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

Abstract The hot band 3 ν 9 − ν 9 of the isotopologue 11 BF 2 OH (difluoroboric acid) located at 1034.78 cm −1 was investigated for the first time by Fourier transform infrared spectroscopy. During previous studies both, the ν 9 mode (OH-torsion relative to the BF 2 moiety, at 522.87 cm −1 ) and the ν 4 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 2 ν 9 and ν 4 bands located at 1042.87 and 961.49 cm −1 , respectively. The present work establishes large amplitude effects also for the 9 3 excited state located at 1557.655 cm −1 . Numerous P and R transitions of the 3 ν 9 – ν 9 hot band were identified in the 2 ν 9 manifold, and doublets corresponding to a torsional splitting of 0.031 cm −1 in the 9 3 state were observed. The vibrational assignment of the 9 3 state was confirmed by the detection of the 3 ν 9 −2 ν 9 hot band Q branch in the 19 μm region.