The Strange Case of the Ethane Radical Cation.

Energetically low-lying structures on the ethane radical cation potential energy surface are compared with the CCSD/DZP, BD/DZP, CCSD(T)/DZP, BD(T)/DZP, CCSD(T)/TZ2P, and CCSD(T)/TZ(2df,7pd)//CCSD(T)/TZ2P methods, and a number of density functional levels as well as second-order perturbation theory. The (2)A(g) state (C-2h symmetry) is only 0.28 kcal mol(-1) lower than the (2)A(1g) state (D-3d symmetry) at BD(T)/DZP and 0.30 kcal mol(-1) at CCSD(T)/TZ(2df,2pd)//CCSD(T)/TZ2P. The two states are separated by a transition state which is less than 0.2 kcal mol(-1) above the energetically higher state, and inclusion of the ZPVE correction can reverse the ordering. The Born-Oppenheimer approximation is therefore not strictly valid and only one thermally averaged C-2h structure should be observed. A third C2H6+ equilibrium geometry is obtained by removal of an electron from the second component of the e(g) orbital of ethane (D-3d symmetry). It corresponds to either a B-2(g) state (C-2h symmetry) or a (2)A " state (C-g symmetry), depending on whether the SOMO is spread out over both carbons or localized on one carbon only. This third minimum lies 3.37 kcal mol(-1) higher in energy than the (2)A(g) state at CCSD(T)/TZ(2df,2pd)//CCSD(T)/TZ2P. The ground state C2H6+ barrier for rotation around the CC bond is estimated to be at least 1.0 kcal mol(-1), in disagreement with the experimental value of only 0.25 kcal mol(-1).