An ab initio molecular orbital study on structures and energetics of a C3H2− anion

Abstract

The geometrical structures of a C3H2− anion are surveyed at the coupled-cluster doubles and coupled-cluster singles, doubles (triples) [CCSD(T)] levels of theory with the aug-cc-pVDZ basis set. The cyclopropenylidene, propadienylidene, and C2, Cs, and C1 propargylene conformers are considered. The final energies are calculated at the CCSD(T) and multireference configuration-interaction levels of theory with the aug-cc-pVTZ basis set. The most stable C3H2− anion is propadienylidene, 2B1. The vertical electron detachment energy of the propadienylidene anion is 1.797 eV, in excellent agreement with the experiment (1.794 eV) by Oakes and Ellison. The most stable cyclopropenylidene anion is 2A1, which is higher in energy than the corresponding neutral. Thus the cyclopropenylidene neutral has no electron affinity. The π electrons on the C3 ring in the cyclopropenylidene anion are equal to 2.0, but the electrostatic repulsion overcomes the aromatic stability based on the 4n+2π-electron rule.