M. Waroquier

Accurate activation energies for 67 hydrocarbon radical addition and beta-scission reactions are calculated with the CBS-QB3 ab initio method. An extension of Benson's group additivity method to activation energies is introduced. The underlying hypotheses, that is, the group concept and the additivity approximation, are validated with ab initio data. Standard activation group additivity values are obtained from the ab initio calculations for reactions involving primary, secondary, tertiary alkylic, allylic, benzylic, and vinylic radicals. The proposed group contribution method yields accurate activation energies for radical addition and for beta-scission reactions. The effect of substituents on the carbon atoms of the reactive center on the activation energy can be as large as 95 kJ/mol for the adding radical, and 187 kJ/mol for the product radical of the P-scission. Non-nearest-neighbor effects such as gauche and cis interactions have an influence of less than 3 kJ/mol per interaction on the activation energies. However, for hydrocarbons that are heavily branched near the reactive center, these interactions can become important. (C) 2004 American Institute of Chemical Engineers.

The symmetry energy for nuclear matter and its relation to the neutron skin in finite nuclei is discussed. The symmetry energy as a function of density obtained in a self-consistent Green function approach is presented and compared to the results of other recent theoretical approaches. A partial explanation of the linear relation between the symmetry energy and the neutron skin is proposed. The potential of several experimental methods to extract the neutron skin is examined.

A fully self-consistent treatment of short-range correlations in nuclear matter is presented. Different implementations of the determination of the nucleon spectral functions for different interactions are shown to be consistent with each other. The resulting saturation densities are closer to the empirical result when compared with (continuous choice) Brueckner-Hartree-Fock values. Arguments for the dominance of short-range correlations in determining the nuclear matter saturation density are presented. A further survey of the role of long-range correlations suggests that the inclusion of pionic contributions to ring diagrams in nuclear matter leads to higher saturation densities than empirically observed. A possible resolution of the nuclear matter saturation problem is suggested.

A quantum chemical investigation is presented for the determination of accurate kinetic and thermodynamic parameters for hydrocarbon radical reactions. First, standard enthalpies of formation are calculated at different levels of theory for a training set of 58 hydrocarbon molecules, ranging from C1 to C10, for which experimental data are available. It is found that the CBS-QB3 method succeeds in predicting standard enthalpies of formation with a mean absolute deviation of 2.5 kJ/mol, after a systematic correction of −1.29 kJ/mol per carbon atom and −0.28 kJ/mol per hydrogen atom. Even after a systematic correction, B3LYP density functional theory calculations are not able to reach this accuracy, with mean absolute deviations of 9.2 (B3LYP/6-31G(d)) and 12.9 kJ/mol (B3LYP/6-311G(d,p)), and with increasing deviations for larger hydrocarbons. Second, high-level transition state geometries are determined for 9 carbon-centered radical additions and 6 hydrogen additions to alkenes and alkynes and 10 hydrogen abstraction reactions using the IRCMax(CBS-QB3//B3LYP/6-311G(d,p)) method. For carbon-centered radical addition reactions, B3LYP/6-311G(d,p) slightly overestimates the length of the forming C−C bond as compared to the IRCMax data. A correlation to improve the agreement is proposed. For hydrogen addition reactions, MPW1K density functional theory (MPW1K/6-31G(d)) is able to locate transition states. However, the lengths of the forming C−H bonds are systematically longer than reference IRCMax data. Here, too, a correlation is proposed to improve the agreement. Transition state geometries for hydrogen abstraction reactions obtained with B3LYP/6-311G(d,p) show good agreement with the IRCMax reference data. Third, the improved transition state geometries are used to calculate activation energies at the CBS-QB3 level. Comparison between both CBS-QB3 and B3LYP density functional theory predictions shows deviations up to 25 kJ/mol. Although main trends are captured by B3LYP DFT, secondary trends due to radical nucleophilic effects are not reproduced accurately.

The thermal degradation process of some new synthesized polycarbonates is studied both from theoretical and experimental points of view to obtain microscopic insight into the factors that govern the reaction rates. Ab initio density functional theory calculations are performed on a set of carbonates, which are model systems for the polycarbonates. Kinetic parameters are determined by transition-state theory and compared with experimental-derived degradation temperatures. The study as presented here gives evidence of the existence of a correlation between these degradation temperatures and the reaction rates resulting from static ab initio calculations on small model systems that are representative for the active area of the degradation process. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003