Vibration–rotation–torsion analysis of the high resolution infrared spectrum of C 2H 6 between 1330 and 1610 cm −1: The ν6, ν8, ν4 + ν12, 2 ν4 + ν9 interacting system and hot transitions from ν4 to ν4 + ν8

Abstract

The room temperature spectrum of ethane in the region of the ν 6 and ν 8 fundamentals, between 1330 and 1610 cm −1, has been re-investigated at a resolution of 0.002 cm −1. It is shown that the complex spectral structure with numerous strong local perturbations, which had hindered a global analysis of this region until now, is caused by a network of l-type resonances acting within the Fermi (or torsional-Coriolis) coupled system ν 8, ν 4 + ν 12. The x, y-Coriolis interaction between ν 6 and ν 8 and vibration–rotation interactions between ν 6 and 2 ν 4 + ν 9 also generate relevant effects on the spectrum. With an appropriate Hamiltonian model accounting for all these effects, we could fit 1814 upper state level energies belonging to ν 6, ν 8 and ν 4 + ν 12 with an overall RMS deviation better than 3.8 × 10 −3 cm −1. The determined values of the intrinsic torsional splittings in the different vibrational states are in agreement with the theoretical expectations. The intrinsic torsional splitting almost vanishes in ν 8, as in digermane and disilane; it is slightly larger in ν 4 + ν 12 than in ν 4, due to the decrease of the torsional barrier height with the excitation of ν 12. For the 2 ν 4 + ν 9 state, the determined values of the vibrational origin and torsional splittings are in good agreement with the large increase of the torsional barrier height expected with the excitation of the ν 9 mode. Several hot transitions originating from the v 4 = 1 torsional state have been observed. An analysis of 294 line positions belonging to the ( ν 4 + ν 8) − ν 4 hot band has been performed, leading to a fit with a RMS deviation of 5.1 × 10 −3 cm −1. It is shown that a complex network of l-type resonances is also active in the ν 4 + ν 8,2 ν 4 + ν 12 system.