The high-resolution infrared spectrum Of Si2H6:: rotation-torsion analysis of the ν5 and ν7 fundamentals, and torsional splittings in the degenerate vibrational states

The infrared active nu(7) and nu(5) fundamentals of disilane, coupled by an x, y Coriolis interaction, have been analysed on a Fourier transform spectrum between 2120 and 2225 cm(-1) at the Doppler limited spectral resolution. A Fermi resonance with 2nu(2)+nu(9) affects the DeltaK=1 side of nu(7), and both nu(7) and nu(5) show the effects of several additional localized perturbations. Line splittings in the nu(5) transitions are not observed, showing that the torsional splitting in the nu(5) excited state and in the vibrational ground state are almost equal. The intrinsic torsional splitting of nu(7) is found to be smaller than in the ground vibrational state by 0.0085 cm(-1). This splitting value and those found for the other two infrared active degenerate fundamentals, nu(8) and nu(9), follow the trend expected from our theoretical predictions. Exploratory numerical calculations show that the decrease of the torsional splittings, observed in the fundamental degenerate vibrational states of disilane, can actually be accounted for by the head-tail and torsional Coriolis coupling of all the degenerate vibrational fundamentals, in several torsional states.