Ab initio investigations of the electronic structure of HeCH+ and HeCH2+

The 1A′ and 3A″ electronic states of HeCH+ and the 2A′ ground electronic state of HeCH2+ have been investigated using an all electron coupled cluster single, double and triple excitation [CCSD(T)] method coupled with an augmented correlation-consistent polarised core valence triple zeta basis set (aug-cc-pCVTZ). For the 1A′ and 3A″ electronic states of HeCH+, the CCSD(T)a aug-cc-pCVTZ model yielded an optimised geometry of Cs symmetry eith a {rCHe, rCH, θHCHe} structural parameters of {2.172 Å, 1.129 Å, 81.4°} and {1.675 Å, 1.121 Å, 101.4°} respectively. Moreover, this electronic model yielded the singlet state to be 26.3 kcal mol−1 lower in energy in energy when compared with the triplet state. For the 2A′ ground electronic state of HeCH2+ the optimised structural parameters were {1.388 Å, 1.253 Å, 110.5°}, respectively. The calculated harmonic frequencies for all these electronic states were found to be real.