Donor vibrational bands

Much earlier information on the structure of diazonium ions than that derived from X-ray analyses (but still useful today) was obtained by infrared spectroscopy. The pioneers in the application of this technique to diazonium and diazo compounds were Le Fevre and his school, who provided the first IR evidence for the triple bonds by identifying the characteristic stretching vibration band at 2260 cm-1 (Aroney et al., 1955 see also Whetsel et al., 1956). Its frequency lies between the Raman frequency of dinitrogen (2330 cm-1, Schrotter, 1970) and the stretching vibration frequency of the C = N group in benzonitrile (2255 cm-1, Aroney et al., 1955). In substituted benzenediazonium salts the frequency of the NN stretching vibration follows Hammett op relationships. Electron donor substituents reduce the frequency, whereas acceptor substituents increase it. The 4-dimethylamino group, for example, shifts it by 103 cm-1 to 2177 cm-1 (Nuttall et al., 1961). This result supports the hypothesis that... 

The assignment of the 809 and 1560 cm-1 bands of the donor-H complexes were based upon the expected frequencies of the wagging and stretching modes and the 2 1 ratio of intensities (Bergman et al., 1988a). A uniaxial stress study of the vibrational bands (Bergman et al., 1988b) will be reviewed here. Only the As—H complex was studied under stress because the donor-H complexes have nearly identical vibrational spectra for the different donors and are expected to behave similarly under stress. 

Alkali-metal-ion-O-donor-solvent cages exhibit low-frequency bands in the i.r. spectrum characteristic of the cation-O polyhedra. Similar bands are seen with N-donors. The bands can be used to establish the nature of cation co-ordination and serve as probes to examine ion-solvent interactions.87 An analysis of the i.r. spectra of ternary mixtures LiClCX-Sj-Sz showed a preferential solvation of the Li+ ion by NH3 and methylamines (St) in MeCN or THF (S2) and by MeCN (S2) in MeN02 (SO. The appearance of wide bands in the ion-cage vibration region is related to the formation of different species [Li(Si) -i(S2)i]+. The Li+-solvent molecule interaction energy decreases when the number of S2 molecules in the first solvation shell increases. The mean composition of the first solvation shell was obtained from intensity measurements of the molecules not bonded to the ions in favourable cases (S2 = ND3 or MeCN), the solvation number of the Li+ ion in the pure solvent... 

The most typical IR spectral characteristics of classical hydrogen bond formation are the changes of XH-stretching vibration bands of the proton donors in the presence of bases decrease in intensity of the free XH band, VxH(free)> nd appearance of a low frequency broad and intense VxH(bonded) band. These changes become... 

The solvatochrome effect may also appear in the infrared spectrum. The acceptor strength scale of Kagiya [Ka 68] is based on the solvent dependence of the C==0 vibration band for acetophenone. As for his donor strength scale, the reference solvent is benzene. The acceptor strength is denoted by the band shift,... 

The vibrational characteristics of the 1560 cm-1 and 809 cm-1 bands are consistent with the donor-H configuration (Fig. 8) proposed by Johnson et al. (1986). The change in the vibrational frequency of the 1560 cm-1 band for the different donors is sufficient to show that the donor is involved in the complex. However, one would have expected the large change in the donor s mass and size when going from P to Sb to have had a greater effect on the vibrational frequency if the H were attached directly to the donor atom. Further evidence that the H is not attached to the donor directly comes from the frequency ratios, r, shown in Table II that are nearly constant for the different donors as would be expected if the H were attached to Si. 

Beyond ethanol, the number of ft-alkanol dimer conformations becomes too large to be vibrationally resolved, even in supersonic jets. For -propanol, more than five isomers are discernible in the donor O—H stretching spectrum (see Fig. 8). For longer chains, there is a smaller number of dominant conformations [69]. Ar relaxation shows that the most stable -propanol and n-butanol dimers are those with the largest observed red shifts. For longer chains, the situation is more complex. However, the window of observed O—H stretching bands is quite independent of chain length beyond propanol. 

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