SPECTRA AND STRUCTURE OF SMALL RING COMPOUNDS .61. IR AND RAMAN SPECRTRA, VIBRATIONAL ASSIGNMENT, CONFORMATIONAL STABILITY AND AB-INITIO CALCULATIONS OF CYCLOPROPYLMETHYLSILANE

The IR (3500-50 cm(-1)) and Raman (3500-20 cm(-1)) spectra of gaseous and solid cyclopropylmethylsilane, c-C3H5-CH2SiH3, have been recorded. Additionally, we have obtained the Raman spectrum of the liquid and t he IR spectrum of the sample trapped in an argon matrix. From these da ta a complete assignment of the normal vibrational modes is provided. The spectra of the gaseous and liquid phases have been interpreted on the basis of the predominance of a conformation having a gauche struct ure and this form exists exclusively in the solid. Ab initio Hartree-F ock structural optimizations with the 3-21G and 6-31G* basis sets are consistent with the gauche rotamer lying similar to 800 cm(-1) lower in energy than the cis conformation which has the SiH3 group cis to th e cyclopropyl ring. The force fields obtained with the 3-21G basis se t have been used to perform a normal coordinate analysis. The cyclopro pyl torsion has been observed as an IR band of hybrid contour centered at 80 cm(-1) in the far-IR spectrum of the gas. Combination band spec tra in the 2300-2000 cm(-1) region of the mid-IR spectrum of the gas c an be attributed to sum and difference bands of the SiH3 and cycloprop yl torsional modes with an SiH3 stretching mode. Analysis of these dat a leads to a barrier governing internal rotation of the silyl group of 687 +/- 50 cm(-1). These results are compared with corresponding quan tities in some similar molecules.