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, H2C=CHF, has been deeply investigated in the nu(4) band region around 6 at a resolution of 0.002 cm(-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 nu(4) ro-vibrational states. Indeed, the nu(4)=1 state is involved in a six-level resonant polyad, which, besides nu(4) (1656.0 cm(-1)), comprises the A' symmetry vibrational states nu(7)+nu(9) (1635.5 cm(-1)), nu(10)+nu(12) (1641.8 cm(-1)) and 2 nu(11) (1733.6 cm(-1)), as well as the nu(8)+nu(12) (1638.7 cm(-1)) and 2 nu(9)+nu(12) (1683.8 cm(-1)) combination bands of A '' symmetry. The ro-vibrational analysis led to the assignment of more than 2100 transitions of nu(4) with J '' <= 57 and K-a '' <= 16, and about 1000 ro-vibrational transitions (J '' <= 47 and K-a '' <= 11) reaching the nu(7)=nu(9)=1 state. 505 transitions of these belong to the nu(7)+nu(9)-nu(9) hot band, which is located at about 1052.4 cm(-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 6 mu m region. (C) 2011 Elsevier Ltd. All rights reserved.