Vibrational Spectroscopy. Infrared Absorption. Raman Spectra

2 Vibrational Spectroscopy. Infrared Absorption. Raman Spectra 

In the infrared (IR) and in Raman spectra of a molecule, bands are observed that correspond to normal modes of vibration of its particular structure. Vibration occurs by oscillation of two atoms in one direction (stretching 

By calculation 12 bands were assigned to different modes of vibrations, in part to their combinations. Here only the strong bands are mentioned. The band at 3236 cm (amide A, cf. 3330 cm above) is completely due to NH stretching, 

Shifts and certain splitting by resonance interactions of amide I and amide II bands in polypeptides in folded and extended conformation are discussed in Ref. 22 and in the early investigations of Miyazawa and of Elkan R. Blout [23], who used IR spectra for the characterization of the conformation of poly-amino acids. 

IR spectra are generally recorded as transmittance bands with 100 at the top and zero at the bottom, (Fig 9). 

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It is important to appreciate that Raman shifts are, in theory, independent of the wavelength of the incident beam, and only depend on the nature of the sample, although other factors (such as the absorbance of the sample) might make some frequencies more useful than others in certain circumstances. For many materials, the Raman and infrared spectra can often contain the same information, but there are a significant number of cases, in which infrared inactive vibrational modes are important, where the Raman spectrum contains complementary information. One big advantage of Raman spectroscopy is that water is not Raman active, and is, therefore, transparent in Raman spectra (unlike in infrared spectroscopy, where water absorption often dominates the spectrum). This means that aqueous samples can be investigated by Raman spectroscopy. 

Infrared spectroscopy is not as inherently informative with regard to metal interactions in highly symmetrical metal-metal bound dimers as is Raman spectroscopy, since the totally symmetric metal-metal stretch is a forbidden absorption in the infrared experiment. Oldham and Ketteringham have prepared mixed-halide dimers of the type Re2ClxBr 2xto lower the symmetry and hence introduce some infrared allowedness into the Re-Re stretching mode (206). Indeed, the appearance of a medium-intensity band at 274 cm 1 in the infrared spectrum of the mixed-halo species was considered to be the result of absorption by the metal—metal stretching vibration, which was also observed in the Raman spectrum at 274 cm ". ... 

These devices are based on the anisotropic absorption of light. Usually molecular crystals exhibit this property and tourmaline is the classical example for this. For practical purposes, however, micro crystals are oriented in polymer sheets. Polymers containing chromophors become after stretching dichroic polarizers. The devices produced in this manner are called polawids. They have found a broad application in many technologies. Their application in spectroscopy is limited to the near ultraviolet and to the visible and near infrared range of the spectrum. In vibrational spectroscopy polaroids are employed as analyzers only for Raman spectroscopy. 

Zeolites. The weak Raman signals arising from the aluminosilicate zeolite framework allow for the detection of vibrational bands of adsorbates, especially below 1200 cm which are not readily accessible to infrared absorption techniques. Raman spectroscopy is an extremely effective characterization method when two or more colored species coexist on the surface, since the spectrum of one of the species may be enhanced selectively by a careful choice of the exciting line. A wide range of adsorbate/zeolite systems have been examined by Raman spectroscopy and include SO2, NO2, acety-lene/polyacetylene, dimethylacetylene, benzene, pyridine, pyrazine, cyclopropane, and halogens. Extensive discussions of these absorbate/zeolite studies are found in a review article by Bartlett and Cooney. ... 

Infrared and Raman spectroscopy are complementary in structural determinations because some molecular vibrations that are inactive in the infrared (that is, do not result in a change in dipole moment and therefore do not cause an absorption band) do have a strong Raman line. The reverse is also true. Some bands that are weak or forbidden in the Raman spectrum are strong in the infrared spectrum. With the combined use of these techniques, the vibrational energies of a molecule can be fully described. 

Infrared spectroscopy and Raman spectroscopy are complementary analytical techniques. Both provide vibrational information about the molecule, but different data are conveyed in the absorption and scattering spectra analyzed, respectively, by IR and Raman spectroscopy. In general, the IR spectrum arises from the absorption of radiation the frequency of which is resonant with a vibrational transition, while the Raman effect results from inelastic scattering of photons to leave a molecule in a vibrationally excited... 

To observe particular rotational isomeric states, the method must be much more rapid than the rate of conformational isomerization. Optical methods such as absorption spectroscopy or light-scattering spectroscopy provide a short-time probe of the molecular conformation. If the electronic states of the molecule are strongly coupled to the backbone conformation, the ultraviolet or visible spectrum of the molecule can be used to study the conformational composition. The vibrational states of macromolecules are often coupled to the backbone conformation. The frequencies of molecular vibrations can be determined by infrared absorption spectroscopy and Raman scattering spectroscopy. The basic principles of vibrational spectres-... 

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