First analysis of the 3ν9−ν9 hot band of difluoroboric acid (BF2OH). Further evidence of large amplitude effects for the OH torsion

The hot band 3ν9−ν9 of the isotopologue 11BF2OH (difluoroboric acid) located at 1034.78 cm−1 was investigated for the first time by Fourier transform infrared spectroscopy. During previous studies both, the ν9 mode (OH-torsion relative to the BF2 moiety, at 522.87 cm−1) and the ν4 mode (in-plane OH bend) had been shown to exert large amplitude motion, and splittings of 0.0051 and 0.0038 cm−1 had been observed in the interacting 2ν9 and ν4 bands located at 1042.87 and 961.49 cm−1, respectively. The present work establishes large amplitude effects also for the 93 excited state located at 1557.655 cm−1. Numerous P and R transitions of the 3ν9-ν9 hot band were identified in the 2ν9 manifold, and doublets corresponding to a torsional splitting of 0.031 cm−1 in the 93 state were observed. The vibrational assignment of the 93 state was confirmed by the detection of the 3ν9−2ν9 hot band Q branch in the 19 μm region.

► Difluoroboric acid (BF2OH). ► The 3ν9−ν9 hot hand, ν9 is the large amplitude OH- torsion. ► 0.031 cm−1 torsional splitting in the 93 excited state. ► Significant differences with nitric acid (isoelectronic molecule) in the 93 state.