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

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Abstract
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 X 3Π. 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 C2v transition structures. For the lowest singlet CCH+ cation, the 1A1 C2v 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 Cs 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+.