Theoretical study of structures and energies of acetylene, ethylene, and vinyl radical and cation

A systematic study of the structures and energies of acetylene, ethylene, and vinyl radical and cation using high levels of ab initio molecular orbital theory, including correlation effects beyond fourth order perturbation theory, is presented. In agreement with previous theoretical studies, the present theory predicts that the bridged form of C2H+3 is more stable than the classical form with an energy difference of 3.1 kcal/mol (including zero point energies). The bond dissociation energy of C2H4,D0(C–H), is calculated to be 110.2 kcal/mol in agreement with the recent photoionization study of Berkowitz et al. and in significant disagreement with several previous experimental studies. Finally, triplet vinyl cation is studied for the first time and is calculated to be 54.5 kcal/mol higher in energy than the singlet ground state.