M. Waroquier

With the aid of density functional theory calculations, all conformers of several single-chain alcohols, thiols, ethers, and sulfides are investigated. Starting from earlier computational works on n-alkanes, we construct an extended set of general rules for predicting the number and occurrence of conformers in these oxygen- or sulfur-containing compounds. In alcohols and thiols, it is found that only the conformers generated by internal rotations in the HXCH2CH2CH2 (X = O or S) top are distinctive from those in n-alkanes. In ethers and sulfides, the primary influence of the heteroelement also extends up to three internal rotations, but many more conformers are possible. However, a number of double gauche sequences are forbidden, and therefore, several conformers can be eliminated. These exclusions in particular make up a set of rules for eventually deducing all possible conformers. Furthermore, on the basis of only an exact calculation of these gg conformations in addition to single gauche conformers, it is possible to make an accurate estimate of the relative energy. This two-dimensional approximation scheme constitutes an effective tool for adequately describing the relative energies of all possible conformers at a minimal computational cost.

A complete and consistent set of 95 Benson group additive values (GAV) for the standard enthalpy of formation of hydrocarbons and hydrocarbon radicals at 298 K and 1 bar is derived from an extensive and accurate database of 233 ab initio standard enthalpies of formation, calculated at the CBS-QB3 level of theory. The accuracy of the database was further improved by adding newly determined bond additive corrections (BAC) to the CBS-QB3 enthalpies. The mean absolute deviation (MAD) for a training set of 51 hydrocarbons is better than 2 kJ mol-1. GAVs for 16 hydrocarbon groups, i.e., C(Cd)3(C), C−(Cd)4, C−(Ct)(Cd)(C)2, C−(Ct)(Cd)2(C), C−(Ct)(Cd)3, C−(Ct)2(C)2, C−(Ct)2(Cd)(C), C−(Ct)2(Cd)2, C−(Ct)3(C), C−(Ct)3(Cd), C−(Ct)4, C−(Cb)(Cd)(C)(H), C−(Cb)(Ct)(H)2, C−(Cb)(Ct)(C)(H), C−(Cb)(Ct)(C)2, Cd−(Cb)(Ct), for 25 hydrocarbon radical groups, and several ring strain corrections (RSC) are determined for the first time. The new parameters significantly extend the applicability of Benson's group additivity method. The extensive database allowed an evaluation of previously proposed methods to account for non-next-nearest neighbor interactions (NNI). Here, a novel consistent scheme is proposed to account for NNIs in radicals. In addition, hydrogen bond increments (HBI) are determined for the calculation of radical standard enthalpies of formation. In particular for resonance stabilized radicals, the HBI method provides an improvement over Benson's group additivity method.

The potential energy of n-hexane is studied since it constitutes a typical example of a single chain molecule in which various internal rotations are present and a large number of conformations are existing, which cannot be reached by using one-dimensional rotational energy profiles. For an accurate reproduction of the global partition function and all derived thermodynamic properties an adequate description of all possible conformers is necessary. The full three-dimensional potential energy surface of the internal rotations in n-hexane (3D-PES) is calculated at an ab initio level and compared with one-dimensional schemes to reproduce the energy. Due to the higher dimensionality of the relevant potential energy surface, the computational cost is very high. A new approximate scheme based on two-dimensional cuts is proposed that gives good accuracy for the relative conformational energies and kinetic energies at a reasonable computational cost. This scheme is of general use for any long chain molecule.

Density functional theory calculations are used to determine the kinetics and reactivity indices of the first propagation steps of the polyethylene and poly(vinyl chloride) polymerization. Transition state theory is applied to evaluate the rate coefficient from the microscopically determined energies and partition functions. A comparison with the experimental Arrhenius plots validates the level of theory. The ability of reactivity indices to predict certain aspects of the studied propagation reactions is tested. Global softnesses of the reactants give an indication of the relative energy barriers of subsequent monomer additions. The correlation between energy and hardness profiles along the reaction path confirm the principle of maximum hardness. Local indices predict the regioselectivity of the attack of the growing radical to vinyl chloride. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Theoretical calculations are presented to determine the structure of radical defects in crystalline lattices. The applications are concentrated on radical defects as they are induced by irradiation in organic crystals and paramagnetic molecular ions embedded in alkali halide lattices. Various approaches are possible to model the molecular environment: the single-molecule approach, the cluster approach, and periodic calculations. The latter are based on a Car-Parrinello formalism in which the molecular orbitals are expanded in a plane-wave basis set and in which the optimized structures at 0 K are obtained by a simulated annealing technique. The pros and cons of the various approaches are highlighted, and where possible comparison with experimental election paramagnetic resonance data are given. Due to the different natures of ionic and organic crystals, specific computational procedures are needed to get good correspondence with the experimental data. In various cases there is experimental evidence that some radical structures are submitted to noticeable changes with increasing temperature. These effects were theoretically reproduced by performing molecular dynamics calculations at elevated temperatures. (C) 2004 Wiley Periodicals, Inc. | Conference: 10th International Conference on the Applications of Density Functional Theory in Chemistry and Physics Location: Brussels, BELGIUM Date: SEP 05-12, 2003