Torsional tunneling splittings in the v₄ = 1 excited torsional state of Si₂H₆: analysis of hot bands accompanying fundamental transitions in the high resolution infrared spectrum

Abstract

The (ν₄ + ν₆) − ν₄, (ν₄ + ν₈) − ν₄ and (ν₄ + ν₉) − ν₄ hot infrared systems of disilane (Si₂H₆) have been analysed at high resolution, and the values of the relative vibration–rotation–torsion parameters have been determined. The torsional splitting is about 0.500 cm⁻¹ in the ν₄ and ν₄ + ν₆ states, and decreases strongly in the vibrationally degenerate upper states ν₄ + ν₈ (about 0.0272 cm⁻¹ on average) and ν₄ + ν₉ (about 0.3019 cm⁻¹), consistent with theoretical predictions. Comparison between the vibrational wavenumbers of cold transitions and hot transitions originating in the excited torsional state v₄ = 1 allows one to determine the change of the fundamental torsional frequency ν₄ caused by the excitation of small amplitude vibrations. A remarkable increase in ν₄ of about 8.599 cm⁻¹ is found in the v₉ = 1 state (E_1d SiH₃-rocking mode, asymmetric to inversion in the staggered geometry), and this corresponds to an increase in the torsional barrier height in this excited fundamental vibrational state by about 48.77 cm⁻¹. The mechanism responsible for the decrease of the torsional splittings in the degenerate vibrational states is briefly outlined by means of second-order perturbation theory, using torsion-hindered vibrational basis functions of E_1d and E_2d symmetries for the degenerate modes.