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

The geometrical structures of a ${\mathrm{C}}_{\mathrm{3}}{\mathrm{H}}_{\mathrm{2}}^{\mathrm{-}}$ 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 ${\mathrm{C}}_{\mathrm{2}},$ ${\mathrm{C}}_s,$ and ${\mathrm{C}}_{\mathrm{1}}$ 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 ${\mathrm{C}}_{\mathrm{3}}{\mathrm{H}}_{\mathrm{2}}^{\mathrm{-}}$ anion is propadienylidene, ${}^2{B}_1.$ 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 ${}^2{A}_1,$ which is higher in energy than the corresponding neutral. Thus the cyclopropenylidene neutral has no electron affinity. The π electrons on the ${\mathrm{C}}_{\mathrm{3}}$ ring in the cyclopropenylidene anion are equal to 2.0, but the electrostatic repulsion overcomes the aromatic stability based on the $\text{4n+2π}$ -electron rule.