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 D_3d 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 E_3d 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 D_3d-forbidden interactions in the spectra of ethane-like molecules is also discussed.