The low-lying electronic states of protonated C₂, CCH ⁺

Complete active space self-consistent field (CASSCF) and multireference configuration interaction (MRCI) calculations have been performed on the energetically lowest lying electronic states of protonated C₂ (CCH⁺), using extended Gaussian basis sets of the general contraction type.The ³Π state has been unequivocally identified as the ground state of CCH⁺, followed by ³Σ^-, which is 0.34 eV higher in energy. The lowest linear singlet state is ¹Π. However, this state is not stable towards Renner-Teller distortion. A bent structure of ¹A′ symmetry is 0.21 eV lower in energy than ¹Π and represents the most stable singlet CCH⁺ cation, 0.91 eV higher than X³Π. A ¹Δ and two ¹Σ⁺ states have also been studied. They are 1.13, 1.54, and 3.55 eV higher in energy than the ³Π ground state. The activation barriers of the degenerate hydrogen migration for ground state CCH⁺, which can occur on the ³A″ or the ³A′ surface, are computed to be 1.27 and 2.43 eV, respectively, with C_2v transition structures. For the lowest singlet CCH ₊ cation, the ¹A₁ C_2v structure is not a transition state, but represents a very shallow energy minimum, 0.26 eV higher than the ¹A′ minimum. The saddle point has C_s geometry and lies 0.30 and 0.04 eV above the ¹A′ and ¹A₁ minima. Our results are compared to earlier theoretical data, obtained at significantly lower levels of ab initio theory, and with the experimentally known translational energy spectrum of CCD ⁺.