Rovibrational analysis of the ν 4,2ν 6 Fermi resonance band of CH35ClF2 by means of a polyad Hamiltonian involving the vibrational levels ν 4, 2ν 6,ν 6+ν 9 and 2ν 9, and comparison with ab initio calculations

Abstract

High resolution FTIR spectra of the ν4/2ν6 band system in the region 750–850 cm−1 were measured with our Bruker IFS 125 HR Zürich prototype (ZP2001) spectrometer using an instrumental resolution of better than 0.001 cm−1 (FWHM, unapodized or 1/MOPD =0.001 cm−1 with the maximum optical path difference MOPD). The spectra were analysed by means of a polyad Hamiltonian built up from the ν4/2ν6 Fermi dyad and the 2ν6,ν6+ν9,2ν9 Coriolis triad which share the level 2ν6. The levels ν6+ν9 and 2ν9 are not directly observed but are included as dark states. Spectroscopic parameters for ν6+ν9 and 2ν9 as well as the Coriolis coupling constants for the Coriolis triad were transferred from the Coriolis dyad ν6,ν9 using previously reported values. The analysis included both isotopomers CH35ClF2, and CH37ClF2. The deperturbed band centres of ν4 and 2ν6 obtained from the fit are cm−1 and cm−1 for CH35ClF2, and cm−1 and cm−1 for CH37ClF2. The Fermi resonance coupling matrix element obtained for CH35ClF2 is F = −7.6839 cm−1. In the corresponding analysis of the isotopomer CH37ClF2, we employed also this value without further adjustment. A comparison with results obtained in the framework of simpler models shows that the inclusion of the full Coriolis triad is essential. We also report ab initio calculations on the MP2 level of theory with aug-cc-pVDZ and aug-cc-pVTZ basis sets pertaining to (q4 ), (q 6) and (q 4 , q 6 ) subspaces. These results agree well with the empirical findings and allow us to assign a sign to certain coupling constants which cannot be obtained from the analysis of the experimental spectra.