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 (E1d 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 E1d and E2d symmetries for the degenerate modes.