THE ISOMERS OF IONIZED ETHANE

The previously reported (2)A(g), (2)A(1g), and B-2(g) states of ionize d ethane are characterized at several levels of theory. The diborane-l ike (2)A(g) state, which gives rise to the observed ESR spectrum, is p redicted by SCF and CCD calculations not to exist in a separate minimu m from the (2)A(1g) state formed by ionization of the C-C bond. Howeve r, as reported by Lunell and Huang, second-order Moller-Plesset theory places the (2)A(g) lowest, provided polarization functions are includ ed on carbon. QCISD theory predicts that both A states correspond to p otential energy minima, but places the long-bond (2)A(1g) state lower, at least with moderately large basis sets. F orbitals on carbon stabi lize the diborane structure more than the long-bond one. When a potent ial energy surface is generated for a series of fixed C-C bond lengths by optimizing all variables except for the C-C bond length with MP2 t heory and calculating the energy with QCISD(T), the (2)A(g) state is p redicted to the lowest energy state with the (2)A(1g) with the (2)A(1g ) state 1.83 kJ/mol above it. The two A states are predicted to be sep arated by a barrier 2.79 kJ/mol above the lower state. This barrier is above the zero-point energy in the C-C stretch for the lower state bu t below the ZPE for the stretch in the upper state, which is therefore predicted not to exist as a stable species. A single quantum of vibra tional excitation in the low frequency C-C stretch is predicted to yie ld an ion with a poorly defined C-C bond length. The highest levels of theory employed give good agreement with the experimental hyperfine c oupling constants. The discrepancy could either be due to neglect of v ibrational effects, to poor inherent accuracy of the calculation, as o ne author has concluded, or to compression of the ion by the matrix as suggested by another. The B-2(g) state is found to be higher in energ y than the A states at all theoretical levels and is predicted to have a large (160.2-177.4 G) hyperfine coupling from four hydrogens. The t ransition state for simultaneous exchange of two hydrogen atoms betwee n the carbons by a diborane structure is predicted to lie above the lo west energy fragmentation threshold, in agreement with experiment. (C) 1996 by John Wiley & Sons, Inc.