High resolution difference bands of ethane C2H6 from torsionally excited lower states: rotation-torsion structure of the ν 2, ν 11 and ν 4 + ν 11 vibrational states

Abstract

A high resolution Fourier transform infrared spectrum of C2H6, measured at a pressure of 173.3 Pa and an optical path of 153.2 m, was analysed between 1050 and 1295 cm−1. Extensive absorption due to the difference bands ν 11–ν 4, and several rotation–torsion lines of the difference band ν 2–ν 4, in the region of the x, y-Coriolis resonance of ν 2 and ν 11, were observed. This allowed a detailed rotation–torsion analysis of the upper states ν 11 and ν 2. The anomalous torsional structure, found in the non-degenerate vibrational state ν 2, can be explained as the effect of an Hamiltonian term accounting for a strong dependence of the torsional barrier height on the normal vibrational coordinate q 2. The value of the barrier height derivative is estimated to be 127 ± 10 cm−1. Also detected and assigned were ‘hot’ difference transitions belonging to the (ν 4 + ν 11)–2ν 4 band, yielding information on the upper state ν 4 + ν 11. It is believed that transitions from 3ν 4 to 2ν 4 + ν 11 are also detectable in the investigated region.