Theory of the ionization of ethane

Ab initio molecular orbital studies on C₂H₆ ⁺ support an assignment of ² A _1g (D _3d point group symmetry) for the ground state of this cation. This differs from the prediction of Koopmans' theorem which suggests a ² E_g state for vertical ionization. Approximate force constants and vibrational frequencies are calculated for the ethane cation in the ² A _1g state and are found to be consistent with the vibrational progression observed in the experimental photoelectron spectrum.