Ab initio molecular orbital studies of H + C₂H₄ and F + C₂H₄. 2. Comparison of the energetics

Heats of reaction and barrier heights have been computed for H + C₂H₄ ⇄ C₂H₅ and F + C₂H₄ → C₂H₄F → H + C₂H₃F by using Møller-Plesset perturbation theory up to fourth order with the 3-21G and 6-31G* basis sets and correcting for zero-point energy. Although energy differences and activation energies for these reactions are difficult to compute directly, reliable comparisons can be made: the CH bond dissociation energy for C₂H₄F is 4.1 ± 0.5 kcal/mol less than C₂H₅ and the exit channel barrier for C₂H₄F → H + C₂H₃F is 3.5 ± 0.5 kcal/mol larger than for C₂H₅. By combining these and related calculations with experimental data, the following estimates are obtained: the heat of reaction for F + C₂H₄ → H + C₂H₃F, -15 ± 2 kcal/mol; the CH bond dissociation energy for C₂H₄F, 31.4 ± 1 kcal/mol; and the exit channel barrier for C₂H₄F → C₂H₃F + H, 5.6 ± 0.5 kcal/mol. The implications for the dynamics of the F + C₂H₄ reaction are discussed.