M. Saeys

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.

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 some model reactions involved in coke formation during the thermal cracking of hydrocarbons. The reactions under consideration are different cyclization pathways for the butylbenzene radical, which can lead to a further growth of the coke layer. This study enables us to gain more microscopic insight into the mechanistic and kinetic aspects of the reactions. Special attention is paid to the exact treatment of internal rotations and their impact on the kinetic parameters. Pre-exponential factors are very sensitive to the accuracy of constructing the microscopic partition functions. In particular, the relative importance of cyclization toward five and six-membered rings is studied on the basis of the calculated rate constants and concentration profiles of the reactants. The influence of the size of the ring and of the relative stability of the primary and secondary butylbenzene radical on the cyclization reaction is discussed. The activation energy for the formation of six-membered rings is approximately 30 kJ/mol lower than that for five-ring formation. The predicted values for the kinetic parameters enable us to validate some basic assumptions on coke formation. The calculations as presented here are especially important for complex reaction schemes, for which experimental data are not always available.