A combined frequency analysis of the nu(3), nu(9), 3 nu(4) and the far-infrared bands of ethane: A reassessment of the torsional parameters for the ground vibrational state

摘要

The lowest frequency parallel fundamental band upsilon(3) 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,.1. Chem. Phys. 97, 7055). The torsion-rotation series in this band with sigma = 3, where sigma = 0, 1, 2, and 3 labels the torsional sublevels, is perturbed by over 1 cm(-1). The lowest frequency-degenerate fundamental upsilon(9) 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 upsilon(9) band with its interacting partner (upsilon(9) = 0, upsilon(4) = 3) a perturbation allowed band 3upsilon(4) has also been observed. We have carried out a combined analysis of upsilon(3), upsilon(9), and 3upsilon(4) 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 upsilon(9) band are attributed to Coriolis-type interactions between the torsional stacks of gs and upsilon(9) = 1 whereas the large shift for the torsion-rotation series with sigma = 3 in the upsilon(3) band is attributed to Fermi-type interactions between the torsional stacks of the gs and upsilon(3) = 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. (C) 2002 Elsevier Science (USA). [References: 18]