Rovibrational analysis of the ν₄,2ν₆ Fermi resonance band of CH³⁵ClF₂ by means of a polyad Hamiltonian involving the vibrational levels ν₄, 2ν₆,ν₆+ν₉ and 2ν₉, 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.

Acknowledgements

Help from Silvan Bäurle in the initial part of the present analysis is gratefully acknowledged as well as discussions with Jürgen Stohner. At earlier stages of this project our work has profited from helpful collaborations with A. Amrein, T. K. Ha, D. Luckhaus, F. Merkt, A. Ross and M. Snels. Our work is supported financially by the ETH Zurich (including C ⁴, AGS and CSCS) and the Schweizerische Nationalfonds.