Si-H stretching

A matrix isolation IR study of cyclic siladienes was more successful (Khabashesku et al., 1992). At first, unstable l-silacyclopenta-2,4-diene [128] was generated by vacuum pyrolysis (800°C 10 -10 Torr) of 5-silaspiro[4.4]nona-2,7-diene [129] or pyrolysis and photolysis (A = 248 nm) of l,l-diazido-l-silacyclopenta-2,4-diene [130] it has been studied by UV and IR spectroscopy in an argon matrix at 12 K. The UV band at Amax = 278 nm and nine IR bands (including two sp Si-H stretching vibrations at 2175 and 2144 cm ) have been recorded in matrix spectra of [128]. Reversible photochemical interconversion of [128] with silacy-... 

In order to document the radical disproportionation reaction, we have used FT-IR spectroscopy to characterize the irradiation products. Upon irradiation of 1 in pentane, the formation of the characteristic peak near 2100 cm-1 due to Si-H stretching vibrations was readily apparent. The IR spectrum obtained in perdeuterated pentane was identical, suggesting that radical processes other than abstraction from the solvent are involved. Furthermore the ESR spectrum obtained in this solvent is identical to that already described. This raises the question whether the initially formed silyl radicals really abstract hydrogen from carbon with the formation of carbon-based radicals as suggested (13), particularly in light of the endothermicity of such a process. 

Fourier transform infrared spectroscopic examination of these polymers also supports this conclusion. As can be observed in Figure 6, a peak occurring at 2086 cm->l is seen in all the copolysilanes. This absorbance has been assigned to a Si-H stretching absorption (15). Such a moiety is expected if silyl radical abstraction of a hydrogen occurs. Examination of the infrared spectra for a Si-CHo-Si vibrational peak which should be located between 1000 and 1100 cm-1 is inconclusive due to the presence of a multitude of absorbances in this region. 

A number of other models were considered and tested (for example, direct B—H bonding). The most significant test was the IR vibrational spectrum, where a sharp absorption band at 1875 cm-1 was found, corresponding to the Si—H stretch mode softened by the proximity of the B-atom. Had the hydrogen been bonded to boron, a sharp absorption band at 2560 cm-1 would have been expected. Also, Johnson (1985) showed that deuteration produced the expected isotopic shift. The most definitive and elegant proof of the correctness of the Si-H-B bonding model was provided by Watkins and coworkers (1990), on the basis of a parametric vibrational interaction between the isotopes D and 10B. 

The first explanation has been discussed in more detail by Lucovsky in Pankove et al. (1985) but can be restated simply as follows the Si—-H force constant is reduced by the slight attraction of the nearby B atom, as shown in Fig. 15b. Hence the frequency of the Si—H stretching vibration is slightly reduced. 

While this expression is seldom sufficiently valid to determine mA accurately, the r s are of use (i) for distinguishing between possible values of mA that differ greatly, (ii) for a comparison of a family of chemically related species, or (iii) for a vibration whose r is well known in other defects. For example, Eq. (1) gives r= 1.39 for Si—H stretching mode. A typical measured value for a longitudinal Si—H vibration is —1.37. 

Fig. 14. Si—H stretching features produced by hydrogen implantation into Si at 80 K (solid line) and after annealing at 200 K (dashed line). [Reprinted with permission from The American Physical Society, Stein, H.J. (1979). Phys. Rev. Lett. 43, 1030.]...

Higher resolution measurements of the Si-H stretching band in model compounds revealed a significant difference in the frequency of this band between terminal and internally placed hydride groups (Figure 3). For terminal hydrides the frequency was about 2127 cm l, while for internal ones it was about 2167 cm l (Table I). The frequency for the two Sylgards was about 2163 cm l, nearly that for internal placements, and in line with a random distribution. 

From that value a force constant of k = 5.6 mdynA 1 for the Si=C double bond is deduced255. This frequency is clearly higher than the usual range for Si—C stretch vibrations but substantially less than for C=C stretches, both because Si is heavier than C and because the Si=C bond is weaker than the C=C bond. More suitable for the experimental characterization is the vinylic Si—H stretch vibration which gives rise to a medium band at 2239 cm-1 (25) or 2187 cm-1 (2)29, hypsochromically shifted by around 100 cm-1 relative to the Si—H stretch in simple silanes. A detailed analysis of the vibrational spectra of matrix-isolated MeHSi=CH2 26 using polarized IR spectroscopy established IR transition moment directions relative to the tot -transition moment (Si-C axis) in 26156. These data provide detailed information about the vibrational modes and about the structure of 26156. The bathochromic shift of the Si=C stretch in the isomeric 1,3-silabuta-l,3-dienes 289 and 290 by around 70 cm 1 compared with the Si=C stretch in simple silenes (Table 15), was interpreted as an indication of Si=C—C=C and C=Si—C=C 7r-conjugation159. 

Silabenzene 24 reveals a characteristic Si—H stretching vibration at 2217 cm-1, as expected for hydrogen attached to a sp2-hybridized silicon atom. Compound 24 shows a typical benzene-type UV spectrum with absorptions at X = 217, 272 and 320 nm, which fit into the series of the already known donor-substituted heterobenzenes30. An additional structural proof was the partially reversible photochemical conversion of 24 into Dewar... 

Family Formula >Si-H Stretch >Si-H Bend >C-Si< Stretch >C-H Bend >Si-0- Stretch -OH Stretch... 

Very low values of the Si-H stretching frequency are found in some H2SiM2 derivatives the value found for H2Si[Fe(CO)2(Cp)]2 of 2042 cm-1 (32) compares quite closely with the value of 2037 cm-1 predicted from simple additivity considerations (235). These low values are consistent with very low effective electronegativities for metal carbonylate groups (32, 235). 

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