Theoretical study of the addition of alkyl and halogenoalkyl radicals to the ethylene double bond: a comparison between Hartree–Fock, perturbation theory and density functional theory

Abstract

The addition reactions of the radicals ˙CH₃, ˙CH₂CH₃, ˙CH(CH₃)₂, ˙C(CH₃)3, ˙CH₂F, ˙CF₃ and ˙CCl₃ to the ethylene double bond have been investigated using traditional ab initio methods (UHF, CASSCF, MP2 and MP4) and density functional theory (DFT). The DFT computations have been performed with different functionals including in all cases non-local corrections. At all levels of theory we have located for each reaction one or two transition states. When two transition states exist they correspond to different conformers which are always very close in energy. The computations have shown that the geometries of the various transition states are not very sensitive to the nature of the attacking radical. The most relevant change is a decrease in the reactant-like character of the transition state with the increasing nucleophilic character of the attacking radical. The various results also show that the topology of the reaction surface is satisfactorily described at the UHF level and that the geometries are not dramatically affected by the correlation energy corrections which cause only an increase in the reactant-like character of the transition states. However, the inclusion of dynamic correlation is essential to obtain reasonable values of the computed activation energies. We have found that the energy barriers computed with the DFT approach are strongly dependent on the type of functional which is used. The best values are provided by the Becke's three parameter hybrid functional (B3LYP). In this case the computed activation energies are in better agreement with experiment than the corresponding MP2 and MP4 values (the difference between the computed and the experimental values is in all cases within 1 kcal mol^–1). The present study indicates the B3LYP functional as a satisfactory calibration of DFT methods suitable for investigating extensively this class of reaction.