G.B. Marin

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 reactivity of polyaromatics involved in various radical reactions is studied. The reactions under study are hydrogen abstractions by a methyl radical and additions to double bonds both intra- and intermolecular. The chemical reactivity of the involved molecules is described through different properties, which are calculated within the density functional theory (DFT) framework. The softness reactivity index is tested on its usefulness and reliability to provide information about the reactivity of the global molecule or about chemical selectivity. The applicability of the hard and soft acids and bases (HSAB) principle for bimolecular radical reactions is illustrated by comparing the results of the softness-matching criterion with kinetic and thermodynamic data. For large polyaromatic molecules several magnetic indices, in particular, magnetic susceptibilities, chemical shifts, and nucleus independent chemical shifts (NICS), are computed to quantify the aromatic character of the involved species. The applicability of these magnetic indices in the case of radical reactions is validated by comparing with kinetic results obtained from transition state theory.

Theoretical ab initio calculations are presented on cyclization reactions of polyaromatics that occur by a radical mechanism. Such processes are one of the elementary steps for polyaromatic hydrocarbon formation in thermal cracking units and during soot formation. Ring closure can take place at various sites of the polyaromatic surface. It is the aim of this study to obtain insight into the influence of the local structure on the reactivity of the polyaromatics. Aromaticity is a determining factor for the reactivity and can be probed by various magnetic properties such as the diamagnetic susceptibilities, proton chemical shifts, and nucear-independent chemical shifts. A correlation is established between the magnetic properties and activation energy of the studied reactions. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

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.

Ab initio density functional theory calculations are presented on cyclization reactions of polyaromatics involved in coke formation during the thermal cracking of hydrocarbons. During coke formation, cyclization can take place at various sites, differing from each other by the local polyaromatic structure. This local structure also determines the minimum number of carbon atoms that must be added to allow the formation of a new ring. Kinetic parameters are calculated for the various ring-closure reactions by means of transition state theory. The activation energy is largely affected by the local structure of the polycyclic aromatic hydrocarbon, whereas the frequency factor varies significantly in terms of the length of the attached alkyl chain. The calculations, as presented, give a microscopic insight into the mechanisms that contribute to barrier formation and to the value of the frequency factor. The relative importance of cyclization at different sites, under conditions typical for an industrial cracking unit, is studied on the basis of the calculated rate constants at various temperatures. The results suggest that the nature of coke formation is autocatalytic: the larger the macroradicals, the faster the subsequent reactions that lead to further growth of the polyaromatic surface. This type of calculation is the first step towards the development of structural relations for the kinetic parameters in terms of the local structure of the coke matrix.