The Si-H stretching-bending overtone polyads of SiHF3: Assignments, band intensities, internal coordinate force field, and ab initio dipole moment surfaces

Fourier transform overtone spectra of SiHF3 were recorded in the region of 2500-9000 cm(-1) and vibrationally assigned. Experimental intensities were estimated. The 3 nu (1) overtone band at 6753 cm(-1) was observed to be mor e than 10 times weaker than the 4 nu (1) band. A reduced three-dimensional Hamiltonian model in terms of internal coordinates was employed to study th e Si-H stretching and bending vibrations including 5 nu (1) and 6 nu (1) wh ich were recently recorded using optoacoustic spectroscopy. Potential energ y parameters were optimized by fitting to experimental band centers. The Fe rmi resonance between the Si-H stretching and bending motions was found to be insignificant. Band intensities were computed using ab initio one- and t hree-dimensional dipole moment surfaces (DMS) expanded to polynomials in te rms of symmetrized internal coordinates. The intensity anomaly of 3 nu (1) is understood as resulting from cancellation of contributions by the linear and quadratic terms in the DMS expansion. The behavior of X-H stretching o vertone intensities as excitation increases was also studied in the low and medium energy regions. Whether a rapid or a slow decrease of intensity occ urs with increasing excitation depends strongly on the nonlinearity of the DMS. For some molecules, there is an almost complete cancellation of contri butions from the lower order terms in the DMS so that the accuracy of the c omputed overtone intensities is mainly limited by the uncertainty of the hi gher order expansion coefficients in the DMS. (C) 2001 American Institute o f Physics.