Toward the understanding of the high resolution infrared spectrum of C 2H 6 near 3.3 μm

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The Fourier transform infrared spectrum of ethane between 2860 and 3060 cm −1 has been re-investigated under high resolution at 229 K. The infrared absorption in this region is due mainly to the CH stretching fundamentals ν 5 (parallel band) and ν 7 (degenerate perpendicular band), and to the parallel combination system ν 8 + ν 11 ( A 4s, A 3s). All the relevant perturbation mechanisms affecting the observed absorption patterns have been clarified. In particular, the main perturbers of the ν 7 state are identified to be the degenerate vibrational combination states ν 8 + ν 11 ( l-type interaction) and ν 3 + 2 ν 4 + ν 8 (Fermi-type interaction). Because of the last interaction, the K″Δ K = −6 transitions occur with intensities comparable to both the infrared active fundamental ν 7 and the almost dark combination ν 3 + 2 ν 4 + ν 8. The parallel combination system ν 8 + ν 11 ( A 4s, A 3s) is overlapped and heavily perturbed by the nearby parallel system ν 4 + ν 11 + ν 12 ( A 4s, A 3s), whose K-structure is spread by the strong z-Coriolis interaction of its two vibrational components. In this work, 95 new transitions to the perturbers of ν 7 have been assigned. They belong mostly to the degenerate vibrational states ν 8 + ν 11 ( E 1d) and ν 3 + 2 ν 4 + ν 8 ( E 1d), and to the parallel system ν 8 + ν 11 ( A 4s, A 3s). A least squares fit calculation, limited to the ν 7 degenerate fundamental and its degenerate perturbers ν 8 + ν 11, ν 3 + 2 ν 4 + ν 8, ν 4 + ν 11 + ν 12, and ν 3 + 3 ν 4 + ν 12 was performed. From the results of this fit, we created a line-by-line database containing the molecular parameters for 4969 transitions in these five bands of 12C 2H 6. Finally, we identified the degenerate combination band ν 2 + ν 8 (62 observed transitions) to be the main perturber ( x, y-Coriolis-type interaction) of the parallel fundamental ν 5.