Wavenumber shift

Because the time scale of the Raman scattering event ( 3.3 x 10-14s for a vibration with wavenumber shift 1000 cm-1 excited in the visible) is much shorter than that of the fastest conformational fluctuations in biomolecules, the ROA spectrum is a superposition of snapshot spectra from all the distinct chiral conformers present in the sample. Together with the dependence of ROA on chirality, this leads to an enhanced sensitivity to the dynamic aspects of biomolecular structure. The two-group model provides a qualitative explanation since it predicts ROA intensities that depend on absolute chirality in the form of a sin x dependence... 

Figure 2. IR spectra showing the substantial wavenumber shift between bands assigned to the V(H-H) vibration of coordinated molecular hydrogen in Mo(CO)5(H2), CrlCOj lH ), and W(CO) (H) in liquid Xe at -70°C. Note that the three traces were recor in separate experiments and that the absorbance scale refers to the Mo trace [28].

TABLE 2. IR carbonyl wavenumber shifts" and equilibrium constants for the reactions of sec-BuIi with substituted phenyl sec-butyl ketones in cyclohexane at 25°C ... 

Indirect experimental evidence implying the presence of a n-bond-containing complexes from ethene on metal catalysts was first produced by Shopov and Palazov and their colleagues (161-163). They pointed out that coadsorption of ethene and CO led to a low-wavenumber shift of the vCO absorption and interpreted this in terms of the effects of -donation by ethene. Subsequently, they suggested that the complex in question was a (di-cr/zr) ethyne surface... 

If a n species from propene were present, this too could be contributing to some of the extra absorptions discussed in the previous paragraph. The weakness of the v=CH absorption above 2980 cm -1 in the spectrum on Pt/Si02 (Fig. 17A) led Shahid and Sheppard to suggest that this is probably the least abundant of the three probable species. On the other hand, Palazov et al. (171) have attributed the low-wavenumber shift of the vCO absorption from carbon monoxide when coadsorbed with propene to the presence of some -adsorbed species, and we have seen above that the MSSR reduces the intensity of v=CH modes. 

A solvatochromic scale, based on the ultraviolet-visible, rather than the infrared, spectral band of suitable probes is that based on the Kamlet -Taft P parameter. This is again an averaged quantity, for which the wavenumber shifts of several protic indicators relative to structurally similar but aprotic homomorphs are used (Kamlet et al. 1983 Kamlet and Taft 1976). It is assumed that the nonspecific effect of a solvent on the protic probe is the same as that on the aprotic one, and that it can be expressed in terms of the n parameter for the solvent, so that the donicity of the solvent, if it is a Lewis base, causes the difference between the responses of the two probes towards the solvent. The probes originally employed were 4-nitrophenol (vs 4-nitroanisole) and 4-nitroaniline (vs 4-nitroN,N-diethylaniline), but once a n scale is known, the need for the specific aprotic homomorph values no longer exists, since the general expression ... 

Another measure of solvent softness proposed is based on Raman spectroscopic measurements. It is the wavenumber shift Av of the Raman band for the symmetrical stretching of Br-Hg-Br in the solvent relative to that of gaseous mercury bromide (Persson 1986). A solvent softness scale, called Ds = [A (Hg-Br)/cm-J] has accordingly been established. An extension of this scale to further solvents is difficult in those cases where the solubility of mercury bromide is insufficient for the ca. 0.2 mol dm-3 required for the Raman spectral measurements. 

Iodoacetylenes as well as iodine cyanide are soft Lewis acids (Laurence etal. 1981), which interact with basic solvents yielding characteristic wavenumber shifts Av (C-I) (e g., for ICN relative to the wavenumber in CCI4 solutions). These shifts differ for soft solvents, with sulfur or selenium donor atoms or n systems, and hard solvents, with oxygen or nitrogen donor atoms. However, these authors have not converted this observation and their data to a solvent softness scale. In fact, if prorated values of A v (O-H), for phenol, relative to CCI4 solutions, see B0 H above, representing the hard basicity of the solvents, are subtracted, the remainder measures the solvent softness. Quantitatively,... 

Wavenumber shifts Av (in cm 1) due to interaction between CDCI3 and SMA23,24... 

Therefore, the wavenumber shift of the free hydroxyl vibration is -according to Gillis-D Hamers- a direct reflection of the population of free and bridged silanols. 

The perturbation of the Si-H vibration of ammoniated trichlorosilylated silica (figures 12.14 and 12.16) consists at first sight of at least 3 distinct bands, indicating that at least 3 different species with an Si-H band exist on the surface. The strong electron donating effect of the amine functions will cause a low wavenumber shift of the Si-H band. 

This linear relationship demonstrates the similarity of the polarity effects on both homomorphic vibrators, trichloroacetic acid and its methyl ester. For HBA solvents B, however, the vc o data points are displaced below the reference line of Eq. (7-37). These deviations are caused by the formation of solute/solvent hydrogen bonds CCI3CO2H B, resulting in a decrease in the C=0 vibration wavenumber. The hydrogen-bond induced wavenumber shift Avc o for a HBA solvent is then calculated by Eq. (7-38) cf Eq. (7-33). 

While the wavenumber shifts were primarily observed... 

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