Theoretical studies on the singlet and triplet cyclopropylidene--allene system

Ab initio SCF calculations employing the STO-3G and 4-31G basis sets have been carried out with full geometry optimization on the singlet and triplet cyclopropylidene-allene systems. Triplet cyclopropylidene (1T) is calculated to be of lower energy than singlet cyclopropylidene (1S) by 8.4 kcal/mol. Triplet allene is calculated to be a bent, planar species, being 8.0 kcal/mol lower in energy than the linear, planar form. Both 1S and 1T undergo disrotatory ring opening, although the intimate details of the two processes differ greatly. In both cases substantial shortening of the C--C bond lengths are observed as the ring opens. Barrier heights for the ring opening of 1S and 1T are calculated to be 18 and 19 kcal/mol, respectively (4 to 31G level). The results of these calculations are significantly different from those reported previously, and the differences are discussed. The structures and ring-opening processes of 1S and 1T are compared with those reported for the cyclopropyl cation and radical