Ab initio study of radical addition reactions: Addition of a primary ethylbenzene radical to ethene (I)

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