A theoretical study of the C 3H 42+ potential energy surface

Abstract

Ab initio molecular orbital theory using basis sets up to 6-311G**, with electron correlation incorporated at the fourth-order Møller-Plesset perturbation level, has been used to examine the C 3H 4 2+ potential energy surface. The allene dication, CH 2CCH 2 2+ ( 1), is predicted to be the most stable C 3H 4 2+ ion. It has a planar D 2h geometry, in contrast to the perpendicular D 2d arrangement of neutral allene. The vinyl methylene dication, CH 2CHCH 2+ ( 2), has an unusual bridged structure at the MP2/6-31G* level, but higher-level calculations suggest that it may collapse without a barrier to 1. The cyclopropene dication, CHCH 2C H 2+ ( 3), is found to prefer a geometry with a planar tetracoordinate carbon. It lies 47 kJ mol −1 above 1, with a calculated barrier to ring opening of 28 kJ mol −1. It is a potentially observable C 3H 4 2+ isomer. The propyne (CH 3CCH 2+) and cyclopropylidene ( CH 2CC H 2+) dications are predicted not to be stable species. Several possible triplet C 3H 4 2+ isomers were examined but were all found to be considerably higher in energy than the singlets. Despite its large estimated heat of formation (2679 kJ mol −1), the allene dication is predicted to be thermodynamically stable towards deprotonation and with respect to CC cleavage into CH 2C +· and CH 2 +·. The predicted stability of the allene dication is consistent with the experimental observation of C 3H 4 2+ in the gas phase. Our calculated ionization energies for the processes C 3H 4 +· → C 3H 4 2+ and C 3H 4 → C 3H 4 2+ are somewhat lower than available experimental values.