SiH stretching anharmonicity, local-mode behaviour and dissociation energies in silane and methyl silanes

Infrared and Quantum-Chemical Studies of Dimethylsilane

“…Samples of DMS-d 0 and d 2 were available from previous work (6,8,9) and were further purified by fractional condensation. Mid infrared spectra were recorded on a Nicolet 7199 FTIR spectrometer, operating at resolutions of 0.25 or 0.5 cm −1 .…”

“…This is matched by a progressive lengthening of the Si-C bond from MS through to TMS (0.0018Å per methyl group according to B3LYP/tz calculations (11,12). The similar change in f SiH is of course just the reflection of the long-recognized decrease in νSiH with α-methyl substitution which has been the subject of many studies (8)(9)(10), to name but a few). The changes in both f SiH and f SiC may be classified together as the effects of α methyl substitution.…”

“…The situation is most acute in treating C-H and C-D stretching frequencies where the respective anharmonicities are about 4% and 2.6%. Si-H and Si-D stretching anharmonicities are slightly less than these (8,9). A palliative measure which has been found effective in many compounds is to divide observed νCD values by 1.011, or νSiD ones by 1.010, for fitting in a refinement which employs observed νCH or νSiH values (15,34).…”

“…Assignments of vibrational modes used in these works were based on the early infrared and Raman studies of (CH 3 ) 2 SiH 2 (hereafter "d 0 ") and (CH 3 ) 2 SiD 2 (hereafter "d 2 ") by Ball et al (6). Subsequent work includes a far infrared study of d 0 by Durig and Hawley (7) and mid and near infrared work focused on SiH and CH stretching transitions (8)(9)(10).…”

Chromophore dependence of intramolecular vibrational relaxation: Si–H stretch second overtone versus C–H stretch first overtone in methylsilane

“…The spectrum of methylsilane in the vϭ3 Si-H overtone region shows just one band, assigned as ͑3,0,0͒. 20 Furthermore, the splitting of the fundamental frequencies of the symmetric and asymmetric Si-H stretches 21 is only 4 cm Ϫ1 . On the other hand, the C-H motion at vϭ2 in methylsilane has a large normal mode component.…”