Cleavage of neutral alkenes and alkene radical cations; hybrid Hartree-Fock/density functional theory results

The cleavage reactions of the 1-butene and 4,4-dimethyl-1-pentene molecules and their cations, to form neutral and charged hydrocarbon products, are i nvestigated using hybrid Hartree-Fock/density functional theory. In compari son with previous theoretical results (Du et al.), the density functional c leavage and ionization energies, including zero-point vibrational energy, l ie in better agreement with experimental and thermochemical data. Assuming vertical ionization processes the mean absolute deviation (MAD) compared wi th experiment is 3.4 kcal/mol for the reaction sequences studied. Using adi abatic ionization processes instead gives a MAD of 5.2 kcal/mol. The larges t deviation from experiment occurs for the cleavage reactions of the neutra l parent molecules, where the difference between theory and experiment is u p to 12.8 kcal/mol. In addition to reaction energies we also report optimiz ed ground-state structures, and for the radicals studied, isotropic hyperfi ne coupling constants that are compared to experimental data. It is found t hat the experimental hyperfine properties of the 1-butene cation can be exp lained by rotational averaging caused by the flat potential surface for rot ation about the C2-C3 bond.