Multimode Jahn–Teller and pseudo-Jahn–Teller effects in BF+3

Abstract

The ionic states of BF₃ exhibit striking vibronic coupling effects. These effects are investigated using ab initio calculated ionization energies and vibronic coupling constants. The two Jahn–Teller active modes show interference effects in all bands of E symmetry. In the 2A₁^′ ionic state single quanta of a degenerate vibration are excited. They borrow their intensities from the adjacent 2E′ state by means of a pseudo‐Jahn–Teller interaction. The symmetry of BF⁺₃ in its nondegenerate ground state is predicted to be lowered due to two‐mode pseudo‐Jahn–Teller interaction with the second excited state. This interaction is surprisingly strong and leads to an interesting vibronic structure. The vibrational fine structure in all experimental spectra is reproduced satisfactorily. The earlier discrepancies in assigning the three lowest lying ionic states are explained.