Volume 83, Issue 8

The IUPAC Conferences on Physical Organic Chemistry are one of several series that arose from Union initiatives in the service of chemistry, and have since become established through regular biennial events on the international stage. Following an inaugural conference in 1972, the enduring topicality of physical organic chemistry in all its expressions has ensured that this series continues to prosper. In a return to Korea after an interval of 14 years since ICPOC-13 (Inchon), the 20th IUPAC Conference on Physical Organic Chemistry (ICPOC-20) was held in the Exhibition & Convention Center, Busan, Korea on 22-28 August 2010. An ambitious scientific program set out to demonstrate the rich diversity of current advances and challenges through the medium of 22 plenary lectures, including presentations by four Nobel laureates, and supported by an extensive program of invited and contributed works in the form of lectures and posters. The conference was well attended by local and international delegates. This collection offers Pure and Applied Chemistry readers a representative glimpse of the scientific program, with papers by six leading authorities on topics such as models for glycosyl transfer in water (I. H. Williams), ballistic conduction in single-molecule conductors (P. W. Fowler), superelectrophilic chemistry (J. Roithová), anion receptors in highly competitive solvents (J. Jurczak), nanotube design for water splitting catalysis (M. Prato), and enzymatic catalysis (J. P. Richard). These fine works contribute new insights into mechanism and reactivity, and will enrich the archival record ((http://www.iupac.org/publications/pac/conferences/family/ICPOC/)) of a series that continues to nurture the rigorous underpinnings of the chemical sciences. The forthcoming 21st event in the ICPOC series will take place at Durham University, UK on 9-13 September 2012 under the Chairmanship of Prof. I. Williams (University of Bath, UK). James R. Bull Scientific Editor, Pure and Applied Chemistry

Superelectrophilic chemistry in the gas phase is driven by the high intrinsic reactivity of dications. The formation of new doubly charged products proceeds via highly internally excited intermediates. Conditions for the formation of the doubly charged intermediates and a “cooling principle” in the reactivity of dications are explained. The reactivity is demonstrated with several examples.

2D free-energy surfaces for transfer of the methoxymethyl cation between two water molecules are constructed from molecular dynamics (MD) simulations in which these atoms are treated quantum-mechanically within a box of 1030 classical solvent water molecules at 300 K. This provides a simple model for glycosyl transfer in water. The AM1/TIP3P surfaces with 2D-spline corrections at either MPWB1K/6-31+G(d,p) or MP2/6-31+G(d,p) contain a shallow free-energy well corresponding to an oxacarbenium ion intermediate in a D N *A N mechanism. MD analysis at three temperatures leads to a classical estimate of the lifetime of the methoxymethyl cation in water; when quantum corrections for vibrational zero-point energy are included, the lifetime is estimated to be about 1 ps, in agreement with the best experimental estimate. This suggests that computational simulation, with appropriate high-level correction, is a reliable tool to obtain detailed and reliable mechanistic descriptions for glycosidases. In view of the importance of developing improved anti-influenza drugs, simulations of sialidases that considered both sialyl oxacarbenium ion and covalent sialyl-enzyme as possible intermediates could provide particular insight.

The tight-binding version of the source-and-sink potential (SSP) model of ballistic conduction can be cast in a graph-theoretical form where the transmission through a molecular wire depends on four characteristic polynomials: those of the molecular graph and the vertex-deleted subgraphs with one or both of the molecular vertices contacting the electrodes removed. This gives an explicit function for the dependence of transmission on energy, one that is well adapted for qualitative description of general classes of conductors and conduction behavior. It also leads directly to a selection-rule criterion for conduction in terms of counting zero roots of the polynomials, which for benzenoids and graphenes is shown to subsume literature approaches based on Kekulé structure counting, bond order, and frontier-orbital matching. As explicitly demonstrated here, the SSP transmission function agrees with that derived by the Green’s function (GF) method.

Carbon nanotubes (CNTs) are versatile nanomaterials with applications spanning from medicinal chemistry and biology, to electronics as field effect transistors or energy as fuel cells. The major drawback stems from the CNT insolubility in most of the organic and aqueous media, which severely hampers the material processability. To overcome this problem, functionalization of CNTs is generally accomplished by either covalent strategies resulting in the modification of the CNT backbone via radical reactions, fluorination, and/or cycloaddition reactions, or noncovalent protocols, exploiting multiple weak interactions (hydrophobic, van der Waals, electrostatic) with suitable reagents. Herein, we highlight that a rewarding approach includes a combination of covalent/noncovalent methods, by a tailored synthetic modification of the CNT surface with polycationic dendrimeric chains, fostering the successive decoration with a multimetallic and polyanionic water oxidation catalyst. The outcome is a hybrid nanomaterial with unperturbed CNT electrical properties, in close contact with a unique multi-electron catalyst enabling electrocatalytic water splitting with high efficiency at low overpotentials.

Neutral anion receptors working in highly demanding solvents are new materials being sought. Benzopyrroles are more acidic than amides and pyrrole itself, and are promising building blocks in the design of host compounds. A whole series of receptors based upon benzopyrroles were synthesized and evaluated. They include carbazole, dipyrrolonaph-thalene, and 7-aminoindole-based hosts. Most of them demonstrate moderate binding affinities in dimethyl sulfoxide (DMSO) and have good selectivity toward tetrahedral oxyanions. Recently, a group of receptors utilizing 7-aminoindole and urea moieties proved to work in a very competitive solvent—methanol.

Deprotonation of carbon and decarboxylation at enzyme active sites proceed through the same carbanion intermediates as for the uncatalyzed reactions in water. The mechanism for the enzymatic reactions can be studied at the same level of detail as for nonenzymatic reactions, using the mechanistic tools developed by physical organic chemists. Triosephosphate isomerase (TIM)-catalyzed interconversion of D-glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP) is being studied as a prototype for enzyme-catalyzed proton transfer, and orotidine monophosphate decarboxylase (OMPDC)-catalyzed decarboxylation of orotidine 5'-monophosphate (OMP) is being studied as a prototype for enzyme-catalyzed decarboxylation. 1 H NMR spectroscopy is an excellent analytical method to monitor proton transfer to and from carbon catalyzed by these enzymes in D 2 O. Studies of these partial enzyme-catalyzed exchange reactions provide novel insight into the stability of carbanion reaction intermediates, which is not accessible in studies of the full enzymatic reaction. The importance of flexible enzyme loops and the contribution of interactions between these loops and the substrate phosphodianion to the enzymatic rate acceleration are discussed. The similarity in the interactions of OMPDC and TIM with the phosphodianion of bound substrate is emphasized.

Supplemented temperature/composition phase diagrams include the non-equilibrium glass-transition temperature ( T g ) curve and equilibrium ice-melting and solubility curves. The inclusion of the non-equilibrium curve allows one to establish relationships with the time coordinate and, thus, with the dynamic behavior of systems, provided that the thermal history of such systems is known. The objective of this report is to contribute to the potential applications of supplemented state diagrams for aqueous glass-formers, in order to describe the influence of water content, nature of vitrifying agents, and temperature on the physico-chemical properties of foods and biological and pharmaceutical products. These data are helpful to develop formulations, processing strategies, or storage procedures in order to optimize the stability of food ingredients and pharmaceutical formulations. Reported experimental data on phase and state transitions for several food and pharmaceutical systems were analyzed. Some methodological aspects and the effect of phase and state transitions on the main potential chemical reactions that can alter those systems during processing and/or storage are discussed.

The term “hydrogen bond” has been used in the literature for nearly a century now. While its importance has been realized by physicists, chemists, biologists, and material scientists, there has been a continual debate about what this term means. This debate has intensified following some important experimental results, especially in the last decade, which questioned the basis of the traditional view on hydrogen bonding. Most important among them are the direct experimental evidence for a partial covalent nature and the observation of a blue-shift in stretching frequency following X – H···Y hydrogen bond formation (XH being the hydrogen bond donor and Y being the hydrogen bond acceptor). Considering the recent experimental and theoretical advances, we have proposed a new definition of the hydrogen bond, which emphasizes the need for evidence. A list of criteria has been provided, and these can be used as evidence for the hydrogen bond formation. This list is followed by some characteristics that are observed in typical hydrogen-bonding environments.

A novel definition for the hydrogen bond is recommended here. It takes into account the theoretical and experimental knowledge acquired over the past century. This definition insists on some evidence. Six criteria are listed that could be used as evidence for the presence of a hydrogen bond.