High-resolution infrared spectrum of CH2DF: analysis of the v5/v6/v9 triad

The very weak v9 fundamental band (1132.78 cm-1) of CH2DF has been studied for the first time at high resolution (0.004 cm-1) by FTIR spectroscopy. This mode of A'' symmetry species produces a special a-type band, induced by Coriolis coupling with v5 (1055.54 cm-1, A' species), following the selection rules DKa=0 and DKc=0, +/-2. The v5 level has, in turn, been found to be coupled with v6 (938.30 cm-1, A' species) through c-type Coriolis and higher-order vibrational resonances. The three bands were treated as a unique band system using the Watson A-reduced Hamiltonian in the Ir representation and five coupling constants; in this way the assigned transitions could be fitted within their experimental accuracy. The analysis yielded the first accurate set of spectroscopic parameters for v9, while the values determined for v5 and v6 represent a significant improvement on those reported previously