Characteristic Group Vibrations

Looking at Hooke s law, we can see that for a strong bond (large ) connected to a light mass it predicts a high-frequency (or wavenumber) vibration, and this is in agreement with the observed wavenumbers for the O-H and N-H absorptions (3600-3200 cm  

Some of the IR absorption bands can be attributed to the vibration of individual bonds, and these bands are the most diagnostically useful since they help in the identification of the functional groups present in the molecule. This is the most important use of IR spectroscopy for the organic chemist. In looking at these characteristic group vibrations, it is convenient to consider the IR spectrum as being divided into five regions  

Vibrations of triple and cumulative bonds Vibrations of C=X Fingerprint region Unsaturated C-H bending 

We will now consider the absorptions in each of these regions in detail, but an indication of where the common functional groups absorb is given in Box 3.1. 

Absorption bands in this region correspond to the stretching vibrations of single bonds to hydrogen C-H, O-H, N-H and S-H. 

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The concept of a chromophore is analogous to that of a group vibration, discussed in Section 6.2.1. Just as the wavenumber of a group vibration is treated as transferable from one molecule to another so is the wavenumber, or wavelength, at which an electronic transition occurs in a particular group. Such a group is called a chromophore since it results in a characteristic colour of the compound due to absorption of visible or, broadening the use of the word colour , ultraviolet radiation. 

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 reorganization free energy /.R represents the electronic-vibrational coupling, ( and y are fractions of the overpotential r] and of the bias voltage bias at the site of the redox center, e is the elementary charge, kB the Boltzmann constant, and coeff a characteristic nuclear vibration frequency, k and p represent, respectively, the microscopic transmission coefficient and the density of electronic levels in the metal leads, which are assumed to be identical for both the reduction and the oxidation of the intermediate redox group. Tmax and r max are the current and the overvoltage at the maximum. 

The characteristic IR vibrations are influenced strongly by small changes in molecular structure, thus making it difficult to identify stmctural fragments from IR data alone. However, there are some groups of atoms that are readily recognised from IR spectra. IR chromophores are most useful for the determination of structure if ... 

Despite the fact that a full assignment of all the observed absorptions to the respective macromolecule s natural frequencies is not possible in all cases - in particular for complex co- and terpolymers, stereoregular polymers, crosslinked systems, composites, compounds or blends this is very difficult - there are many bands caused by local group vibrations of a few atoms which can be interpreted very nicely. As an example, the C=0 band (stretching vibration) is usually observed as an intense absorption between v = 1850-1650 cm. Because of the coupling with other vibrations of the molecule its frequency is characteristic for the constitution and the neighborhood of the observed atom group. 

Useful characteristic group frequency bands often involve coupled vibrations. The spectra of alcohols have a strong band in the region between 1260 and 1000 cm-1, which is usually designated as the C—O stretching band. In the spectrum of methanol this band... 

Characteristic stretching vibrations of the nitro group of 5(6)-nitrobenzimidazoles lie in frequency ranges of 1540-1510 (vj and 1350-1330 cm-1 (vs) [709, 715, 1085-1090], The N02 stretching vibration frequencies of 5(6)-nitrobenzimidazoles and 5- and 6-nitroindazole have been measured (Table 3.62). 

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