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 wi th the hybrid, gradient-corrected, and local spin approximation density fun ctional methods to explore the possibility of tripler acetylene transformat ion in metastable vinylidene. For stationary points (triplet trans-acetylen e, tripler cis-acetylene, H and CCH radicals, and tripler vinylidene) in th eir minima with several transition state structures that combine this minim a were located on the potential energy surface. It was estimated that a glo bal minima on the potential energy surface is the triplet cis-acetylene. Ev en if the triplet trans-acetylene is formed by excitation of the singlet ac etylene, it should be transferred into the more stable triplet cis-acetylen e without barrier. Triplet cis-acetylene and tripler vinylidene have almost identical stabilities, but the activation barrier for the formation of tri pler vinylidene from tripler cis-acetylene through the 1,2-hydrogen shift i s estimated to be 50.4 kcal/mol. The C-H dissociation-recombination mechani sm for the tripler cis-acetylene transformation into triplet vinylidene is even higher, suggesting that direct transformation of acetylene into vinyli dene through acetylene excitation and then relaxing the formed products sho uld not be an energetically preferable path. Derailed analysis of the tripl et acetylene-vinylidene potential energy surface with both the ab initio an d density functional theory methods were discussed. (C) 2000 Elsevier Scien ce B.V. All rights reserved.