The electronic spectrum of the fluoroborane free radical. I. Theoretical calculation of the vibronic energy levels of the ground and first excited electronic states

The fluoroborane HBF free radical has a large vibronic interaction which splits the orbitally degenerate 2Pi state in the linear configuration into two separate electronic states, one strongly bent and one linear. The observed vibrational structure of the electronic transition between the Renner–Teller pair of states is very complex. As an aid to understanding the spectrum, the vibronic energy levels of the ground and first excited states have been calculated from high-level ab initio potential energy surfaces using a variational method. The vibrational frequencies and anharmonicities have been derived from these energy levels and the boron and hydrogen isotope shifts have been predicted. Although the ground state energy levels are for the most part well behaved, the excited state levels show substantial Renner–Teller mixing with nearby ground vibrational states. The calculations in the present work have been successfully used in the companion paper to make vibrational assignments of the laser-induced fluorescence spectra of HBF and DBF.