WG02 15min1:47
AN AB INITIO CALCULATION OF BH2- ROVIBRONIC ENERGIES: A VERY SMALL SINGLET-TRIPLET SPLITTING.

J.-P. GU, G. HIRSCH, R. J. BUENKER AND PER JENSEN, FB 9 - Theoretische Chemie, Bergische Universität - Gesamthochschule Wuppertal, D-42097 Wuppertal, Germany; P. R. BUNKER, Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R6.

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\providecommand\JMSJ.~ Mol. Spectrosc. \providecommand\JCPJ.~ Chem. Phys. For each of the \widetilde X3 B1, \widetilde a1 A1, and \widetilde b1 B1 electronic states of the BH2- molecular ion we have calculated over 50 ab initio points on the potential energy surfaces using the MRD-CI method for all eight electrons. Analytical functions were fitted through the points for each state, and the functions for the \widetilde a and \widetilde b states were such that the surfaces remain degenerate at all linear configurations. For the \widetilde X state the rotation-vibration energies were calculated from the surface using the MORBID \footnote P. Jensen, J.~ Mol. Spectrosc. 128, 478--501 (1988). Hamiltonian, and for the \widetilde a and \widetilde b states the rovibronic energies were calculated using the RENNER \footnote P. Jensen, M. Brumm, W. P. Kraemer, and P. R. Bunker, J.~ Mol. Spectrosc. 171, 31--57 (1995); M. Kolbuszewski, P. R. Bunker, W. P. Kramer, G. Osmann, and P. Jensen, Mol. Phys., in press. Hamiltonian. For the \widetilde X state we obtain re = 1.2034 Å, \alphae = 129.5\deg and a barrier to linearity of 2226 cm-1. The vibrational wavenumbers are determined to be nu1 = 2353 cm-1, nu2 = 883 cm-1 and nu3 = 2498 cm-1. For the \widetilde a-state we determine re = 1.2369 Å, \alphae = 105.7\deg, a barrier height to linearity of 6071 cm-1 and the vibrational wavenumbers nu1 = 2070 cm-1, nu2 = 986 cm-1, and nu3 = 2059 cm-1. For the \widetilde b-state we determine re = 1.1936 Å, \alphae = 135.9\deg, a barrier height to linearity of 1069 cm-1 and the vibrational wavenumbers nu1 = 2399 cm-1, nu2 = 915 cm-1, and nu3 = 2591 cm-1. The most interesting result we obtain is that the singlet-triplet splitting T0 \left( \widetilde a1 A1 \right) is only 153 cm-1 for 11BH2-. Hence singlet-triplet perturbations will significantly affect all levels of the ground state.