Rotation–torsion analysis of the Si2H6 infrared fundamental ν9, perturbed by excited torsional levels of the vibrational ground state

Abstract

The lowest infrared active perpendicular fundamental ν9 of disilane has been analysed on a Fourier transform spectrum between 320 and 430 cm−1, at the spectral resolution 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 3ν4 and 4ν4 in the high barrier limit. The interaction of ν9 and 4ν4, forbidden in the D3d symmetry limit, is allowed between components of E torsional symmetry under the G36(EM) extended molecular group, because of the large amplitude of the internal rotation motion. We could determine the values of the main vibration–rotation–torsion parameters of ν9, interaction parameters, and the vibrational wavenumbers of the four torsional components of 3ν4 and of the E3d component of 4ν4. The intrinsic torsional splitting of ν9 is found to be smaller than in the ground vibrational state by 0.0066 cm−1, in good agreement with our theoretical predictions. The possibility of observing the effects of D3d-forbidden interactions in the spectra of ethane-like molecules is also discussed.