Ab Initio Studies of the C₃H₄ Surface. 2. MCSCF and CI Study of Structures of Vinylmethylene and Ring Opening of Cyclopropene

Abstract

MCSCF and CI calculations have been performed to investigate the potential energy surface of vinylmethylene and rearrangement paths from cyclopropene to vinylmethylene and propenylidene. The results show that triplet vinylmethylene has two isoenergetic minima (trans and cis) that have allylic planar structures and lie 45.9 kcal/mol above the X 1A1 stateof methylacetylene. Singlet vinylmethylene has four isoenergetic local minima (two for cis and two for trans) that have allylic-like nonplanar structures and lie 12 kcal/mol above triplet vinylmethylene. These singlet species are rather unstable with respect to the ring closures to cyclopropene, with potential energy barriers of 1.3 and 4.7 kcal/mol for the cis and trans species, respectively. It is found that a direct reaction path from cyclopropene to cis-vinylmethylene exists with the potential energy barrier of 36.5 kcal/mol while the barrier for the cyclopropene to trans-vinylmethylene is 40.2 kcal/mol. The rearrangement of cyclopropene to propenylidene is a concerted reaction with the potential energy barrier of 41.5 kcal/mol, in which ring opening is accompanied by a 1,2-hydrogen migration, and the barrier for the reverse reaction is 20.5 kcal/mol, indicating that this ring-opening process is product forming. We also found a surface crossing between the singlet excited and ground states of C3H4 occurring in the vicinity of the singlet vinylmethylene structure, providing a mechanism for radiationless decay of excited cyclopropene species. © 1989, American Chemical Society. All rights reserved.