An ab initio calculation of the rovibronic energies of the BH₂ molecule

Abstract

The [Xtilde]²A₁ and òB₁ electronic states of the BH₂ molecule become degenerate (²Π) when the molecule is linear and the electronic angular momentum has an important effect (the Renner effect) on the rovibronic term values. This paper reports the results of an ab initio calculation of the two potential surfaces, and of calculating the rovibronic energies allowing for the effect of the electronic angular momentum and for spin-orbit coupling. A direct calculation is made of the rovibronic term values from the three-dimensional potential surfaces of the two electronic states and from the value of the spin–orbit coupling strength. The theoretical approach used is that described previously (Jensen, P., Brumm, M., Kraemer, W. P., and Bunker, P. R., 1995, J. molec. Spectrosc., 171, 31) but an improvement is made to the basis set contraction scheme used in a manner that is particularly important for quasilinear linear Renner systems like BH₂ and CH⁺ ₂. The experimental data available for BH₂ come from a flash photolysis absorption spectrum for the òB₁← [Xtilde]²A₁ band system (Herzberg, G., and Johns, J. W. C., 1967, Proc. R. Soc. Lond. A, 298, 142). These data yield only rotational term values for the ground vibronic state and rotation–vibration term values for some bending excited vibrational levels of the excited electronic state. The ab initio barrier is adjusted to linearity in the electronic ground state, and the spin–orbit coupling strength also is modified, in order to improve agreement with available experimental data. As a result, the predictions herein should be useful indicators of the rovibronic energies and spin splittings, and in particular they should assist in a more detailed experimental characterization of the ground electronic state.