An ab initio MO study on structures and energetics of C3H−, C3H, and C3H+

The geometrical structures of cyclic and linear conformers in C3H−, C3H, and C3H+ are optimized using the full-valence CASSCF and CCSD(T) levels of theory. The atomic natural orbital ([5s3p2d1f/3s2p1d]) and correlation consistent basis (aug-cc-pVTZ) sets are used. The relative stability between the cyclic and linear conformers is investigated using the CCSD(T) and multireference CI levels of theory with the aug-cc-pVTZ basis set. The basis set dependency is checked with the 6-311+G(3d2 f,2pd) basis set. The most stable conformer in C3H− and C3H is C2v cyclic with a C3-ring, and that in C3H+ C∞v linear. The energy difference between the cyclic and linear in C3H radical is really small, being around 1.0 kcal/mol. The π-electron population on the C3-ring in cyclic C3H− is 2.00, which is a typical value predicted on the basis of the 4n+2 (n=0) aromatic-rule. The π-electron population on the C3-ring decreases in the order of C3H−, C3H, and C3H+, consistent with the order of the stability of the cyclic conformers. The adiabatic electron affinity and ionization potential in C3H are calculated to be 2.01 and 9.06 eV, respectively. The excitation energies from the most stable isomer are also calculated at the multireference CI level of theory with the aug-cc-pVTZ basis set.