Complete basis set, G1, G2, G2MP2, and density functional theory computational studies of the lowest energy triplet potential energy surface for acetylene-vinylidene rearrangement

The triplet acetylene-vinylidene potential energy surface was explored with the complete basis set, G1, G2, and G2MP2 ab initio methods, as well as with the hybrid, gradient-corrected, and local spin approximation density functional methods to explore the possibility of tripler acetylene transformation in metastable vinylidene. For stationary points (triplet trans-acetylene, tripler cis-acetylene, H and CCH radicals, and tripler vinylidene) in their minima with several transition state structures that combine this minima were located on the potential energy surface. It was estimated that a global minima on the potential energy surface is the triplet cis-acetylene. Even if the triplet trans-acetylene is formed by excitation of the singlet acetylene, it should be transferred into the more stable triplet cis-acetylene without barrier. Triplet cis-acetylene and tripler vinylidene have almost identical stabilities, but the activation barrier for the formation of tripler vinylidene from tripler cis-acetylene through the 1,2-hydrogen shift is estimated to be 50.4 kcal/mol. The C-H dissociation-recombination mechanism for the tripler cis-acetylene transformation into triplet vinylidene is even higher, suggesting that direct transformation of acetylene into vinylidene through acetylene excitation and then relaxing the formed products should not be an energetically preferable path. Derailed analysis of the triplet acetylene-vinylidene potential energy surface with both the ab initio and density functional theory methods were discussed. (C) 2000 Elsevier Science B.V. All rights reserved.