An ab initio molecular orbital study of the C 3H +3 potential energy surface

Abstract

The C 3H + 3 potential energy surface was restudied using moderately high level ab initio molecular orbital calculations. Full geometry optimization was performed for equilibrium structures and saddle points at the Hartree-Fock level with the 4-31G and 6-31G ∗ basis sets and, in most cases, at the correlated level MP2/6-31G ∗; improved energy comparisons were obtained at the correlated level MP4/6-31G ∗∗. Four C 3H + 3 equilibrium structures were characterized and their heats of formation calculated. In ascending energy order they are the cyclopropenylium ion, the propargylium ion, 1-propen-3-ylidynylium ion, and the 1-propynylium ion. In addition, four saddle points were located. Two of these correspond to transition structures for internal rotation of the methylene group in the 1-propen-3-ylidynylium ion along alternative pathways, and a third is the transition structure for 1,2-methyl migration in the 1-propynylium ion; a second possible transition structure for the methyl migration has been characterized as a second-order saddle point. These computational results are compared with those from previous theoretical investigations and with those from a recent mass spectroscopic study in which all four of the above-mentioned equilibrium species were observed.