A Combined Frequency Analysis of the ν3, ν9, 3ν4 and the Far-Infrared Bands of Ethane: A Reassessment of the Torsional Parameters for the Ground Vibrational State

Abstract The lowest frequency parallel fundamental band ν3 of ethane is Raman active. A stimulated Raman spectrum of the Q branch for this band at a resolution of 0.0055 cm−1 has been measured by D. Bermejo et al. (1992, J. Chem. Phys.97, 7055). The torsion–rotation series in this band with σ=3, where σ=0, 1, 2, and 3 labels the torsional sublevels, is perturbed by over 1 cm−1. The lowest frequency-degenerate fundamental ν9 is infrared active. A high-resolution (0.0014 cm−1) Fourier transform spectrum of this band has been measured by N. Moazzen-Ahmadi et al. (1999, J. Chem. Phys.111, 9609). The observed torsional splittings for this band are substantially larger than expected from the observed barrier height. Because of a near-degeneracy of the upper level in the ν9 band with its interacting partner (v9=0, v4=3) a perturbation allowed band 3ν4 has also been observed. We have carried out a combined analysis of ν3, ν9, and 3ν4 together with the far-infrared torsional spectra in the ground vibrational state (gs). A fit to within the experimental error was achieved using 37 parameters. The large torsional splittings in the ν9 band are attributed to Coriolis-type interactions between the torsional stacks of gs and v9=1 whereas the large shift for the torsion–rotation series with σ=3 in the ν3 band is attributed to Fermi-type interactions between the torsional stacks of the gs and v3=1. The introduction of the Fermi-type interactions causes a considerable change in the leading terms in the torsional Hamiltonian for the gs. These changes are quantitatively explained.

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