Vinylmethylene: Theoretical Investigations1a

Generalized valence bond calculations on cyclopropene and vinylmethylene (the open-shell species formed when cyclopropene undergoes ring cleavage) lead to the following conclusions: (1) the allyl-type -π system is strongly distorted by the presence of the unpaired σ electron, leading to a methylene-like triplet, 3A” (4), but a 1,3-diradical-like singlet state, 1A” (3); (2) the lowest-lying singlet state of vinylmethylene has the form of a singlet methylene 1A' (6) lying 12 kcal/mol above the 3A” ground state, while the diradical singlet state 1A” (3) lies at 14 kcal/mol; (3) the 3A” ground state of 4 should exist in two geometrical forms (syn and anti) having nearly the same energy and separated by a large barrier (9 kcal); and (4) the diradical singlet state has only a low barrier (2 kcal/mol) between the syn and anti forms while the methylene triplet and singlet states have large barriers. These results suggest that the ring opening of cyclopropene proceeds directly to a diradical planar intermediate.