Rotation-torsion analysis of the Si2H6 infrared fundamental n9, perturbed by excited torsional levels of thevibrational ground state

The lowest IR active perpendicular fundamental n9 of disilane has been analyzed on a Fourier transform spectrum between 320 and 430 cm-1, at the spectral resoln. of 0.0012 cm-1. The rotation-torsion structure of this band is affected by x,y Coriolis interactions with excited torsional levels of the vibrational ground state, correlating with components of 3n4 and 4n4 in the high barrier limit. The interaction of n9 and 4n4, forbidden in the D3d symmetry limit, is allowed between components of E torsional symmetry under the G36(EM) extended mol. group, because of the large amplitude of the internal rotation motion. We could det. the values of the main vibration-rotation-torsion parameters of n9, interaction parameters, and the vibrational wavenumbers of the four torsional components of 3n4 and of the E3d component of 4n4. The intrinsic torsional splitting of n9 is found to be smaller than in the ground vibrational state by 0.0066 cm-1, in good agreement with our theor. predictions. The possibility of observing the effects of D3d-forbidden interactions in the spectra of ethane-like mols. is also discussed.