D. Van Neck

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.

Ab initio density functional theory calculations have been carried out on a model reaction involved in coke formation during the thermal cracking of hydrocarbons, namely, the addition of the ethylbenzene radical to ethene. This study enables one to get more microscopic insight into the mechanistic and kinetic aspects of the reaction. A profound ab initio conformational analysis of the formed products, reactants, and transition states is made. The impact of internal rotations on the two kinetic parameters deduced from transition state theory (TST), the activation energy and the preexponential factor, has been studied in detail. Furthermore, we report on the various components that govern the kinetic parameters. Preexponential factors are very sensitive to the accuracy of constructing the microscopic partition functions. Internal rotations play a dominant role in the reaction mechanism, and their impact on the preexponential factor is large. Hence, a very accurate handling of internal rotations is of crucial importance. We present a new algorithm to extract exactly on a quantum mechanical basis the partition functions of the internal rotations. The calculations as presented here are especially important for complex reaction schemes, for which experimental data are not always available.

A variational optimization of the second-order density matrix under the P-, Q-, and G-conditions was carried out for a set of diatomic 14-electron molecules, including N2, O22+, NO+, CO, and CN−. The dissociation of these molecules is studied by analyzing several chemical properties (dipole moments, population analysis, and bond indices) up to the dissociation limit (10 and 20 Å). Serious chemical flaws are observed for the heteronuclear diatomics in the dissociation limit. A careful examination of the chemical properties reveals that the origin of the dissociation problem lies in the flawed description of fractionally occupied species under the P-, Q-, and G-conditions. A novel constraint is introduced that imposes the correct dissociation and enforces size consistency. The effect of this constraint is illustrated with calculations on NO+, CO, CN−, N2, and O22+.