Si force constants

As expected, the strongest SiC l bond is observed for the SSiCl2 molecule. This is in line with an increase of ionic contributions going from SSiH2 to SSi(H)Cl and then to SSiCl2. Furthermore the strongest SiS bond is formed in the molecule SSiC. The values of the SiS force constants are of the same order as the SiS "double" bonds in the molecules SiS and SiS2. 

With the aid of a normal coordinate analysis involving different isotopomers a linear structure of the Pd-Si-0 molecule is deduced. The results of ab initio MP2 calculations (Tab. 4) confirm the experimentally obtained IR spectra and their interpretation. The Pd-C bond in PdCO is similar to the Pd-Si bond in PdSiO which means, that the donor bond is strengthened by x acceptor components. This conclusion is in line with the high value of the Pd-Si force constant (exp. f(PdSi) = 2.69, f(SiO) = 8.92 mdyn/A) as well as with the energy of PdSiO (Pd + SiO —> PdSiO + 182 kJ/mol for comparison Pd + CO —> PdCO + 162 kJ/mol, MP2 level of theory). 

Molecular Cl(H)Si=S (126) was also formed in an argon matrix in a photochemically induced reaction of SiS with HC1. From the isotopic splittings (H/D and 35C1/37C1) of the IR absorptions the Cs structure of the species with silicon as the central atom is deduced. By a normal coordinate analysis a value of 4.83 mdynA-1 is obtained for the SiS force constant, a value which was confirmed by ab initio SCF calculations of the IR spectrum51. 

Calculations of force constants show that the strength of the Si—Si linkage is easily influenced by substituents. The Si—Si force constant is now expected to range from 1.44 to 2.5 mdyn/A. 

To judge the bonding properties of SiO and SiS, we compare their experimentally derived force constants and bond energies with those of CO and CS [10]. Further insight into the bonding characteristics is gained from molecular parameters such as geometry and force constant data as well as electron distributions (Tab. 1), which are derived from ab initio quantum chemical calculations. 

In contrast to carbon compounds, force constants and bond distances of SiO and SiOz - as well as of SiS and SiS2 - are equal. On the other hand, the trend within the bond energies shows the expected decrease from SiO to Si02 and from SiS to SiS2, respectively. 

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. 

The transformed weight corresponding to 5, is the wave function (4.1) normalization condition w = w + W3 = 1. Thus, the solvent force constant matrix elements Km and K m, m = [1,3], bear no dependence on the solute electronic structure, since their components K% and KP°J, are zero [cf. (3.5)]. Then, Si cannot couple to the solute electronic structure, and is unable to monitor any rearrangement — due to the variation of the coefficients Ci and c2 — of the solute total charge distribution p. By contrast, s3 is associated with Kp - = -r) 3,Wi c -c, and is therefore sensitive to the relative change of the weights of the states 1) and 2). 

In this study the authors develop simplified equations relating equilibrium fractionations to mass-scaling factors and molecular force constants. Equilibrium isotopic fractionations of heavy elements (Si and Sn) are predicted to be small, based on highly simplified, one-parameter empirical force-field models (bond-stretching only) of Sip4, [SiFJ, SnCl4, and [SnCl,] -. 

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. 

In addition, these experiments were the first proof for the simplest of all silanimines, H2Si=NH (735). The results were interpreted with the help of corresponding calculations and supported by deuterium labeling experiments. The bond orders, as determined from the force constants (experimental IR spectra), are 2.0 for HSi=N 733 and 2.3 for HN=Si 737. 

Under similar reaction conditions, Cl2Si=S (127) was formed in a matrix reaction between SiS and CI2. The formation of 127 was also concluded from some isotopic shifts in the IR spectra. The force constant of the Si—S bond in 127 has a value of... 

The magnitude of the Si—Si vibrational force constants in cyclopolysilane rings also depends on the ring size and on the substituents attached to silicon. Compared with other silanes like disilanes, the force constants are generally weaker. Detailed results are summarized elsewhere5d. 

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