Modelling the anharmonic and Coriolis resonances within the six level polyad involving the ν4 fundamental in the ro-vibrational spectrum of vinyl fluoride

The Fourier transform infrared (FTIR) spectrum of vinyl fluoride, H 2C CHF, has been deeply investigated in the ν 4 band region around 6μm at a resolution of 0.002cm -1. This normal mode, of A' symmetry species and corresponding to the C C stretching motion, yields an a/b hybrid band with a prevalent a-type character. In the present contribution, a systematic investigation is carried out by explicitly taking into account the strong and invasive anharmonic and Coriolis resonances, which perturb all of the ν 4 ro-vibrational states. Indeed, the ν 4=1 state is involved in a six-level resonant polyad, which, besides ν 4 (1656.0cm -1), comprises the A' symmetry vibrational states ν 7+ν 9 (1635.5cm -1), ν 10+ν 12 (1641.8cm -1) and 2ν 11 (1733.6cm -1), as well as the ν 8+ν 12 (1638.7cm -1) and 2ν 9+ν 12 (1683.8cm -1) combination bands of A' symmetry. The ro-vibrational analysis led to the assignment of more than 2100 transitions of ν 4 with J'≤57 and Ka'≤16, and about 1000 ro-vibrational transitions (J'≤47 and Ka'≤11) reaching the ν 7=ν 9=1 state. 505 transitions of these belong to the ν 7+ν 9-ν 9 hot band, which is located at about 1052.4cm -1 within the atmospheric window. No features are detected for the remaining vibrational states involved in the polyad, which therefore behave as dark states. By adopting an interaction model involving fourteen different anharmonic and Coriolis resonances, the assigned transitions are simultaneously fitted within the Watson's A-reduction Hamiltonian in the I r representation up to the sixth power of angular momentum operators. As demonstrated by spectral simulations, the obtained ro-vibrational Hamiltonian well reproduces the ro-vibrational spectrum of vinyl fluoride in the 6μm region.