Raman and infrared spectra,conformational stability,ab initio calculations and vibrational assignment of dimethylsilylisocyanate

The Raman (3200‐30 cm⁻¹) and/or infrared spectra (3500 to 400 cm⁻¹) of gaseous, liquid and solid dimethylsilylisocyanate, (CH₃)₂ Si(H)NCO, have been recorded. The MP2(full) calculations, employing a variety of basis sets with and without diffusion functions, have been used to predict the structural parameters, conformational stability, vibrational fundamental wavenumbers, Raman activities, depolarization values and infrared intensities to support the vibrational assignment. The low wavenumber Raman spectrum of the gas with a significant number of Q‐branches for the SiNC(O) bend is consistent with an essentially linear SiNCO moiety. The ab initio calculations supported this conclusion as all possible orientations of the NCO moiety lead to nearly the same energy. This result is at variance with the conclusion from the electron diffraction study that the heavy atom skeleton was bent with an angle of 152(5)° with one stable cis conformer. It is believed that this reported angle difference from 180° is due to the shrinkage effect. The SiH distance of 1.486 Å has been obtained from the isolated SiH stretching wavenumber. From the adjustment of the ab initio MP2(full)/6‐311+G(d,p) predicted structural parameters, a proposed structure is reported, which is expected to give rotational constants within a few megahertz of the actual ones. These experimental and theoretical results are compared with the corresponding quantities of similar molecules. Copyright © 2009 John Wiley & Sons, Ltd.