High resolution infrared study of the 2ν9 and ν4 bands of 10BF2OH and 11BF2OH: evidence of large amplitude effects for the OH-torsion and OH-bending modes in the 92 and 41 excited states

Abstract

High resolution (2–3 × 10−3 cm−1) Fourier transform infrared spectra of gas phase 10B and 11B enriched and natural samples of BF2OH (difluoroboric acid) were recorded at Wuppertal and Richland. Starting from the results of previous studies, it was possible to perform the first rovibrational analysis of the 2ν9 (first overtone of ν9, the OH torsion) and ν4 (BOH bending) bands located at about 1043.9 and 961.7 cm−1 and 1042.9 and 961.5 cm−1 for the 10BF2OH and 11BF2OH isotopic species, respectively. Numerous ‘classical’ perturbations were observed in the analysis of the 2ν9 and ν4 bands. The energy levels of the 92 bright state are indeed involved in a B-type Coriolis resonance with those of the 6191 dark state. The 41 levels are perturbed by a B-type Coriolis resonance and by an anharmonic resonance with the levels of the 7191 and the 6171 dark states, respectively. In addition large amplitude effects were observed for the 2ν9 and also, more surprisingly, the ν4 bands. This results in splittings of the energy levels of about 0.005 and 0.0035 cm−1 for the 92 and 41 states, respectively, which are easily observable in the P and R branches for both bands. The theoretical model used to reproduce the experimental levels accounts for the classical vibration–rotation resonances. Also the large amplitude torsional (or bending) effects are accounted for within the frame of the IAM (Internal Axis Method)-like approach. The Coriolis resonances between the two torsional (or bending) substates are taken into account by {Jx, Jz} non orthorhombic terms in the v-diagonal blocks. This means that the z-quantification axis deviates from the a inertial axis by an axis switching effect of ∼35° for the {92, 6191} system and of ∼16.6° for the {41, 7191, 6171} system of interacting vibrational states. The calculation of the relative line intensities for the 2ν9 and ν4 bands accounts for these axis switching effects as well as for the intensity alternation which is due to the nuclear spin statistical weights since the OH large amplitude torsion and/or bending motion results indeed in an exchange of the two fluorine nuclei.