Vibration-rotation-torsion analysis of the high resolution infrared spectrum of C2H6 between 1330 and 1610cm-1: The n6, n8, n4 + n12, 2n4 + n9 interacting system and hot transitions from n4 to n4 + n8

The room temp. spectrum of ethane in the region of the n 6 and n 8 fundamentals, between 1330 and 1610 cm-1, has been re-investigated at a resoln. of 0.002 cm-1. It is shown that the complex spectral structure with numerous strong local perturbations, which had hindered a global anal. of this region until now, is caused by a network of l-type resonances acting within the Fermi (or torsional-Coriolis) coupled system n8, n4 + n12. The x,y-Coriolis interaction between n6 and n 8 and vibration-rotation interactions between n 6 and 2n4 + n9 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 n6, n8 and n4 + n12 with an overall RMS deviation better than 3.8 × 10-3 cm-1. The detd. values of the intrinsic torsional splittings in the different vibrational states are in agreement with the theor. expectations. The intrinsic torsional splitting almost vanishes in n 8, as in digermane and disilane; it is slightly larger in n4 + n12 than in n4, due to the decrease of the torsional barrier height with the excitation of n12. For the 2n4 + n9 state, the detd. 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 n 9 mode. Several hot transitions originating from the v4 = 1 torsional state have been obsd. An anal. of 294 line positions belonging to the (n 4 + n 8) - n 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 n 4 + n8,2n4 + n12 system.