Ab initio study of radical reactions: Cyclization pathways for the butylbenzene radical (II)

Ab initio density functional theory calculations are presented on some mode l reactions involved in coke formation during the thermal cracking of hydro carbons. The reactions under consideration are different cyclization pathwa ys 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 pai d to the exact treatment of internal rotations and their impact on the kine tic parameters. Pre-exponential factors are very sensitive to the accuracy of constructing the microscopic partition functions. In particular, the rel ative importance of cyclization toward five and six-membered rings is studi ed 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 relativ e stability of the primary and secondary butylbenzene radical on the cycliz ation reaction is discussed. The activation energy for the formation of six -membered ring's is approximately 30 kJ/mol lower than that for five-ring f ormation. The predicted values for the kinetic parameters enable us to vali date some basic assumptions on coke formation. The calculations as presente d here are especially important for complex reaction schemes, for which exp erimental data are not always available.