Ab initio and hybrid density functional theory studies of the forward and reverse barriers for the C2H4 + H → C 2H5 reaction

Abstract

Ab initio and density functional theory (DFT) methods have been used to compute the potential energy surface of the C₂H₄ + H → C ₂H₅ reaction. It was demonstrated that the computation of the forward reaction barrier is a very difficult problem. Even high level ab initio methods, such as MP2 and QCISD, had problems coming close to the experimental value. It was demonstrated that the G2 computational method predicts a very accurate energy profile for the reaction. DFT methods have a considerable problem in finding and optimizing the transition state structure. Many of these produced negative activation barriers. It was suggested that DFT methods overestimate the total energy of the hydrogen radical. Because of this effect, the heat of the reaction and the barrier of the reaction are too low. If this problem is solved, the energies generated by hybrid DFT methods for reactions which involve the hydrogen radical should be very close to experimental values.