Volume 79, Issue 6

The 18th International Conference on Physical Organic Chemistry (ICPOC-18) took place at the Gromada Hotel in Warsaw, Poland on 20-25 August 2006 under the local auspices of Warsaw University and the Polish Chemical Society. It was organized by a local Organizing Committee from the Department of Chemistry of Warsaw University led by Prof. Tadeusz M. Krygowski. Although physical organic chemistry began in the 1930s and at the beginning was concerned mostly with the mechanisms and kinetics of organic reactions and their dependence on structural and medium effects, a great extension of the field toward bioorganic, organic, organometallic, theoretical, catalytic, supramolecular, and photochemistry has been observed for decades now. Representative topics for modern physical organic chemistry include: reaction mechanisms; reactive intermediates; bioprocesses; novel structures; reactivity relationships; solvent, substituent, isotope, and solid-state effects; long-lived charges; sextet or open-shell species; magnetic, nonlinear optical, and conducting molecules; and molecular recognition. Contributions from all of these fields were presented. About 220 researchers, representing 31 countries, participated in the conference. The following eight plenary lectures were presented: R. Huber (Nobel laureate, Germany): "Molecular machines in biology" A. Yonath (Israel): "The spectacular ribosomal architecture: Nascent proteins voyage towards folding via antibiotics binding-pockets" P. Coppens (USA): "Time-resolved diffraction studies of molecular excited states and beyond" K. S. Kim (South Korea): "De novo design based on nano-recognition: Functional molecules/materials and nanosensors/nanodevices" I. P. Beletskaya (Russia): "Mechanistic aspects and synthetic application of carbon-carbon and carbon-heteroatom bonds formation in substitution and addition reactions catalyzed by transition-metal complexes" S. Fukuzumi (Japan): "New development of electron-transfer catalytic systems" D. Braga (Italy): "Making crystals from crystals: A green route to crystal engineering and polymorphism" L. Latos-Grażyński (Poland): "Carbaporphyrinoids: Exploring metal ion-arene interaction in a macrocyclic environment" Additionally, 17 invited talks and, during two parallel sessions, 51 oral communications were presented. There were more than 100 poster presentations. I am pleased to introduce a representative selection of outstanding papers based on plenary and invited lectures delivered at ICPOC-18. In addition to the contributions mentioned above, this volume contains: a discussion of modern understanding of aromaticity (P. Fowler, UK); fascinating studies of new mechanisms focused on reactive intermediates (R. Moss, USA); interpretation of acidity, basicity, and hydride affinity by the trichotomy paradigm (Z. Maksić, Croatia); a quantum approach to proton transfer across hydrogen bond (F. Fillaux, France); a discussion of self-assembly of nickel(II) pseudorotaxene nanostructures on Au surface (R. Bilewicz, Poland); a discussion of synthesis and properties of macrocyclic receptors for anions (J. Jurczak, Poland); a description of novel organic-inorganic frameworks (J. Klinowski, UK); an application of microemulsions as microreactors (J. R. Leis, Spain); a discussion of silicon rehybridization and molecular rearrangements in hypercoordinate silicon dichelates (D. Kost, Israel); and a description of solvation in pure and mixed solvents (O. El Seoud, Brazil). All of these papers exemplify the broad range and diversity of interests of the participants and characterize the present and future challenges in physical organic chemistry. The social program of the conference included: a welcome reception; a Chopin music concert organized in cooperation with the Frederic Chopin Society; conference excursions, including Warsaw Old Town and Żelazowa Wola, the house where Chopin was born; the Warsaw Uprising (1944) Museum and the Heroes of Ghetto Memorial; and folk music dances during the conference dinner. Because ICPOC-18 was attended by quite a number of young chemists from all over the world, it can be expected that the next conference in this series, ICPOC-19, which will be held in July 2008 and is being organized by Profs. J. Ramon Leis from the University of Santiago de Compostela and A. Santaballa from the University of A Coruna (Spain), will not only reflect recent developments and the rich potential of physical organic chemistry, but will also demonstrate the aspirations of younger generations of scientists in this field. Krzysztof Woźniak Conference Editor

In comparison to existing structural, biochemical, and therapeutical data, the crystal structures of large ribosomal subunit from the eubacterial pathogen model Deinococcus radiodurans in complex with the 14-membered macrolides erythromycylamine, RU69874, and the 16-membered macrolide josamycin, highlighted the similarities and differences in macrolides binding to the ribosomal tunnel. The three compounds occupy the macrolide binding pocket with their desosamine or mycaminose aminosugar, the C4-C7 edge of the macrolactone ring and the cladinose sugar sharing similar positions and orientations, although the latter, known to be unnecessary for antibiotic activity, displays fewer contacts. The macrolactone ring displays altogether few contacts with the ribosome and can, therefore, tilt in order to optimize its interaction with the 23S rRNA. In addition to their contacts with nucleotides of domain V of the 23S RNA, erythromycylamine and RU69874 interact with domain II nucleotide U790, and RU69874 also reaches van der Waals distance from A752, in a fashion similar to that observed for the ketolides telithromycin and cethromycin. The variability in the sequences and consequently the diversity of the conformations of macrolide binding pockets in various bacterial species can explain the drug's altered level of effectiveness on different organisms and is thus an important factor in structure-based drug design.

The magnetic criterion of aromaticity is based on the ability of a cyclic system to support a ring current. If the ring current has the diatropic sense, the system is aromatic; if the sense is paratropic, the system is antiaromatic. Attribution of aromaticity ab initio therefore reduces to the calculation and visualization of induced current density. This can be achieved at modest computational cost within the ipsocentric approach, where current density at any point in space is calculated with that point as origin of vector potential. This choice leads to accurate maps of current density and a uniquely simple interpretation in terms of nonredundant orbital contributions governed by translational and rotational selection rules. For example, four-electron diatropicity and two-electron paratropicity are predicted for delocalized forms of [4n+2]- and [4n]-annulenes, respectively. A new application to homoaromatic systems is described. Two model tris-homoaromatics, both neutral molecules, are shown to exhibit a through-space, highest occupied molecular orbital (HOMO)-dominated, four-electron ring current, in a direct analogy with the conventional "aromatic" benzene π-current.

As an alternative to conventional charge-separation functional molecular models based on multi-step long-range electron transfer (ET) within redox cascades, simple donor-acceptor dyads have been developed to attain a long-lived and high-energy charge-separated (CS) state without significant loss of excitation energy. In particular, a simple molecular electron donor-acceptor dyad, 9-mesityl-10-methylacridinium ion (Acr + -Mes), is capable of fast charge separation but extremely slow charge recombination. Such a simple molecular dyad has significant advantages with regard to synthetic feasibility, providing a variety of applications for photoinduced ET catalytic systems, including efficient photocatalytic systems for the solar energy conversion and construction of organic solar cells.

Laser flash photolysis (LFP) of appropriate diazirine precursors afforded the UV spectra of methylchlorocarbene, benzylchlorocarbene, cyclopropylchlorocarbene, and 1-adamantylchlorocarbene. The σ → p transitions of these carbenes were used to directly monitor their reaction kinetics. The unusual and highly reactive N 2 CO compound, diazirinone, was obtained from the reaction of fluoride ion with p -nitrophenoxychlorodiazirine. Diazirinone fragmented to CO and N 2 in a reaction that was exothermic by >90 kcal/mol. Dichlorodiazirine (DCD) was generated by the reaction of chloride ion with p -nitrophenoxychlorodiazirine. Reactions of dichlorocarbene generated from DCD are discussed.

Some of the most important aspects of modeling in chemistry are discussed in detail. It is argued that the interpretive side of (quantum) chemistry is indispensable, since it gives sense to a myriad of experimental and computational results. The usefulness of some physical modeling is illustrated by the trichotomy approach in rationalizing acidity, basicity, and hydride affinities of neutral organic compounds. According to trichotomy paradigm, the simple chemical reaction of protonation and H - attachment can be decomposed into three separate sequential steps, which in turn mirror the initial-, intermediate-, and final-state effects. Ample evidence is given, which convincingly shows that the trichotomy approach has some distinct advantages in interpreting aforementioned properties that belong to the most important ones in chemistry and biochemistry.

We review recent studies of the interconversion mechanism of OH···O hydrogen-bonded centrosymmetric dimers through proton transfer in the prototype crystals of potassium hydrogen carbonate (KHCO 3 ) and benzoic acid (C 6 H 5 COOH). The point at issue is whether the proton distributions at various temperatures arise from classical statistical mixtures of tautomers or quantum mechanical superposition states. A related issue is whether it is possible to probe a quantum superposition without inducing decoherence and classicality. We show that neutron diffraction can realize decoherence-free measurements for strictly defined scattering geometries and thus evidence macroscopic quantum correlations. We present a theoretical framework for decoherence-free macroscopically entangled states of the sublattice of protons. The neutron diffraction cross-section of protons is enhanced by a factor of ~45, compared to regular Bragg diffraction, and quantum correlations are observed with remarkable contrast. At elevated temperatures, up to 300 K, quantum correlations are unaffected by proton transfer. The crystal is a coherent superposition of macroscopic tunnelling states, like Schrödinger's cat in a superposition of dead and alive states.

The synthetic application and mechanistic aspects of transition-metal (Ni, Pd, Pt) catalyzed addition of E-E and E-H (E=S, Se) bonds to alkynes were investigated in detail. This study revealed major factors controlling the selectivity of such addition reactions. A new Ni-based catalytic system with a self-organized nanostructured catalyst has been designed to perform chemical transformation in high yield, under mild conditions.

For the design of functional molecules and nanodevices, it is very useful to utilize nanorecognition (which is governed mainly by interaction forces such as hydrogen bonding, ionic interaction, π-H/π-π interactions, and metallic interactions) and nanodynamics (involving capture, transport, and release of electrons, photons, or protons). The manifestation of these interaction forces has led us to the design and realization of diverse ionophores/receptors, organic nanotubes, nanowires, molecular mechanical devices, molecular switches, enzyme mimetics, protein folding/unfolding, etc. In this review, we begin with a brief discussion of the interaction forces, followed by some of our representative applications. We discuss ionophores with chemo-sensing capability for biologically important cations and anions and explain how the understanding of hydrogen bonding and π-interactions has led to the design of self-assembled nanotubes from calix[4]hydroquinone (CHQ). The binding study of neutral and cationic transition metals with the redox system of hydroquinone (HQ) and quinone (Q) predicts what kind of nanostructures would form. Finally, we look into the conformational changes between stacked and edge-to-face conformers in π-benzoquinone-benzene complexes controlled by alternating electrochemical potential. The resulting flapping motion illustrates a promising pathway toward the design of mobile nanomechanical devices.

Tetraazamacrocyclic complexes of Ni II and Cu II can be used as components of catenanes or rotaxanes showing electrochemically switched intramolecular motion in solution. In our present studies, we modify these compounds with organothiol chains to attach them to the surface of the electrode using the self-assembly method and employ them next as molecular switches, which change conductivity upon applying appropriate potential. The electrochemical properties of these compounds are studied in the solution and, in the case of thiol derivative, immobilized on the electrode surface. The macrocyclic complex of Ni II , immobilized on the Au surface, forms the axis of the rotaxane. This compound can be anchored to the surface by one or two thiol groups. The data obtained from scanning tunneling microscopy (STM) experiments using colloidal Au confirm that the orientation normal to the surface dominates. The electrochemical experiments reveal reversible one-electron oxidation of metal center from +2 to +3. The behavior of the electrode modified with the macrocyclic complex of Ni II upon immersion in a solution containing bismacrocyclic complex of Ni II points to the formation of a new rotaxane-like nanostructure on the surface of the electrode.

Understanding of structure-affinity relationships is crucial for rational receptor design, however, such studies for anion receptors are still limited. Therefore, we investigated this issue in the case of amide-based macrocyclic receptors derived from aromatic diacids (i.e., isophthalic and dipicolinic). Using these model compounds, we examined the macrocyclic effect, the influence of intramolecular hydrogen bonds, and the correlation between the ring size and anion affinity. We found that in contrast to what was known for acyclic diamides, macrocyclic isophthalamide receptors bind anions more weakly than their dipicolinic analogs. Comprehensive structural studies revealed that such behavior is due to intramolecular hydrogen bonds present in isophthalamide receptors. Furthermore, we demonstrated how this obstacle can be overcome by the preparation of a hybrid macrocycle based on both building blocks.

Crystal engineering of coordination frameworks is a topical and rapidly advancing field, and a variety of entirely new solid materials have been synthesized. We describe the main challenges involved in the hydrothermal approach to these materials using, as an example, cadmium- and zinc-based coordination frameworks. We discuss the unusual crystal architectures of the products, and show that the kind and quality of the resulting crystals are directly controlled by the composition of the ternary reactive mixture and the temperature regime used in the hydrothermal synthesis.

Microemulsions are very versatile reaction media which nowadays find many applications, ranging from nanoparticle templating to preparative organic chemistry. The thermodynamically stable and microheterogeneous nature of microemulsions, used as reaction media, induces drastic changes in the reagent concentrations, and this can be specifically used for tuning the reaction rates. In particular, amphiphilic organic molecules can accumulate and orient at the oil-water interface, inducing regiospecificity in organic reactions. In this review, we will show the recent tendencies of the use of microemulsions as organic reaction media.

Hydrazide-based hypercoordinate silicon dichelates are remarkably flexible in terms of geometry and reactivity: this paper demonstrates how rather subtle constitutional changes result in dramatic geometrical and reactivity changes. A change of ligand-donor group from NMe 2 to N=CMe 2 changes the solid-state geometry from bicapped tetrahedral to octahedral. These two geometries are shown to coexist in solution in dynamic equilibrium. Hexacoordinate complexes are shown to dissociate to pentacoordinate compounds in two distinct ways: ionic or neutral, depending on substitution. Hexacoordinate dichelates with imino-donor groups undergo a skeletal rearrangement (intramolecular aldol-type condensation of imines), catalyzed by their dissociated halide counterions. However, even in the absence of counterions, silacyclobutane dichelates undergo a similar rearrangement.

The effect of solvents on the spectra, absorption, or emission of substances is called solvatochromism; it is due to solute/solvent nonspecific and specific interactions, including dipole/dipole, dipole-induced/dipole, dispersion interactions, and hydrogen bonding. Thermo-solvatochromism refers to the effect of temperature on solvatochromism. The molecular structure of certain substances, polarity probes, make them particularly sensitive to these interactions; their solutions in different solvents have distinct and vivid colors. The study of both phenomena sheds light on the relative importance of the solvation mechanisms. This account focuses on recent developments in solvation in pure and binary solvent mixtures. The former has been quantitatively analyzed in terms of a multiparameter equation, modified to include the lipophilicity of the solvent. Solvation in binary solvent mixtures is complex because of the phenomenon of "preferential solvation" of the probe by one component of the mixture. A recently introduced solvent exchange model allows calculation of the composition of the probe solvation shell, relative to that of bulk medium. This model is based on the presence of the organic solvent (S), water (W), and a 1:1 hydrogen-bonded species (S-W). Solvation by the latter is more efficient than by its precursor solvents, due to probe/solvent hydrogen-bonding and hydrophobic interactions. Dimethylsulfoxide (DMSO) is an exception, because the strong DMSO/W interactions probably deactivate the latter species toward solvation. The relevance of the results obtained to kinetics of reactions is briefly discussed by addressing temperature-induced desolvation of the species involved (reactants and activated complexes) and the complex dependence of kinetic data (observed rate constants and activation parameters) in binary solvent mixtures on medium composition.