HIGH RESOLUTION INFRARED STUDY OF THE $\nu_4$ AND 2$\nu_9$ BANDS OF $^{11}$BF$_2$OH AND $^{10}$BF$_2$OH: EVIDENCE OF LARGE AMPLITUDE EFFECTS FOR THE OH-TORSION-BENDING MODES IN THE 9$^2$ AND 4$^1$ STATES FOR THE OH-TORSION-BENDING MODES IN THE 9$^2$ AND 4$^1$ STATES

{A.Perrin, M.Carvajal-Zaera, Z.Dutkiewicz, J.M.Flaud, D.Collet, H.Burger, J.Demaison, F.Willaert, H.Mader, and N.W.Larsen, Mol. Phys. 102 , 1641 (2004), J.Breidung, J.Demaison, J.F.D'Eu, J.M.Flaud, L.Margules, D.Collet, E.B.Mkadmi, A.Perrin, and W.Thiel, J. Mol. Spectrosc. 228, 7 (2004).{L.H.Coudert and J.T.Hougen, J. Mol. Spectrosc. 130, 86 (1988)Author Institution: Laboratoire Inter Universitaire des Systemes Atmospheriques,; CNRS, Universite Paris 12, 61 Av du General de Gaulle, 94010 Creteil; Cedex France; Anorganische Chemie, FBC, Universitat, D-42097 Wuppertal, Germany; Laboratoire PhLAM, CNRS, Universite de Lille I, Bat. P5,; 59655 Villeneuve d'Ascq Cedex, France; Pacific Northwest National Laboratory, 902 Battelle Boulevard,; P.O. Box 999, Richland, WA 99352, USA; Department of Chemistry, University of Texas, Austin, Texas, 78712-1167; USAHigh resolution (2-3 x 10$^{-3}$ cm$^{-1}$) Fourier transform infrared spectra of gas phase $^{11}$B and $^{10}$B enriched and natural samples of BF$_2$OH (difluoroboric acid) were recorded at Wuppertal and Richland. Starting from the results of previous studies}, it has been possible to perform the first rovibrational analysis of the $\nu_4$ (BOH bending) and 2$\nu_9$ (first overtone of $\nu_9$, the OH torsional mode) bands located at 961.5 and 1042.9 cm$^{-1}$) and at 961.7 and 1043.9 cm$^{-1}$) for the $^{11}$BF$_2$OH and $^{10}$BF$_2$OH isotopic species respectively. In addition to various "classical" vibration-rotation resonances, large amplitude effects were observed for both the 2$\nu_9$ and $\nu_4$ bands. These result in a doubling of levels of about 0.004 and 0.003 cm$^{-1}$) for the 9$^2$ and 4$^1$ states respectively. The energy level calculations account for these large amplitude torsion-bending perturbations using the IAM- type method}, and for the "classical" vibration -rotation resonances