Infrared Studies of the Symmetry Changes of the 28SiH4 Molecule in Low-Temperature Matrices. Fundamental, Combination and Overtone Transitions

IR spectra of 28SiH4 in argon and nitrogen matrices at low temperature 6.5 – 20 K in the region of overtone and combination transitions were recorded for the first time. Additionally, the high resolution spectra were obtained in the fundamental region. The frequencies and the relative intensities of all bands were determined. The set of experimental data suggests that the symmetry of molecules studied in the matrices is different from the symmetry of the free molecules because of an interaction with the environment. The symmetry of 28SiH4 changes from Td to C3v on transition from the gas phase to a nitrogen matrix and to D2d on transition to an argon matrix. A modeling of SiH4 molecule force fields explains the experimental data as a change of a force constant of the selected SiH bond in the case of SiH4 in the nitrogen matrix or force constants of two opposite angles in the case of SiH4 in the argon matrix. In spite of small values of these changes, they result in to noticeable spectroscopic effects: the...