and a π electron system.

See [105].

chain reaction

Reaction in which one or more reactive reactive intermediates (frequently radicals) are continuously regenerated through a repetitive cycle of elementary steps (“propagation steps”).

Example: Chlorination of methane by a radical mechanism, where Cl^· is continually regenerated in the chain-propagation steps:

Note: In chain polymerization reactions, reactive intermediates of the same types, generated in successive steps or cycles of steps, differ in molecular mass, as in

See [106].

See also chain transfer, initiation, termination.

[3]

chain transfer

Chemical reaction during a chain polymerization in which the active center is transferred from the growing macromolecule to another molecule or to another site on the same molecule, often by abstraction of an atom by the radical end of the growing macromolecule.

Note 1: The growth of the polymer chain is thereby terminated but a new radical, capable of chain propagation and polymerization, is simultaneously created. For the example of alkene polymerization cited for a chain reaction, the reaction

represents a chain transfer, with the radical Cl₃C^⋅ inducing further polymerization.

Note 2: Chain transfer also occurs in other chain reactions, such as cationic or anionic polymerization, in which case the abstraction is by the reactive cationic or anionic end of the growing chain.

See [106].

See also telomerization.

[3]

charge density

See electron density.

See [11].

[3]

charge-transfer (CT) complex

Ground-state adduct that exhibits an electronic absorption corresponding to light-induced transfer of electronic charge from one region of the adduct to another.

See [8].

rev[3]

chelation

Formation or presence of bonds (or other attractive interactions) between a single central atom (or ion) and two or more separate binding sites within the same ligand.

Note 1: A molecular entity in which there is chelation (and the corresponding chemical species) is called a chelate, while the species that binds to the central atom is called a chelant.

Note 2: The terms bidentate, tridentate, … multidentate are used to indicate the number of potential binding sites of the ligand, at least two of which must be used by the ligand in forming a chelate. For example, the bidentate ethylenediamine forms a chelate with Cu⁺² in which both nitrogen atoms of ethylenediamine are bonded to copper.

Note 3: The use of the term is often restricted to metallic central atoms or ions.

Note 4: The phrase “separate binding sites” is intended to exclude cases such as [PtCl₃(CH₂=CH₂)]^–, ferrocene, and (benzene)tricarbonylchromium, in which ethene, the cyclopentadienyl group, and benzene, respectively, are considered to present single binding sites to the respective metal atom.

See also ambident, cryptand.

See also [28].

[3]

cheletropic reaction

Cycloaddition across the terminal atoms of a π system with formation of two new σ bonds to a single atom of a monocentric reagent. There is formal loss of one π bond in the substrate and an increase in the coordination number of the relevant atom of the reagent.

Example: addition of sulfur dioxide to butadiene:

Note: The reverse of this type of reaction is designated “cheletropic elimination”.

See [107].

[3]

chelotropic reaction

Alternative (and etymologically more correct) name for cheletropic reaction.

See [50].

[3]

chemical flux

ϕ

Unidirectional rate of reaction, applicable to the progress of component reaction steps in a complex system or to the progress of reactions in a system at dynamic equilibrium (in which there are no observable concentration changes with time), excluding the reverse reaction and other reaction steps.

Note 1: Chemical flux is a derivative with respect to time, and has the dimensions of amount of substance per volume transformed per time. For example, for the mechanism

Note 2: The sum of all the chemical fluxes leading to C is designated the “total chemical flux into C” (symbol Σϕc), and the sum of all the chemical fluxes leading to destruction of C is designated the “total chemical flux out of C” (symbol Σϕ−c), and similarly for A and B. It then follows for this example that Σϕc=ϕ1 and Σϕ−c=ϕ−1+ϕ2 ,

Note 3: The net rate of appearance of C is then given by

Note 4: In this system, ϕ₁ (or Σϕc) can be regarded as the hypothetical rate of formation of C due to the single (unidirectional) reaction 1 proceeding in the assumed absence of all other reactions, and Σϕ−c can be regarded as the hypothetical rate of destruction of C due to the two (unidirectional) reactions −1 and 2.

Note 5: Even when there is no net reaction, chemical flux can often be measured by NMR methods.

See [1].

See also order of reaction, rate-limiting step, steady state.

rev[3]

chemical reaction

Process that results in the interconversion of chemical species.

Note 1: This definition includes experimentally observable interconversions of conformers and degenerate rearrangements.

Note 2: Chemical reactions may be elementary reactions or stepwise reactions.

Note 3: Detectable chemical reactions normally involve sets of molecular entities, as indicated by this definition, but it is often conceptually convenient to use the term also for changes involving single molecular entities (i.e., “microscopic chemical events”), whose reactions can now be observed experimentally.

See also identity reaction.

rev[3]

chemical relaxation

Passage of a perturbed system toward or into chemical equilibrium.

Note 1: A chemical reaction at equilibrium can be disturbed from equilibrium by a sudden change of some external parameter such as temperature, pressure, or electric-field strength.

Note 2: In many cases, and in particular when the displacement from equilibrium is slight, the progress of the system towards equilibrium can be expressed as a first-order process

where (c_eq)₁ and (c_eq)₂ are the equilibrium amount concentrations of one of the chemical species before and after the change in the external parameter, and c(t) is its amount concentration at time t. The time parameter τ, called relaxation time, is related to the rate coefficients of the chemical reaction involved. Such measurements are commonly used to follow the kinetics of very fast reactions.

Note 3: Relaxation, or the passage toward equilibrium, is more general than chemical relaxation, and includes relaxation of nuclear spins.

See [108, 109].

See relaxation.

rev[3]

chemical shift (NMR)

δ

Variation of the resonance frequency of a nucleus in nuclear magnetic resonance (NMR) spectrometry as a consequence of its environment.

Note 1: The chemical shift of a nucleus X, δ_X, expressed as its frequency, ν_X, relative to that of a standard, ν_ref, and defined as

 For ¹H and ¹³C NMR, the reference signal is usually that of tetramethylsilane (SiMe₄).

Note 2: Chemical shift is usually reported in “parts per million” or ppm, where the numerator has unit Hz, and the denominator has unit MHz, like the spectrometer’s operating frequency.

Note 3: For historical reasons that predate Fourier-transform NMR, if a resonance signal occurs at higher frequency than a reference signal, it is said to be downfield, and if resonance occurs at lower frequency, the signal is upfield. Resonances downfield from SiMe₄ have positive δ-values, and resonances upfield from SiMe₄ have negative δ-values. These terms have been superseded, and deshielded and shielded are preferred for downfield and upfield, respectively.

See [110].

See also shielding.

rev[3]

chemical species

Ensemble of chemically identical molecular entities that can explore the same set of molecular energy levels on the time scale of an experiment. The term is applied equally to a set of chemically identical atomic or molecular structural units in a solid array.

Note 1: For example, conformational isomers may be interconverted sufficiently slowly to be detectable by separate NMR spectra and hence to be considered to be separate chemical species on a time scale governed by the radiofrequency of the spectrometer used. On the other hand, in a slow chemical reaction, the same mixture of conformers may behave as a single chemical species, i.e., there is virtually complete equilibrium population of the total set of molecular energy levels belonging to the two conformers.

Note 2: Except where the context requires otherwise, the term is taken to refer to a set of molecular entities containing isotopes in their natural abundance.

Note 3: The definition given is intended to embrace not only cases such as graphite and sodium chloride but also a surface oxide, where the basic structural units may not be capable of isolated existence.

Note 4: In common chemical usage, and in this Glossary, generic and specific chemical names (such as radical or hydroxide ion) or chemical formulae refer either to a chemical species or to a molecular entity.

[3]

chemically induced dynamic nuclear polarization (CIDNP)

Non-Boltzmann nuclear spin-state distribution produced in thermal or photochemical reactions, usually from colligation and diffusion or disproportionation of radical pairs, and detected by NMR spectroscopy as enhanced absorption or emission signals.

See [111, 112].

rev[3]

chemiexcitation

Generation, by a chemical reaction, of an electronically excited molecular entity from reactants in their ground electronic states.

See [8].

[3]

chemoselectivity

Feature of a chemical reagent that reacts preferentially with one of two or more different functional groups.

Note 1: A reagent has a high chemoselectivity if reaction occurs with only a limited number of different functional groups. For example, sodium tetrahydridoborate (NaBH₄) is a more chemoselective reducing agent than is lithium tetrahydridoaluminate (LiAlH₄). The concept has not been defined in more quantitative terms.

Note 2: The term is also applied to reacting molecules or intermediates that exhibit selectivity towards chemically different reagents.

Note 3: Usage of the term chemospecificity for 100 % chemoselectivity is discouraged.

See [113].

See also regioselectivity, stereoselectivity, stereospecificity.

[3]

chemospecificity

obsolete

See chemoselectivity.

[3]

chirality

Property of a structure that is not superimposable on its mirror image.

See [10, 45].

[3]

chirality center

chiral center (superseded)

Atom with attached groups such that the arrangement is not superimposable on its mirror image.

Note: Often this is a tetrahedral atom with four different groups attached, such as CHBrClF, or C2 of CH₃CHBrCH₂CH₃, or the sulfur of CH₃S(=O)Ph, where the lone pair is considered as a fourth group.

See [10].

See also stereogenic center.

[3]

chiral feature

Structural characteristic rendering a molecule chiral.

Examples: four different substituents on a carbon atom (chirality center), conformational helix, and chiral axis (as in allenes XCH=C=CHX).

chiral recognition

Attraction between molecules through noncovalent interactions that exhibit complementarity only between partners with specific chirality.

See also molecular recognition.

chromophore

Part (atom or group of atoms) of a molecular entity in which the electronic transition responsible for a given spectral band is approximately localized.

Note 1: The term arose originally to refer to the groupings that are responsible for a dye’s color.

Note 2: The electronic transition can often be assigned as involving n, π, π*, σ, and/or σ* orbitals whose energy difference falls within the range of the visible or UV spectrum.

Note 3: The term has been extended to vibrational transitions in the infrared.

See [8, 114, 115].

[3]

CIDNP

Acronym for chemically induced dynamic nuclear polarization.

[3]

cine-substitution

Substitution reaction (generally aromatic) in which the entering group takes up a position adjacent to that occupied by the leaving group.

Example:

See also tele-substitution.

[3]

classical carbocation

Carbocation whose electronic structure can be adequately described by two-electron-two-center bonds, i.e., synonymous with carbenium ion.

clathrate

See host, inclusion compound.

[3]

coarctate

Feature of a concerted transformation in which the primary changes in bonding occur within a cyclic array of atoms but in which two nonbonding atomic orbitals on an atom interchange roles with two bonding orbitals.

Example: epoxidation with dimethyldioxirane

Note: Because the atomic orbitals that interchange roles are orthogonal, such a reaction does not proceed through a fully conjugated transition state and is thus not a pericyclic reaction. It is therefore not governed by the rules that express orbital symmetry restrictions applicable to pericyclic reactions.

See [116].

See also pseudopericyclic.

colligation

Formation of a covalent bond by combination or recombination of two radicals.

Note: This is the reverse of unimolecular homolysis.

Example:

[3]

collision complex

Ensemble formed by two reaction partners, where the distance between them is equal to the sum of the van der Waals radii of neighboring atoms.

See also encounter complex.

[3]

common-ion effect (on rates)

Reduction in the rate of certain reactions of a substrate RX in solution [by a path that involves a pre-equilibrium with formation of R⁺ (or R^–) ions as reaction intermediates] caused by the addition to the reaction mixture of an electrolyte solute containing the “common ion” X^– (or X⁺).

Example: the rate of solvolysis of chlorodiphenylmethane in acetone–water is reduced by the addition of salts of the common ion Cl^–, which causes a decrease in the steady-state concentration of the diphenylmethyl cation:

Note: This retardation due to a common ion should be distinguished from the acceleration due to a salt effect of all ions.

rev[3]

compensation effect

Observation that a plot of T∆_rS vs. ∆_rH (frequently TΔ‡S vs. Δ‡H) for a series of reactions with a range of different substituents (or other unique variable such as solvent or dissolved salt), are straight lines of approximately unit slope, so that, e.g., the terms Δ‡H and TΔ‡S partially compensate, and Δ‡G=Δ‡H−TΔ‡S shows less variation than Δ‡H or TΔ‡S separately.

Note: Frequently, such Δ‡S vs. Δ‡H correlations are statistical artifacts, arising if entropy and enthalpy are extracted from the variation of an equilibrium constant or a rate constant with temperature, so that the slope and intercept of the van’t Hoff plots are correlated. The problem is avoided if the equilibrium constant and the enthalpy are measured independently, for example by spectrophotometry and calorimetry, respectively.

See [117], [118], [119], [120].

See also isoequilibrium relationship, isokinetic relationship.

rev[3]

complex

Molecular entity formed by loose association involving two or more component molecular entities (ionic or uncharged) or the corresponding chemical species. The attraction between the components is often due to hydrogen-bonding or van der Waals attraction and is normally weaker than a covalent bond.

Note 1: The term has also been used with a variety of meanings in different contexts: it is therefore best avoided when a more explicit alternative is applicable, such as adduct when the association is a consequence of bond formation.

Note 2: In inorganic chemistry, the term “coordination entity” is recommended instead of “complex” [28, 121].

See also activated complex, adduct, charge transfer complex, electron-donor-acceptor complex, encounter complex, hydrogen bond, inclusion complex, σ-adduct, π-adduct, transition state, van der Waals forces.

[3]

composite reaction

Chemical reaction for which the expression for the rate of disappearance of a reactant (or rate of appearance of a product) involves rate constants of more than a single elementary reaction.

Examples: “opposing reactions” (where rate constants of two opposed chemical reactions are involved), “parallel reactions” (for which the rate of disappearance of any reactant is governed by the rate constants relating to several simultaneous reactions that form different products from a single set of reactants), and stepwise reactions.

[3]

comproportionation

Any chemical reaction of the type A′ + A″ → 2 A

Example:

Note: Other stoichiometries are possible, depending on the oxidation numbers of the species.

Reverse of disproportionation. The term “symproportionation” is also used.

See [122].

rev[3]

concerted

Feature of a process in which two or more primitive changes occur within the same elementary reaction. Such changes will normally be “energetically coupled”.

Note 1: The term “energetically coupled” means that the simultaneous progress of the primitive changes involves a transition state of lower energy than that for their successive occurrence.

Note 2: In a concerted process, the primitive changes may be synchronous or asynchronous.

See also bifunctional catalysis, potential-energy surface.

[3]

condensation

Reaction (usually stepwise) in which two or more reactants (or remote reactive sites within the same molecular entity) yield a product with accompanying formation of water or some other small molecule, e.g., ammonia, ethanol, acetic acid, hydrogen sulfide.

Note 1: The mechanism of many condensation reactions has been shown to comprise consecutive addition and elimination reactions, as in the base-catalyzed formation of (E)-but-2-enal (crotonaldehyde) from acetaldehyde, via dehydration of 3-hydroxybutanal (aldol). The overall reaction in this example is known as the aldol condensation.

Note 2: The term is sometimes also applied to cases where the formation of water or other simple molecule does not occur, as in “benzoin condensation”.

[3]

condensed formula

Linear representation of the structure of a molecular entity in which bonds are omitted.

Example: methyl 3-methylbutyl ether (isoamyl methyl ether, (CH₃)₂CHCH₂CH₂OCH₃, sometimes condensed further to (CH₃)₂CH[CH₂]₂OCH₃)

Note: This term is sometimes also called a line formula, because it can be written on a single line, but the line formula explicitly shows all bonds.

configuration (electronic)

Distribution of the electrons of an atom or a molecular entity over a set of one-electron wavefunctions called orbitals, according to the Pauli principle.

Note: From one configuration several states with different multiplicities may result. For example, the ground electronic configuration of the oxygen molecule (O₂) is

resulting in

See [7, 8].

[3]

configuration (molecular)

Arrangement in space of the atoms of a molecular entity that distinguishes it from any other molecular entity having the same molecular formula and connectivity and that is not due to conformational differences (rotation about single bonds).

See [10].

[3]

conformations

Different spatial arrangements of a molecular entity that can be interconverted by rotation about one or more formally single bonds.

Note 1: Different conformations are often not considered to be stereoisomeric, because interconversion is rapid at the temperature under consideration.

Note 2: Different or equivalent spatial arrangements of ligands about a central atom, such as those interconverted by pyramidal inversion (of amines) or Berry pseudorotation (as of PF₅) and other “polytopal rearrangements”, are sometimes considered conformations, but they are properly described as configurations.

See [10].

rev[3]

conformational isomers

conformers

conformer

Conformation of a molecular entity that corresponds to a minimum on the potential-energy surface of that molecular entity.

Note: The distinction between conformers and isomers is the height of the barrier for interconversion. Isomers are stable on macroscopic timescales because the barrier for interconversion is high, whereas a conformer cannot persist on a macroscopic timescale because the interconversion between conformations is achieved rapidly.

See [10].

conjugate acid

Brønsted acid BH⁺ formed on protonation (hydronation) of the base B.

Note 1: B is called the conjugate base of the acid BH⁺.

Note 2: The conjugate acid always carries one unit of positive charge more than the base, but the absolute charges of the species are immaterial to the definition. For example: the Brønsted acid HCl and its conjugate base Cl^– constitute a conjugate acid–base pair and so do NH₄⁺ and its conjugate base NH₃.

rev[3]

conjugated system, conjugation

Molecular entity whose structure may be represented as a system of alternating multiple and single (or σ) bonds: e.g.,

Note: In such systems, conjugation is the interaction of one p-orbital with another p-orbital (or d-orbital) across an intervening σ bond, including the analogous interaction involving a p-orbital containing an unshared electron pair, e.g.,

or the interaction across a double bond whose π system does not interact with the p orbitals of the conjugated system, as in

See also cross-conjugation, delocalization, resonance, through-conjugation.

[3]

connectivity

Description of which atoms are bonded to which other atoms.

Note: Connectivity is often displayed in a line formula or other structure showing which atoms are bonded to which other atoms, but with minimal or no indication of bond multiplicity.

Example: The connectivity of propyne is specified by CH₃CCH.

rev[3]

conrotatory

Stereochemical feature of an electrocyclic reaction in which the substituents at the interacting termini of the conjugated system rotate in the same sense (both clockwise or both counterclockwise).

See also disrotatory.

rev[3]

conservation of orbital symmetry

An approach to understanding pericyclic reactions that focuses on a symmetry element (e.g., a reflection plane) that is retained along a reaction pathway. If each of the singly or doubly occupied orbitals of the reactant(s) is of the same symmetry as a similarly occupied orbital of the product(s), that pathway is “allowed” by orbital symmetry conservation. If instead a singly or doubly occupied orbital of the reactant(s) is of the same symmetry as an unoccupied orbital of the product(s), and an unoccupied orbital of the reactant(s) is of the same symmetry as a singly or doubly occupied orbital of the product(s), that pathway is “forbidden” by orbital symmetry conservation.

Note 1: This principle permits the qualitative construction of correlation diagrams to show how molecular orbitals transform and how their energies change during chemical reactions.

Note 2: Considerations of orbital symmetry are frequently grossly simplified in that, for example, the π and π* orbitals of a carbonyl group in an asymmetric molecule are treated as having the same topology (pattern of local nodal planes) as those of a symmetric molecule (e.g., CH₂=O), despite the absence of formal symmetry elements.

See [107, 123].

See also orbital symmetry.

constitutional isomers

Species (or molecular entities) with the same atomic composition (molecular formula) but with different connectivity.

Note: The term structural isomers is discouraged because all isomers differ in structure and because isomers may be constitutional, configurational, or conformational.

See [10].

contributing structure

resonance form

rev[3]

coordinate covalent bond

dative bond

coordination

Formation of a covalent bond, the two shared electrons of which come from only one of the two partners linked by the bond, as in the reaction of a Lewis acid and a Lewis base to form a Lewis adduct; alternatively, the bonding formed in this way.

Note: In the former sense, it is the reverse of unimolecular heterolysis.

See also dative bond, π-adduct.

rev[3]

coordination number

Number of other atoms directly linked to a specified atom in a chemical species regardless of the number of electrons in the bonds linking them [28, Rule IR-10.2.5]. For example, the coordination number of carbon in methane or of phosphorus in triphenylphosphane oxide (triphenylphosphine oxide) is four whereas the coordination number of phosphorus is five in phosphorus pentafluoride.

Note: The term is used in a different sense in the crystallographic description of ionic crystals.

rev[3]

coronate

See crown ether.

rev[3]

correlation analysis

Use of empirical correlations relating one body of experimental data to another, with the objective of finding quantitative relationships among the factors underlying the phenomena involved. Correlation analysis in organic chemistry often uses linear Gibbs-energy relations (formerly linear free-energy relation, LFER) for rates or equilibria of reactions, but the term also embraces similar analysis of physical (most commonly spectroscopic) properties and of biological activity.

See [124], [125], [126], [127], [128].

See also linear free-energy relation, quantitative structure–activity relationship (QSAR).

[3]

coupling constant

spin-spin coupling constant

J

(unit: Hz)

Quantitative measure of nuclear spin–spin coupling in nuclear magnetic resonance spectroscopy.

Note: Spin–spin coupling constants have been correlated with atomic hybridization and with molecular conformations.

See [129], [130], [131].

rev[3]

covalent bond

Stabilizing interaction associated with the sharing of electron pairs between two atomic centers of a molecular entity, leading to a characteristic internuclear distance.

See also agostic, coordination, hydrogen bond, multi-center bond.

See [7].

rev[3]

Cox–Yates equation

Generalization of the Bunnett–Olsen equation of the form

where [H⁺] is the amount concentration of acid and X is the activity-coefficient ratio

lg(γ_sγ_H+/γ_SH+) for an arbitrary reference base, pK_SH+ is the thermodynamic dissociation constant of SH⁺, and m* is an empirical parameter derived from linear regression of the left-hand side vs. X. Arguments in the lg functions should be unitless. Thus, the reduced quantities should be used: {[H⁺]} is [H⁺] divided by its unit.

Note: The function X is called excess acidity because it gives a measure of the difference between the acidity of a solution and that of an ideal solution of the same concentration. In practice, X = –(H_o + lg{[H⁺]}) and m* = 1 − ϕ, where H_o is the Hammett acidity function and ϕ is the slope in the Bunnett–Olsen equation.

See [132], [133], [134].

See Bunnett–Olsen equation.

rev[3]

critical micellization concentration (cmc)

critical micelle concentration

Relatively small range of concentrations separating the limit below which virtually no micelles are detected and the limit above which virtually all additional surfactant molecules form micelles.

Note 1: Many physical properties of surfactant solutions, such as conductivity or light scattering, show an abrupt change at a particular concentration of the surfactant, which can be taken as the cmc.

Note 2: As values obtained using different properties are not quite identical, the method by which the cmc is determined should be clearly stated.

See [135].

See also inverted micelle.

rev[3]

cross-conjugation

Phenomenon of three conjugated groups, two pairs of which exhibit conjugation but without through-conjugation of all three, as in 2-phenylallyl, benzoate anion, divinyl ether, or m-xylylene [1,3-C₆H₄(CH₂)₂].

See [64].

rev[3]

crown ether

Molecular entity comprising a monocyclic ligand assembly that contains three or more binding sites held together by covalent bonds and capable of binding a guest in a central (or nearly central) position. The adducts formed are sometimes known as “coronates”. The best known members of this group are macrocyclic polyethers, such as 18-crown-6, containing several repeating units –CR₂–CR₂O– (where R is most commonly H).

See [136, 137].

See also host.

rev[3]

cryptand

Molecular entity comprising a cyclic or polycyclic assembly of binding sites, which contains three or more binding sites held together by covalent bonds and which defines a molecular cavity in such a way as to bind (and thus “hide” in the cavity) another molecular entity, the guest (a cation, an anion, or a neutral species), more strongly than do the separate parts of the assembly (at the same total concentration of binding sites).

Note 1: The adduct thus formed is called a “cryptate”. The term is usually restricted to bicyclic or oligocyclic molecular entities.

Example:

Note 2: Corresponding monocyclic ligand assemblies (crown ether) are sometimes included in this group, if they can be considered to define a cavity in which a guest can hide. The terms “podand” and “spherand” are used for certain specific ligand assemblies. Coplanar cyclic polydentate ligands, such as porphyrins, are not normally regarded as cryptands.

See [138].

See also host. See also [139].

[3]

curly arrows

Symbols for depicting the flow of electrons in a chemical reaction or to generate additional resonance forms.

Note 1: The tail of the curly arrow shows where an electron pair originates, and the head of the curly arrow shows where the electron pair goes.

Note 2: Single-headed curly arrows are used to depict the flow of unpaired electrons.

Examples:

See electron pushing.

Curtin–Hammett principle

Statement that in a chemical reaction that yields one product (X) from one isomer (A′) and a different product (Y) from another isomer (A″) (and provided these two isomers are rapidly interconvertible relative to the rate of product formation, whereas the products do not undergo interconversion) the product composition is not directly related to the relative concentrations of the isomers in the substrate; it is controlled only by the difference in standard Gibbs energies (GA′‡-GA″‡) of the respective transition states.

Note 1: The product composition is given by [Y]/[X] = (k_Yk₁)/(k₋₁k_X) or K_ck_Y/k_X, where K_c = [A″]/[A′] is the equilibrium constant, and where k_Y and k_X are the respective rate constants of their reactions; these parameters are usually unknown.

Note 2: The energy diagram below represents the transformation of rapidly interconverting isomers A′ and A″ into products X and Y.

Note 3: A related concept is the Winstein-Holness equation for the overall rate of product formation in a system of rapidly interconverting isomers A′ and A″. The overall rate constant k is given by

where x′ and x″ are the respective mole fractions of the isomers at equilibrium [14]. However, because x′ and x″ are simple functions of K_c, the apparent dependence of the overall rate on the relative concentrations of A′ and A″ disappears. The overall rate constant is the sum k_X + K_ck_Y, which is a function of the same quantities that determine the product ratio.

See [140]. See also [14, 141, 142].

rev[3]

cybotactic region

That part of a solution in the vicinity of a solute molecule in which the ordering of the solvent molecules is modified by the presence of the solute molecule. The term solvent “cosphere” of the solute has also been used.

See [143, 144].

[3]

cyclization

Formation of a ring compound from a chain by formation of a new bond.

See also annulation.

[3]

cycloaddition

Reaction in which two or more unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity.

Note 1: The following two systems of notation have been used for the more detailed specification of cycloadditions, of which the second, more recent system (described under (2)) is preferred:

1. An (i+j+…) cycloaddition is a reaction in which two or more molecules (or parts of the same molecule), respectively, provide units of i, j,… linearly connected atoms: these units become joined at their respective termini by new σ bonds so as to form a cycle containing (i+j+…) atoms. In this notation, (a) a Diels–Alder reaction is a (4+2) cycloaddition, (b) the initial reaction of ozone with an alkene is a (3+2) cycloaddition, and (c) the reaction of norbornadiene below is a (2+2+2) cycloaddition. (Parentheses are used to indicate the numbers of atoms, but brackets are also used.)

1. The symbolism [i+j+…] for a cycloaddition identifies the numbers i, j,… of electrons in the interacting units that participate in the transformation of reactants to products. In this notation the reaction (a) and (b) of the preceding paragraph would both be described as [2+4] cycloadditions, and (c) as a [2+2+2] cycloaddition. The symbol a or s (a: antarafacial, s: suprafacial) is often added (usually as a subscript) after the number to designate the stereochemistry of addition to each fragment. A subscript specifying the orbitals, viz., σ, π (with their usual significance), or n (for an orbital associated with a single atom), may be added as a subscript before the number. Thus, the normal Diels–Alder reaction is a [4_s+2_s] or [_π4_s + _π2_s] cycloaddition, whereas the reaction

Note 2: Cycloadditions may be pericyclic reactions or (non-concerted) stepwise reactions. The term “dipolar cycloaddition” (Huisgen reaction) is used for cycloadditions of 1,3-dipolar compounds.

See [107, 145, 146].

See also cheletropic reaction, ene reaction, pericyclic reaction.

rev[3]

cycloelimination

Reverse of cycloaddition. The term is preferred to the synonyms “cycloreversion”, “retro-addition”, and “retrocycloaddition”.

[3]

cycloreversion

obsolete

See cycloelimination.

[3]

dative bond

obsolescent

Coordination bond formed between two chemical species, one of which serves as a donor and the other as an acceptor of the electron pair that is shared in the bond.

Examples: the N–B bond in H₃N⁺–B^–H₃ and the S–O bond in (CH₃)₂S⁺–O^–.

Note 1: A distinctive feature of dative bonds is that their minimum-energy rupture in the gas phase or in inert solvent follows the heterolytic bond-cleavage path.

Note 2: The term is obsolescent because the distinction between dative bonds and ordinary covalent bonds is not useful, in that the precursors of the bond are irrelevant: H₃N⁺–B^–H₃ is the same molecule, with the same bonds, regardless of whether the precursors are considered to have been H₃N + BH₃ or H₃N⁺ + ^–BH₃.

See [7].

See coordination.

rev[3]

degenerate chemical reaction

See identity reaction.

[3]

degenerate rearrangement

Chemical reaction in which the product is indistinguishable (in the absence of isotopic labelling) from the reactant.

Note 1: The term includes both “degenerate intramolecular rearrangements” and reactions that involve intermolecular transfer of atoms or groups (“degenerate intermolecular reactions”): both are degenerate isomerizations.

Note 2: The occurrence of degenerate rearrangements may be detectable by isotopic labelling or by dynamic NMR techniques. For example: the [3,3]sigmatropic rearrangement of hexa-1,5-diene (Cope rearrangement),

Note 3: Synonymous but less preferable terms are “automerization”, “permutational isomerism”, “isodynamic transformation”, and “topomerization”.

See [147].

See also fluxional, molecular rearrangement, narcissistic reaction, valence isomer.

[3]

delocalization

Quantum-mechanical concept most usually applied in organic chemistry to describe the redistribution of π electrons in a conjugated system, where each link has a fractional double-bond character or a non-integer bond order, rather than π electrons that are localized in double or triple bonds.

Note 1: There is a corresponding “delocalization energy”, identifiable with the stabilization of the system relative to a hypothetical alternative in which formal (localized) single and double bonds are present. Some degree of delocalization is always present and can be estimated by quantum mechanical calculations. The effects are particularly evident in aromatic systems and in symmetrical molecular entities in which a lone pair of electrons or a vacant p-orbital is conjugated with a double bond (e.g., carboxylate ions, nitro compounds, enamines, and the allyl cation).

Note 2: Delocalization in such species may be represented by partial bonds or by resonance (here symbolized by a two-headed arrow) between resonance forms.

These examples also illustrate the concomitant delocalization of charge in ionic conjugated systems. Analogously, delocalization of the spin of an unpaired electron occurs in conjugated radicals.

Note 3: Delocalization is not limited to π electrons. Hyperconjugation is the delocalization of electrons of σ bonds.

See [7].

[3]

dendrimer

Large molecule constructed from a central core with repetitive branching and multiple functional groups at the periphery.

See [148].

Example: (with 48 CH₂OH at the periphery)

Note: The name comes from the Greek δενδρον (dendron), which translates to “tree”. Synonymous terms include arborols and cascade molecules.

See [148], [149], [150], [151], [152].

deshielding

See shielding.

[3]

detachment

Reverse of attachment.

[3]

detailed balancing

Principle that when equilibrium is reached in a reaction system (containing an arbitrary number of components and reaction paths), as many atoms, in their respective molecular entities, will pass forward in a given finite time interval as will pass backward along each individual path.

Note 1: It then follows that the reaction path in the reverse direction must in every detail be the reverse of the reaction path in the forward direction (provided that the system is at equilibrium).

Note 2: The principle of detailed balancing is a consequence for macroscopic systems of the principle of microscopic reversibility.

[3]

diamagnetism

Property of substances having a negative magnetic susceptibility (χ), whereby they are repelled out of a magnetic field.

See also paramagnetism.

[3]

diastereoisomerism

Stereoisomerism other than enantiomerism.

See diastereoisomers [10].

[3]

diastereomeric excess

diastereoisomeric excess

x ₁ − x₂, where x₁ and x₂ (with x₁ + x₂ = 1) are the mole fractions of two diastereoisomers in a mixture, or the fractional yields of two diastereoisomers formed in a reaction.

Note: Frequently this term is abbreviated to d.e.

See stereoselectivity, diastereoisomers.

See [10].

diastereomeric ratio

x ₁/x₂, where x₁ and x₂ are the mole fractions of two diastereoisomers in a mixture formed in a reaction.

Note: Frequently this term is abbreviated to d.r.

See stereoselectivity, diastereoisomers.

See [10].

diastereoisomers

diastereomers

Stereoisomers not related as mirror images of each other.

Note: Diastereoisomers are characterized by differences in physical properties and by differences in chemical behavior toward chiral as well as achiral reagents.

See [10].

diastereoselectivity

Preferential formation in a chemical reaction of one diastereoisomer over another.

Note: This can be expressed quantitatively by the diastereoisomeric excess or by the diastereomeric ratio, which is preferable because it is more closely related to a Gibbs-energy difference.

See [10].

See selectivity.

dielectric constant

obsolete

See [11].

See permittivity (relative).

rev[3]

dienophile

Ene or yne component of a Diels–Alder reaction, including compounds with hetero-double bonds and hetero-triple bonds.

See cycloaddition.

rev[3]

diffusion-controlled rate

See encounter-controlled rate, microscopic diffusion control. Contrast mixing control.

[3]

dimerization

Transformation of a molecular entity A to give a molecular entity A₂.

Examples:

See also association.

[3]

Dimroth–Reichardt E_T parameter

Quantitative measure of solvent polarity, based on the wavelength, λ_max, of the longest-wavelength intramolecular charge-transfer absorption band of the solvatochromic betaine dye 2,6-diphenyl-4-(2,4,6-triphenylpyridin-1-ium-1-yl)phenolate.

See [16, 153], [154], [155].

See solvent parameter.

rev[3]

dipolar aprotic solvent

See dipolar non-HBD solvent.

rev[3]

dipolar non-HBD solvent

Non-hydrogen-bond donating solvent

Solvent with a comparatively high relative permittivity (“dielectric constant”), greater than ca. 15, and composed of molecules that have a sizable permanent dipole moment and that, although it may contain hydrogen atoms, cannot donate suitably labile hydrogen atoms to form strong solvent–solute hydrogen bonds.

Examples: dimethyl sulfoxide, acetonitrile, acetone, as contrasted with methanol and N-methylformamide.

Note 1: The term “dipolar” refers to solvents whose molecules have a permanent dipole moment, in contrast to solvents whose molecules have no permanent dipole moment and should be termed “apolar” or “nonpolar”.

Note 2: Non-HBD solvents are often called aprotic, but this term is misleading because a proton can be removed by a sufficiently strong base. The aprotic nature of a solvent molecule means that its hydrogens are in only covalent C–H bonds and not in polar O–H^δ+ or N–H^δ+ bonds that can serve as hydrogen-bond donors. Use of “aprotic” is therefore discouraged, unless the context makes the term unambiguous.

Note 3: It is recommended to classify solvents according to their capability to donate or not donate, as well as to accept or not accept, hydrogen bonds to or from the solute, as follows:

Hydrogen-bond donating solvents (short: HBD solvents), formerly protic solvents

Non-hydrogen-bond donating solvents (short: non-HBD solvents), formerly aprotic solvents

Hydrogen-bond accepting solvents (short: HBA solvents)

Non-hydrogen-bond accepting solvents (short: non-HBA solvents)

See [156, 157].

dipole–dipole excitation transfer

Förster resonance-energy transfer (FRET)

See [8].

dipole–dipole interaction

Intermolecular or intramolecular interaction between molecules or groups having a permanent electric dipole moment. The strength of the interaction depends on the distance and relative orientation of the dipoles.

Note: A dipole/dipole interaction is a simplification of the electrostatic interactions between molecules that originate from asymmetries in the electron densities. Such interactions can be described more correctly by the use of higher-order multipole moments.

See also van der Waals forces.

rev[3]

dipole-induced dipole forces

See van der Waals forces.

[3]

diradical

biradical

Even-electron molecular entity with two (possibly delocalized) radical centers that act nearly independently of each other, e.g.,

Note 1: Species in which the two radical centers interact significantly are often referred to as “diradicaloids”. If the two radical centers are located on the same atom, the species is more properly referred to by its generic name: carbene, nitrene, etc.

Note 2: The lowest-energy triplet state of a diradical lies below or at most only a little above its lowest singlet state (usually judged relative to k_BT, the product of the Boltzmann constant k_B and the absolute temperature T). If the two radical centers interact significantly, the singlet state may be more stable. The states of those diradicals whose radical centers interact particularly weakly are most easily understood in terms of a pair of local doublets.

Note 3: Theoretical descriptions of low-energy states of diradicals display two unsaturated valences: the dominant valence-bond structures have two dots, the low-energy molecular orbital configurations have only two electrons in two approximately nonbonding molecular orbitals, and two of the natural orbitals have occupancies close to one.

See [8, 158], [159], [160], [161], [162], [163].

See also carbene, nitrene.

[3]

dispersion forces

See London forces, van der Waals forces.

[3]

disproportionation

Any chemical reaction of the type A+A⇋A′+A″, where A, A′, and A″ are different chemical species.

Examples:

Note 1: The reverse of disproportionation is called comproportionation.

Note 2: A special case of disproportionation (or “dismutation”) is “radical disproportionation”, exemplified by

Note 3: A somewhat more restricted usage of the term prevails in inorganic chemistry, where A, A′, and A″ are of different oxidation states.

[3]

disrotatory

Stereochemical feature of an electrocyclic reaction in which the substituents at the interacting termini of the conjugated system rotate in opposite senses (one clockwise and the other counterclockwise).

See also conrotatory.

rev[3]

dissociation

Separation of a molecular entity into two or more molecular entities (or any similar separation within a polyatomic molecular entity).

Example:

Note 1: Although the separation of the constituents of an ion pair into free ions is a dissociation, the ionization that produces the ion pair is not a dissociation because the ion pair is a single molecular entity.

Note 2: The reverse of dissociation is association.

rev[3]

distonic ion

Radical ion in which charge and radical sites are separated.

Example: ·CH₂CH₂OCH₂⁺

See [164, 165].

rev[3]

distortion interaction model

activation strain model

Method for analyzing activation energy as the sum of the energies to distort the reactants into the geometries they have in transition states plus the energy of interaction between the two distorted reactants.

See [166].

distribution ratio

partition ratio

Ratio of concentrations of a solute in a mixture of two immiscible phases at equilibrium.

See also Hansch constant.

See [167].

di-π-methane rearrangement

Photochemical reaction of a molecular entity with two π-systems separated by a saturated carbon, to form an ene-substituted cyclopropane.

Pattern:

See [8, 168].

donicity

also called donor number, DN, which is a misnomer

Quantitative experimental measure of the Lewis basicity of a molecule B, expressed as the negative enthalpy of the formation of the 1:1 complex B·SbCl₅.

Note: donicity is often used as a measure of a solvent’s basicity.

See also acceptor parameter, Lewis basicity.

See [63, 169, 170].

downfield

superseded but still widely used in NMR to mean deshielded.

See chemical shift, shielding.

rev[3]

driving force

1. Negative of the Gibbs energy change (∆_rG°) on going from the reactants to the products of a chemical reaction under standard conditions. Also called affinity.

2. Qualitative term that relates the favorable thermodynamics of a reaction to a specific feature of molecular structure, such as the conversion of weaker bonds into stronger (CH₃–H + Br–Br → CH₃–Br + H–Br), neutralization of an acid (Claisen condensation of 2 CH₃COOEt + EtO^– to CH₃COCH^–COOEt + EtOH), or increase in entropy (cycloelimination of cyclohexene to butadiene + ethylene).

Note 1: This term is a misnomer, because favorable thermodynamics is due to energy, not force.

Note 2: This term has also been used in connection with photoinduced electron transfer reactions, to indicate the negative of the estimated standard Gibbs energy change for the outer sphere electron transfer (∆_ETG°) [8].

rev[3]

dual substituent-parameter equation

Any equation that expresses substituent effects in terms of two parameters.

Note: In practice, the term is used specifically for an equation for modeling the effects of meta- and para-substituents X on chemical reactivity, spectroscopic properties, etc. of a probe site in benzene or other aromatic system.

where P_X is the magnitude of the property for substituent X, expressed relative to the property for X = H; σ_I and σ_R are inductive (or polar) and resonance substituent constants, respectively, there being various scales for σ_R; ρ_I and ρ_R are the corresponding regression coefficients.

See [171], [172], [173].

See also extended Hammett equation, Yukawa–Tsuno equation.

[3]

dynamic NMR

NMR spectroscopy of samples undergoing chemical reactions.

Note: Customarily, this does not apply to samples where the composition of the sample, and thus its NMR spectrum, changes with time, but rather to samples at equilibrium, without any net reaction. The occurrence of chemical reactions is manifested by features of the NMR line-shape or by magnetization transfer.

See chemical flux.

dyotropic rearrangement

Process in which two σ bonds simultaneously migrate intramolecularly.

Example

See [174], [175], [176].

rev[3]

educt

deprecated: usage strongly discouraged

starting material, reactant

Note: This term should be avoided and replaced by reactant, because it means “something that comes out”, not “something that goes in”.

rev[3]

effective charge

Z _eff

Net positive charge experienced by an electron in a polyelectronic atom, which is less than the full nuclear charge because of shielding by the other electrons.

rev[3]

effective molarity

effective concentration

Ratio of the first-order rate constant or equilibrium constant of an intramolecular reaction involving two functional groups within the same molecular entity to the second-order rate constant or equilibrium constant of an analogous intermolecular elementary reaction.

Note: This ratio has unit of concentration, mol dm⁻³ or mol L⁻¹, sometimes denoted by M.

See [177].

See also intramolecular catalysis.

[3]

eighteen-electron rule

Electron-counting rule that the number of nonbonding electrons at a metal plus the number of electrons in the metal–ligand bonds should be 18.

Note: The 18-electron rule in transition-metal chemistry is an analogue of the Lewis octet rule.

[3]

electrocyclic reaction

electrocyclization

Molecular rearrangement that involves the formation of a σ bond between the termini of a fully conjugated linear π-electron system (or a linear fragment of a π-electron system) and a decrease by one in the number of π bonds, or the reverse of that process.

Examples:

Note: The stereochemistry of such a process is termed “conrotatory” if the substituents at the interacting termini of the conjugated system both rotate in the same sense, as inor “disrotatory” if one terminus rotates in a clockwise and the other in a counterclockwise sense, as in

See also pericyclic reaction.

[3]

electrofuge

Leaving group that does not carry away the bonding electron pair.

Examples: In the nitration of benzene by NO₂⁺, H⁺ is the electrofuge, and in an S_N1 reaction, the carbocation is the electrofuge.

Note 1: Electrofugality characterizes the relative rates of atoms or groups to depart without the bonding electron pair. Electrofugality depends on the nature of the reference reaction and is not the reverse of electrophilicity [178].

Note 2: For electrofuges in S_N1 reactions, see [179].

See also electrophile, nucleofuge.

rev[3]

electromeric effect

obsolete

rev[3]

electron acceptor

Molecular entity to which an electron may be transferred.

Examples: 1,4-dinitrobenzene,1,1’-dimethyl-4,4’-bipyridinium dication, and benzophenone.

Note 1: A group that accepts electron density from another group is not called an electron acceptor but an electron-withdrawing group.

Note 2: A Lewis acid is not called an electron acceptor but an electron-pair acceptor.

rev[3]

electron affinity

Energy released when an additional electron (without excess energy) attaches itself to a molecular entity (often electrically neutral but not necessarily).

Note 1: Equivalent to the minimum energy required to detach an electron from a singly charged negative ion.

Note 2: Measurement of electron affinities is possible only in the gas phase, but there are indirect methods for evaluating them from solution data, such as polarographic half-wave potentials or charge-transfer spectra.

See [7, 180, 181].

rev[3]

electron capture

Transfer of an electron to a molecular entity, resulting in a molecular entity of (algebraically) increased negative charge.

electron-deficient bond

Bond between adjacent atoms that is formed by fewer than two electrons.

Example:

Note: The B–H–B bonds are also called “two-electron three-center bonds”.

[3]

electron density

The electron density at a point with coordinates x, y, z in an atom or molecular entity is the product of the probability P(x,y,z) (unit: m⁻³) of finding an electron at that point with the volume element dx dy dz (unit: m³).

Note: For many purposes (e.g., X-ray scattering and forces on atoms), the system behaves as if the electrons were spread out into a continuous distribution, which is a manifestation of the wave–particle duality.

See also atomic charge, charge density.

rev[3]

electron donor

Molecular entity that can transfer an electron to another molecular entity or to the corresponding chemical species.

Note 1: After the electron transfer, the two entities may separate or remain associated.

Note 2: A group that donates electron density to another group is an electron-donating group, regardless of whether the donation is of σ or π electrons.

Note 3: A Lewis base is not called an electron donor, but an electron-pair donor.

See also electron acceptor.

rev[3]

electron-donor-acceptor complex

obsolete

charge-transfer complex.

See also adduct, coordination.

electron-pair acceptor

Lewis acid.

[3]

electron-pair donor

Lewis base.

[3]

electron pushing

Method using curly arrows for showing the formal movement of an electron pair (from a lone pair or a σ or π bond) or of an unpaired electron, in order to generate additional resonance forms or to denote a chemical reaction.

Note 1: The electron movement may be intramolecular or intermolecular.

Note 2: When a single electron is transferred, a single-headed curly arrow or “fish-hook” is used, but the electron movement in the opposite direction is redundant and sometimes omitted.

Examples:

electron transfer

Transfer of an electron from one molecular entity to another or between two localized sites in the same molecular entity.

See also inner sphere (electron transfer), Marcus equation, outer sphere (electron transfer).

[3]

electron-transfer catalysis

Process describing a sequence of reactions such as shown in equations (1)–(3), leading from A to B, via species A·^– and B·^– that have an extra electron:

Note 1: An analogous sequence involving radical cations (A·⁺, B·⁺) also occurs.

Note 2: The most notable example of electron-transfer catalysis is the S_RN1 (or T+D_N+A_N) reaction of haloarenes with nucleophiles.

Note 3: The term has its origin in an analogy to acid–base catalysis, with the electron instead of the proton. However, there is a difference between the two catalytic mechanisms, as the electron is not a true catalyst but rather behaves as the initiator of a chain reaction. “Electron-transfer induced chain reaction” is a more appropriate term for the mechanism described by equations (1)–(3).

See [182, 183].

[3]

electronation

obsolete

See reduction.

rev[3]

electronegativity

Measure of the power of an atom to attract electrons to itself.

Note 1: The concept has been quantified by a number of authors. The first, due to Pauling, is based on bond-dissociation energies, E_d (unit: eV), and is anchored by assigning the electronegativity of hydrogen as χ_r,H = 2.1. For atoms A and B

where χ_r denotes the Pauling relative electronegativity; it is relative in the senses that the values are of dimension 1 and that only electronegativity differences are defined on this empirical scale. See [11], section 2.5.

Note 2: Alternatively, the electronegativity of an element, according to the Mulliken scale, is the average of its atomic ionization energy and electron affinity. Other scales have been developed by Allred and Rochow, by Sanderson, and by Allen.

Note 3: Absolute electronegativity is property derived from density functional theory [7].

See [75, 184], [185], [186], [187], [188], [189], [190].

rev[3]

electronic effects (of substituents)

Changes exerted by a substituent on a molecular property or molecular reactivity, often distinguished as inductive (through-bond polarization), through-space electrostatics (field effect), or resonance, but excluding steric.

Note 1: The obsolete terms mesomeric and electromeric are discouraged.

Note 2: Quantitative scales of substituent effects are available.

See [126, 173, 191].

See also polar effect.

rev[3]

electrophile (n.), electrophilic (adj.)

Reagent that forms a bond to its reaction partner (the nucleophile) by accepting both bonding electrons from that partner.

Note 1: Electrophilic reagents are Lewis acids.

Note 2: “Electrophilic catalysis” is catalysis by Lewis acids.

Note 3: The term “electrophilic” is also used to designate the apparent polar character of certain radicals, as inferred from their higher relative reactivities with reaction sites of higher electron density.

See also electrophilicity.

[3]

electrophilic substitution

Heterolytic reaction in which the entering group adds to a nucleophile and in which the leaving group, or electrofuge, relinquishes both electrons to its reaction partner, whereupon it becomes another potential electrophile.

Example (azo coupling):

Note: It is arbitrary to emphasize the electrophile and ignore the feature that this is also a nucleophilic substitution, but the distinction depends on the electrophilic nature of the reactant that is considered to react with the substrate.

See also substitution.

electrophilicity

Relative reactivity of an electrophile toward a common nucleophile.

Note: The concept is related to Lewis acidity. However, whereas Lewis acidity is measured by relative equilibrium constants toward a common Lewis base, electrophilicity is measured by relative rate constants for reactions of different electrophilic reagents towards a common nucleophilic substrate.

See [192], [193], [194].

See also nucleophilicity.

rev[3]

element effect

Ratio of the rate constants of two reactions that differ only in the identity of the element in the leaving group.

Example: k_Br/k_Cl for the reaction of N₃^– (azide) with CH₃Br or CH₃Cl.

rev[3]

elementary reaction

Reaction for which no reaction intermediates have been detected or need to be postulated in order to describe the chemical reaction on a molecular scale. An elementary reaction is assumed to occur in a single step and to pass through only one transition state or none.

See [12].

See also composite reaction, stepwise reaction.

rev[3]

elimination

Reverse of an addition reaction.

Note 1: In an elimination, two groups (called eliminands) are lost, most often from two different centers [1,2-elimination (β-elimination) or 1,3-elimination, etc.] with concomitant formation of an unsaturation (double bond or triple bond) in the molecule, or formation of a ring.

Note 2: If the groups are lost from a single carbon or nitrogen center (1,1-elimination, α-elimination), the resulting product is a carbene or nitrene, respectively.

rev[3]

empirical formula

List of the elements in a chemical species, with integer subscripts indicating the simplest possible ratios of all elements.

Note 1: In organic chemistry, C and H are listed first, then the other elements in alphabetical order.

Note 2: This differs from the molecular formula, in which the subscripts indicate how many of each element is included and which is an integer multiple of the empirical formula. For example, the empirical formula of glucose is CH₂O while its molecular formula is C₆H₁₂O₆.

Note 3: The empirical formula is the information provided by combustion analysis, which has been largely superseded by mass spectrometry, which provides the molecular formula.

enantiomer

One of a pair of stereoisomeric molecular entities that are non-superimposable mirror images of each other.

See [10].

See also stereoisomers.

[3]

enantiomeric excess

Absolute value of the difference between the mole fractions of two enantiomers:

where x₊ + x₋ = 1.

Note: Enantiomeric excess can be evaluated experimentally from the observed specific optical rotatory power [α]_obs, relative to [α]_max, the (maximum) specific optical rotatory power of a pure enantiomer:

and also by chiral chromatography, NMR, and MS methods.

See [10].

rev[3]

enantiomeric ratio

mole fraction of one enantiomer in a mixture divided by the mole fraction of the other.

where x₊ + x_– = 1

See [10].

rev[3]

enantioselectivity

See stereoselectivity.

[3]

encounter complex

Complex of molecular entities produced at an encounter-controlled rate, and which occurs as an intermediate in a reaction.

Note 1: When the complex is formed from two molecular entities, it is called an “encounter pair”. A distinction between encounter pairs and (larger) encounter complexes may be relevant for mechanisms involving pre-association.

Note 2: The separation between the entities is small compared to the diameter of a solvent molecule.

See also [8].

rev[3]

encounter-controlled rate

Rate of reaction corresponding to the rate at which the reacting molecular entities encounter each other. This is also known as the “diffusion-controlled rate”, as rates of encounter are themselves controlled by diffusion rates (which in turn depend on the viscosity of the medium and the dimensions of the reacting molecular entities).

Note: At 25 °C in most solvents, including water, a bimolecular reaction that proceeds at an encounter-controlled rate has a second-order rate constant of 10⁹ to 10¹⁰ dm³ mol⁻¹ s⁻¹.

See also microscopic diffusion control.

[3]

ene reaction

Addition of a compound with a double bond and an allylic hydrogen (the “ene”) to a compound with a multiple bond (the “enophile”) with transfer of the allylic hydrogen and a concomitant reorganization of the bonding.

Example, with propene as the ene and ethene as the enophile.

Note: The reverse is a “retro-ene” reaction.

[3]

energy of activation

E _a or E_A

Arrhenius energy of activation

activation energy

(derived SI unit: kJ mol⁻¹)

Operationally defined quantity expressing the dependence of a rate constant on temperature according to

as derived from the Arrhenius equation, k(T) = A exp(−E_a/RT), where A is the pre-exponential factor and R the gas constant. As the argument of the ln function, k should be divided by its unit, i.e., by [k].

Note 1: According to collision theory, the pre-exponential factor A is the frequency of collisions with the correct orientation for reaction and E_a (or E₀) is the threshold energy that collisions must have for the reaction to occur.

Note 2: The term Arrhenius activation energy is to be used only for the empirical quantity as defined above. There are other empirical equations with different activation energies, see [11].

See [12].

See also enthalpy of activation.

rev[3]

energy profile

See Gibbs energy diagram, potential-energy profile.

[3]

enforced concerted mechanism

Situation where a putative intermediate possesses a lifetime shorter than a bond vibration, so that the steps become concerted.

See [195], [196], [197].

rev[3]

enthalpy of activation

standard enthalpy of activation

(derived SI unit: kJ mol⁻¹)

Standard enthalpy difference between the transition state and the ground state of the reactants at the same temperature and pressure. It is related experimentally to the temperature dependence of the rate coefficient k according to equation (1) for first-order rate constants:

and to equation (2) for second order rate coefficients

This quantity can be obtained, along with the entropy of activation ∆^‡S°, from the slope and intercept of the linear least-squares fit of rate coefficients k to the equation

for a first-order rate coefficient

and

for a second-order rate coefficient where k_B is Boltzmann constant, h is Planck constant, and k_B/h = 2.08366… × 10¹⁰ K⁻¹ s⁻¹.

Note 1: An advantage of the least-squares fit is that it can also give error estimates for Δ‡H° and Δ‡S°.

Note 2: It is also given by

where E_a is the energy of activation, provided that the rate coefficients for reactions other than first-order are expressed in temperature-independent concentration unit (e.g., mol kg⁻¹, measured at a fixed temperature and pressure). The argument in a logarithmic function should be of dimension 1. Thus, k is divided by its unit, i.e., by [k], as in the alternative form, in terms of reduced variables.

See also entropy of activation, Gibbs energy of activation.

See [11, 12].

rev[3]

entropy of activation

standard entropy of activation

(derived SI unit: J mol⁻¹ K⁻¹)

Standard entropy difference between the transition state and the ground state of the reactants, at the same temperature and pressure. It is related to the Gibbs energy of activation and enthalpy of activation by the equation

provided that rate coefficients for reactions other than first-order reactions are expressed in temperature-independent concentration unit (e.g., mol dm⁻³, measured at fixed temperature and pressure). The numerical value of S depends on the standard state (and therefore on the concentration unit selected).

Note 1: It can also be obtained from the intercept of the linear least-squares fit of rate coefficients k to the equation

where k_B/h = 2.08366… × 10¹⁰ K⁻¹ s⁻¹. k should be divided by its unit, i.e., by [k] = s⁻¹ for first-order, and by [k] = (s⁻¹ mol⁻¹ dm³) for second-order rate coefficients and T should be divided by its unit [T] = K, as in the second equation, in terms of reduced variables.

Note 2: The information represented by the entropy of activation may alternatively be conveyed by the pre-exponential factor A, which reflects the fraction of collisions with the correct orientation for reaction (see energy of activation).

See [11, 12].

rev[3]

epimer

Diastereoisomer that has the opposite configuration at only one of two or more tetrahedral stereogenic centers in the respective molecular entity.

See [10].

[3]

epimerization

Interconversion of epimers by reversal of the configuration at one of the stereogenic centers.

See [10].

[3]

equilibrium, chemical

Situation in which reversible processes (processes that may be made to proceed in either the forward or reverse direction by the infinitesimal change of one variable) have reached a point where the rates in both directions are identical, so that the amount of each species no longer changes.

Note 1: In this situation, the Gibbs energy, G, is a minimum. Also, the sum of the chemical potentials of the reactants equals that of the products, so that

where the thermodynamic equilibrium constant, K (of dimension 1), is the product of product activities divided by the product of reactant activities.

Note 2: In dilute solutions, the numerical values of the thermodynamic activities may be approximated by the respective concentrations.

rev[3]

equilibrium control

See thermodynamic control.

[3]

E _T-value

See Dimroth–Reichardt E_T parameter, Z-value.

[3]

excess acidity

See Bunnett–Olsen equations, Cox–Yates equation.

[3]

excimer (“excited dimer”)

Complex formed by the interaction of an excited molecular entity with another identical molecular entity in its ground state.

Note: The complex is not stable in the ground state.

See [8].

See also exciplex.

[3]

exciplex

Electronically excited complex of definite stoichiometry that is non-bonding in the ground state.

Note: An exciplex is a complex formed by the noncovalent interaction of an excited molecular entity with the ground state of a different molecular entity, but an excimer, formed from two identical components, is often also considered to be an exciplex.

See [8].

See also excimer.

[3]

excited state

Condition of a system with energy higher than that of the ground state. This term is most commonly used to characterize a molecular entity in one of its electronically excited states, but may also refer to vibrational and/or rotational excitation in the electronic ground state.

See [8].

[3]

EXSY

NMR exchange spectroscopy

Two-dimensional NMR technique producing cross peaks corresponding to site-to site chemical exchange. The cross-peak amplitudes carry information about exchange rates.

See [198].

extended Hammett equation

Multiparameter extension of the Hammett equation for the description of substituent effects.

Note 1: The major extensions using two parameters (dual substituent-parameter equations) were devised for the separation of inductive and steric effects (Taft equation) or of inductive (or field) and resonance effects.

Note 2: Other parameters may be added (polarizability, hydrophobicity, etc.) when additional substituent effects are operative.

See [191].

See also Yukawa–Tsuno equation.

[3]

external return

external ion-pair recombination

Recombination of free ions formed in an S_N1 reaction (as distinguished from ion-pair collapse).

See ion-pair recombination.

rev[3]

extrusion

Transformation in which an atom or group Y connected to two other atoms or groups X and Z is lost from a molecule, leading to a product in which X is bonded to Z, i.e.,

Example:

Note 1: When Y is a metal, this process is called a reductive elimination.

Note 2: The reverse of an extrusion is called an insertion.

See also cheletropic reaction.

[3]

field effect

Experimentally observable substituent effect (on reaction rates, etc.) of intramolecular electrostatic interaction between the center of interest and a monopole or dipole, by direct electric-field action through space rather than through bonds.

Note 1: The magnitude of the field effect depends on the monopole charge or dipole moment, on the orientation of the dipole, on the distance between the center of interest and the monopole or dipole, and on the effective dielectric constant that reflects how the intervening bonds are polarized.

Note 2: Although a theoretical distinction may be made between the field effect and the inductive effect as models for the Coulomb interaction between a given site and a remote monopole or dipole within the same entity, the experimental distinction between the two effects has proved difficult because the field effect and the inductive effect are ordinarily influenced in the same direction by structural changes.

Note 3: The substituent acts through the electric field that it generates, and it is an oversimplification to reduce that interaction to that of a monopole or dipole.

See also electronic effect, inductive effect, polar effect.

See [172, 191, 199, 200].

rev[3]

flash photolysis

Spectroscopic or kinetic technique in which an ultraviolet, visible, or infrared radiation pulse is used to produce transient species.

Note 1: Commonly, an intense pulse of short duration is used to produce a sufficient concentration of transient species, suitable for spectroscopic observation. The most common observation is of the absorption of the transient species (transient absorption spectroscopy).

Note 2: If only photophysical processes are involved, a more appropriate term would be “pulsed photoactivation”. The term “flash photolysis” would be correct only if chemical bonds are broken (the Greek “lysis” means dissolution or decomposition and in general lysis is used to indicate breaking). However, historically, the name has been used to describe the technique of pulsed excitation, independently of the process that follows the excitation.

See [8].

flash vacuum pyrolysis (FVP)

Thermal reaction of a molecule by exposure to a short thermal shock at high temperature, usually in the gas phase.

Example:

See [201], [202], [203], [204].

rev[3]

fluxionality

Property of a chemical species that undergoes rapid degenerate rearrangements (generally detectable by methods that allow the observation of the behavior of individual nuclei in a rearranged chemical species, e.g., NMR, X-ray).

Example: tricyclo[3.3.2.0^2,8]deca-3,6,9-triene (bullvalene), which has 1 209 600 (= 10!/3) interconvertible arrangements of the ten CH groups.

Note 1: Fluxionality differs from resonance, where no rearrangement of nuclear positions occurs.

Note 2: The term is also used to designate positional change among ligands of complex compounds and organometallics. In these cases, the change is not necessarily degenerate.

See also valence tautomerization.

rev[3]

force-field calculations

See molecular mechanics calculation.

GB

formal charge

Quantity (omitted if zero) attached to each atom in a Lewis structure according to

Examples: CH₂=N⁺=N^–, H₃O⁺, (CH₃)₂C=N-O^–, etc.

Note: This formalism assumes that electrons in bonds are shared equally, regardless of electronegativity.

Förster resonance-energy transfer (FRET)

dipole–dipole excitation transfer

Nonradiative mechanism for transfer of electronic excitation energy from one molecular entity to another, distant one. It arises from the interaction between the transition dipole moments of the two entities.

See [8].

fractionation factor, isotopic

Ratio [x₁(A)/x₂(A)]/[x₁(B)/x₂(B)], where x is the mole fraction of the isotope designated by the subscript, when the two isotopes are equilibrated between two different chemical species A and B (or between specific sites A and B in the same chemical species).

Note 1: The term is most commonly met in connection with deuterium solvent isotope effects, where the fractionation factor, symbolized by ϕ, expresses the ratio

for the exchangeable hydrogen atoms in the chemical species (or sites) concerned.

Note 2: The concept is also applicable to transition states.

See [1].

[3]

fragmentation

1. Heterolytic or homolytic cleavage of a molecule into more than two fragments, according to the general reaction (where a, b, c, d, and X are atoms or groups of atoms)

Examples:

See [205].

1. Breakdown of a radical or radical ion into a closed-shell molecule or ion and a smaller radical

Examples:

[3]

Franck–Condon Principle

Approximation that an electronic transition is most likely to occur without change in nuclear positions.

Note 1: The resulting state is called a Franck–Condon state, and the transition involved is called a vertical transition.

Note 2: As a consequence, the intensity of a vibronic transition is proportional to the square of the overlap integral between the vibrational wavefunctions of the two states that are involved in the transition.

See [8].

free energy

The thermodynamic function Gibbs energy (symbol G) or Helmholtz energy (symbol A) specifically defined as

and

where H is enthalpy, U is internal energy, and S is entropy. The possibility of spontaneous motion for a statistical distribution of an assembly of atoms (at absolute temperature T above 0 K) is governed by free energy and not by potential energy.

1. The IUPAC recommendation is to use the specific terms Gibbs energy or Helmholtz energy whenever possible. However, it is useful to retain the generic term “free energy” for use in contexts where the distinction between (on the one hand) either Gibbs energy or Helmholtz energy and (on the other hand) potential energy is more important than the distinction between conditions either of constant pressure or of constant volume; e.g., in computational modelling to distinguish between results of simulations performed for ensembles under conditions of either constant P or constant V at finite T and calculations based purely on potential energy.

2. Whereas motion of a single molecular entity is determined by the force acting upon it, which is obtained as the negative gradient of the potential energy, motion for an assembly of many molecular entities is determined by the mean force acting upon the statistical distribution, which is obtained as the negative gradient of the potential of mean force.

free radical

See radical.

[3]

frontier orbitals

Highest-energy Occupied Molecular Orbital (HOMO) and Lowest-energy Unoccupied Molecular Orbital (LUMO) of a molecular entity.

1. These terms should be limited to doubly occupied orbitals, and not to a singly occupied molecular orbital (sometimes designated as a SOMO), because HOMO and LUMO are ambiguous for molecular orbitals that are half filled and thus only partly occupied or unoccupied.

2. Examination of the mixing of frontier molecular orbitals of reacting molecular entities affords an approach to the interpretation of reaction behavior; this constitutes a simplified perturbation theory of chemical behavior.

3. In some cases, a subjacent orbital (Next-to-Highest Occupied Molecular Orbital (NHOMO) or a Second Lowest Unoccupied Molecular Orbital (SLUMO)) may affect reactivity.

See [7, 206, 207].

See also SOMO, subjacent orbital.

rev[3]

frustrated Lewis acid–base pair

Acid and base for which adduct formation is prevented by steric hindrance.

See [208, 209].

fullerene

Molecular entity composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube.

Note: Spherical fullerenes are also called buckyballs, and cylindrical ones are called carbon nanotubes or buckytubes.

See [39, 210].

functional group

Atom or group of atoms within molecular entities that are responsible for the characteristic chemical reactions of those molecular entities. The same functional group will undergo the same or similar chemical reaction(s) regardless of the size of the molecule it is a part of. However, its relative reactivity can be modified by nearby substituents.

rev[3]

gas-phase acidity

Standard reaction Gibbs energy (∆_rG°) change for the gas-phase reaction.

Note 1: The symbol often found in the literature is ∆_acidG or GA.

Note 2: The corresponding enthalpy ∆_rH° is often symbolized by ∆_acidH and called “enthalpy of acidity” or deprotonation enthalpy, abbreviated DPE.

See [211, 212].

rev[3]

gas-phase basicity

Negative of the standard reaction Gibbs energy (∆_rG°) change for the gas-phase reaction

Note: An acronym commonly used in the literature is GB. The corresponding enthalpy ∆_rH° is called proton affinity, PA, even though affinity properly refers to Gibbs energy. Moreover, such acronyms are not accepted by IUPAC.

See [213, 214].

rev[3]

Gaussian orbital

Function centered on an atom of the form ϕ(r) ∝xiyjzkexp(−ζr2), used to approximate atomic orbitals in the LCAO-MO method.

See [7].

geminate pair

Pair of molecular entities in close proximity within a solvent cage and resulting from reaction (e.g., bond scission, electron transfer, group transfer) of a precursor.

Note: Because of the proximity, the pair constitutes only a single kinetic entity.

See also ion pair, radical pair.

rev[3]

geminate recombination

Recombination reaction of a geminate pair.

Example:

rev[3]

general acid catalysis

Catalysis of a chemical reaction by Brønsted acids (which may include the solvated hydrogen ion), where the rate of the catalyzed part of the reaction is given by Σ_HAk_HA[HA] multiplied by some function of substrate concentrations.

Note 1: General acid catalysis can be experimentally distinguished from specific catalysis by hydrogen cations (hydrons) if the rate of reaction increases with buffer concentration at constant pH and ionic strength.

Note 2: The acid catalysts HA are unchanged by the overall reaction. This requirement is sometimes relaxed, but the phenomenon is then properly called pseudo-catalysis.

See also catalysis, catalytic coefficient, intramolecular catalysis, pseudo-catalysis, specific catalysis.

rev[3]

general base catalysis

Catalysis of a chemical reaction by Brønsted bases (which may include the lyate ion), where the rate of the catalyzed part of the reaction is given by Σ_Bk_B[B] multiplied by some function of substrate concentrations.

See also general acid catalysis.

[3]

Gibbs energy diagram

Diagram showing the relative standard Gibbs energies of reactants, transition states, reaction intermediates, and products, in the same sequence as they occur in a chemical reaction.

Note 1: The abscissa expresses the sequence of reactants, products, reaction intermediates, and transition states and is often an undefined “reaction coordinate” or only vaguely defined as a measure of progress along a reaction path. In some adaptations, the abscissas are however explicitly defined as bond orders, Brønsted exponents, etc.

Note 2: These points are often connected by a smooth curve (a “Gibbs energy profile”, commonly referred to as a “free-energy profile”, a terminology that is discouraged), but experimental observation can provide information on relative standard Gibbs energies only at the maxima and minima and not at the configurations between them.

Note 3: It should be noted that the use of standard Gibbs energies implies a common standard state for all chemical species (usually 1 M for reactions in solution). Contrary to statements in many textbooks, the highest point on a Gibbs energy diagram does not automatically correspond to the transition state of the rate-limiting step. For example, in a stepwise reaction consisting of two elementary reaction steps

• A+B⇄C    1

•                    C + D → E    2

one of the two transition states must (in general) have a higher standard Gibbs energy than the other. Under experimental conditions where all species have the standard-state concentration, the rate-limiting step is that whose transition state is of highest standard Gibbs energy. However, under (more usual) experimental conditions where all species do not have the standard-state concentration, the value of the concentration of D determines which reaction step is rate-limiting. However, if the particular concentrations of interest, which may vary, are chosen as the standard state, then the rate-limiting step is indeed the one of highest Gibbs energy.

See also potential-energy profile, potential-energy surface, reaction coordinate.

rev[3]

Gibbs energy of activation

standard free energy of activation

(derived SI unit: kJ mol⁻¹)

Standard Gibbs energy difference between the transition state of an elementary reaction and the ground state of the reactants for that step. It is calculated from the rate constant k via the absolute rate equation:

where k_B is Boltzmann’s constant, [k] represents the unit of k, and h is Planck’s constant. The values of the rate constants, and hence the Gibbs energies of activation, depend on the choice of concentration unit (or of the thermodynamic standard state).

Note 1: For a complex stepwise reaction, composed of many elementary reactions, ∆^‡G°, the observed Gibbs energy of activation (activation free energy), can be calculated as RT [ln(k_B/J K⁻¹)/(h/J s) − ln(k′/[k′]/T/K)], where k′ is the observed rate constant, k′ should be divided by its unit, and T/K is the absolute temperature, of dimension 1, as the argument of a logarithmic function should be of dimension 1, as expressed by the reduced variables in the second form of the equation.

Note 2: Both Δ‡G° and k′ are non-trivial functions of the rate constants and activation energies of the elementary steps.

See also enthalpy of activation, entropy of activation.

rev[3]

graphene

Allotrope of carbon, whose structure is a one-atom-thick planar sheet of sp²-bonded carbon atoms in a honeycomb (hexagonal) crystal lattice.

ground state

State of the lowest Gibbs energy of a system [1].

Note: In photochemistry and quantum chemistry, the lowest-energy state of a chemical entity (ground electronic state) is usually meant.

See [8].

See also excited state.

rev[3]

group

See functional group, substituent.

rev[3]

Grunwald–Winstein equation

Linear Gibbs-energy relation (Linear free-energy relation)