PNNL

Abstract

High resolution (2-3x10-3cm-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 [A.Perrin, M.Carvajal-Zaera, Z.Dutkiewicz, J.-M.Flaud, D.Collet, H.Bürger, J.Demaison, F.Willaert, H.Mäder, and N.W.Larsen, Mol. Phys. 102 , 1641 (2004); J. Breidung, J. Demaison, J.-F. D’Eu, L. Margulès, D. Collet, E.B. Mkadmi, A. Perrin and W. Thiel, J. Mol. Spectrosc. 228, 7, (2004)], it was possible to perform the first rovibrational analysis of the 2?9 (first overtone of ?9, the OH torsion) and ?4 (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 “classic” perturbations were observed in the analysis of the 2?9 and ?4 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 2?9 and also, more surprisingly, the ?4 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 classic 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 zquantification 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 2?9 and ?4 bands accounts for these axis switching effects as well as for the intensity alternation which is due to the nuclear spin statistics since the OH large amplitude torsion and/or bending motion results indeed in an exchange of the two fluorine nuclei.