Vinylidene: a very shallow minimum on the C2H2 potential energy surface

Abstract

The potential energy surface for the singlet vinylidene → acetylene rearrangement has been investigated in this study by using nonempirical molecular electronic structure theory. A double-ζ plus polarization basis set was used in conjunction with configuration interaction (CI) including single and double excitations, a total of 13,861 configurations. Newly developed analytic CI gradient techniques were used to locate precisely the vinylidene and acetylene minima and the transition state connecting them. Single point calculations were carried out with a larger triple-ζ plus polarization basis. The classical barrier height is calculated to be 6.4 kcal, or 5.4 kcal after correction for the effects of higher excitations, and our best estimate of the true classical barrier is 4 kcal. Harmonic vibrational analyses were carried out about each of the three stationary points, and zero-point energy effects lower the effective barrier by 1.8 kcal. Even for energies below this, however, tunneling through the barrier is found to be extremely rapid; for instance, with no vibrational excitation energy (above its zero point energy) the lifetime of vinylidene with respect to rearrangement to acetylene is calculated to be only ~ 10^-9 s, and with 2 kcal of excitation energy this decreases to ~ 10^-12 s. In addition, these predictions appear to be consistent with the experimental findings of Skell (1972) and Steinfeld (1980).