Ab Initio Studies of the C₃H₄ Surface. 1. SCF and CI Study of Structures and Stabilities of Isomers

Abstract

SCF and CI calculations have been performed to study structures and stabilities of possible C3H4 isomers. The stable singlet isomers considered are methylacetylene (0), allene (0.7), cyclopropene (22.5), propenylidene (43.6), and cyclopropylidene (63.3), where calculated relative energies in kcal/mol are given in parentheses. For reactive intermediates, vinylmethylenes, low-lying states for six possible conformations (trans or cis planar carbene; trans or cis planar and bisected diradicals) are studied. Our best results show that the 3 A” states of trans and cis planar carbene structures are the most stable and isoenergetic lying 50.5 kcal/mol above the singlet methylacetylene. The lowest singlet vinylmethylene is the 1A' state of planar carbene which lies 10 kcal/mol above the 3 A” states of vinylmethylene. A study of the rotation of the CH2 group revealed that both singlet and triplet bisected diradicals and also the triplet planar diradicals are not stable. A study of cis-trans isomerization for vinylmethylene conformers indicates 7–8 kcal/mol potential energy barriers for the singlet carbenes and diradicals and 5 kcal/mol for the triplet carbenes. From the analysis of the relative stabilities of the singlet C3H4 isomers, a new possible mechanism that involves propenylidene emerged for the thermal cyclopropene to methylacetylene isomerization. © 1985, American Chemical Society. All rights reserved.