Torsional splittings in the v₁₂=1 vibrational state of Si₂H₆: analysis of the ν₆+ν₁₂ and ν₉+ν₁₂(E) combination bands in the high resolution infrared spectrum

Abstract

The ν₆ + ν₁₂ and ν₉ + ν₁₂(E) vibrational combination bands of disilane (Si₂H₆) have been measured at high resolution and analysed. A few K-branches of the hot band associated with ν₉ + ν₁₂(E), originated from the torsional state v₄ = 1, have also been observed and analysed. These observations give information on the effects of the v₁₂ = 1 vibrational state, not directly accessible in the infrared, on the molecular internal rotation. From the vibrational frequencies of ν₉ + ν₁₂(E) and its related hot band, determined in this work, and those of ν₉ and its hot band determined previously, we can determine that the torsional barrier height changes by −19.25 cm⁻¹ if the ν₁₂ mode is excited by one quantum. This causes the torsional splitting to increase by about 0.0038 cm⁻¹. The observed torsional splitting in the ν₆ + ν₁₂ vibrational state is larger than in the ground vibrational state by 0.0024 cm⁻¹, with a 0.0033 cm⁻¹ change caused by the lowering of the barrier height and a −0.0009 cm⁻¹ change caused by the head–tail and torsional Coriolis mechanisms. The torsional splitting observed in the ν₉ + ν₁₂(E) state, −0.0043 cm⁻¹, is inverted with respect to the ground vibrational state (0.0150 cm⁻¹ in the lowest torsional state), showing that this state is closer to the E_2s than to the E_1d vibrational symmetry description. The analysis of perturbations observed in the rotation–torsion structure of ν₉ + ν₁₂(E) yields information on the vibrational states ν₂ + ν₄ and ν₄ + ν₁₁, and on the E_3d torsional component of 4ν₄ + ν₁₂.