High resolution infrared spectroscopy and global vibrational analysis for the CH₃D and CHD₃ isotopomers of methane

Abstract

We report infrared spectra of CH₃D and CHD₃ in the range 2900 to 9000 cm⁻¹ measured with the Zürich high resolution Fourier transform infrared (FTIR) interferometer Bruker IFS 125 prototype (ZP 2001) at 80 K in a collisional-cooling cell with optical paths ranging from 5 to 10 m. In all, 57 new ro-vibrational bands of CH₃D and 40 for CHD₃ were assigned and analysed. Using a strategy of the direct assignment of the J = 0 states of excited vibrational levels, precise experimental values of the band centres with uncertainties in the range of about 0.0001 to 0.0003 cm⁻¹ were obtained. Including 15 previously known band centres of CH₃D and 12 previously known band centres of CHD₃, these data were used as the initial information for the determination of the harmonic frequencies, anharmonic coefficients, and vibrational resonance interaction parameters in an effective hamiltonian. A joint set of 64 parameters reproduces the 124 experimental vibrational energies of both molecules up to 6500 cm⁻¹ with a root mean deviation of d _rms = 0.73 cm⁻¹. The results are discussed in relation to intramolecular dynamics on a global potential hypersurface of methane, intramolecular vibrational redistribution, and the spectroscopy of the atmospheres of the earth and planetary systems.

Keywords:

• infrared spectroscopy
• vibration–rotation spectra
• CH₃D
• CHD₃
• overtone and combination bands
• spectroscopic parameters
• isotopes
• methane
• FTIR
• intramolecular quantum dynamics
• potential hypersurfaces
• resonances

Acknowledgements

Our work is supported financially by ETH Zürich and the Schweizerischer Nationalfonds. Part of the work benefited from the joint PICS grant of CNRS (France) and RFBR (Russia), 4221N0000211752a and from the Russian Science and Innovations Federal Agency under contract No. 02.740.11.0238. We gratefully acknowledge discussions with Tucker Carrington, Veronika Horká Zelenková, Roberto Marquardt, Hans Martin Niederer, and Georg Seyfang. We enjoyed many years of fruitful scientific exchange and friendship with Richard N. Zare to whom this paper is thus justly dedicated.