Ab initio study of the addition reaction of the methyl radical to ethylene and formaldehyde

Abstract

A detailed study of the potential energy surfaces for methyl radical plus ethylene and methyl radical plus formaldehyde has been carried out with the 3-21G and 6-31G* basis sets at the Hartree-Fock level. Heats of reaction and barrier heights have been computed with the Moeller-Plesset perturbation theory up to the fourth order with and without annihilation of spin contamination. The results of the calculations indicate the formation of an early transition state with reactant-like structure for both reactions. In the case of methyl radical plus ethylene, spin annihilation lowers the barrier height by 7 kcal/mol, while in the reaction between methyl radical and formaldehyde the barrier is lowered by 6 kcal/mol when spin annihilation is considered. A comparison between the calculated and experimental values of the barrier height for the methyl addition to ethylene (6.8 kcal/mol vs 7.9 kcal/mol) and formaldehyde (6.3 kcal/mol vs. 6.8 kcal/mol) indicates very good agreement between theory and experiment.