Raman functional group frequencies

Raman spectra arc similar to IR spectra in that they have regions useful for functional group detection and fingerprint regions that permit the idcntiflcaiion of specific compounds. Uaimay et al. have published a comprehensive treatment of Raman functional group frequencies,"... 

A group of investigators recently suggested that the density-functional theory (DFT), which calculates IR and Raman spectra, is a useful tool for direct characterization of the structures of diamondoids with increasing complexity [66]. They applied DFT to calculate Raman spectra whose frequencies and relative intensities were shown to be in excellent agreement with the experimental Raman spectra for C26H30, thus providing direct vibrational proof of its existence. 

TABLE 3.1. Example of Raman frequency for several functional groups. 

The interactions of photons with molecules are described by molecular cross-sections. For IR spectroscopy the cross-section is some two orders of magnitude smaller with respect to UV or fluorescence spectroscopy but about 10 orders of magnitude bigger than for Raman scattering. The peaks in IR spectra represent the excitation of vibrational modes of the molecules in the sample and thus are associated with the various chemical bonds and functional groups present in the molecules. The frequencies of the characteristic absorption bands lie within a relatively narrow range, almost independent of the composition of the rest of the molecule. The relative constancy of these group frequencies allows determination of the characteristic... 

Further details of the theory and application of Raman spectroscopy in polymer studies can be found elsewhere (1. 9). However, vibrational frequencies of functional groups in polymers can be characterized from the spacing of the Raman lines and thus information complementary to IR absorption spectroscopy can be obtained. In addition, since visible radiation is used the technique can be applied to aqueous media in contrast to IR spectroscopy, allowing studies of synthetic polyelectrolytes and biopolymers to be undertaken. Conformation and crystallinity of polymers have also been shown to influence the Raman spectra Q.) while the possibility of studying scattering from small sample volumes in the focussed laser beam (-100 pm diameter) can provide information on localized changes in chemical structure. 

Both Raman and infrared spectroscopy provide qualitative and quantitative information about ehemieal species through the interaetion of radiation with molecular vibrations. Raman spectroscopy complements infrared spectroscopy, particularly for the study of non-polar bonds and certain functional groups. It is often used as an additional technique for elueidating the molecular structure and symmetry of a eompound. Raman spectroseopy also provides facile access to the low frequency region (less than 400 cm Raman shift), an area that is more difficult for infrared speetroseopy. 

In this example, the relation between 19 chemicals and 23 physicochemical parameters was examined ( ). PLS, unlike canonical correlation, permits use of more chemical parameters than stimuli. The twenty-three physicochemical variables included molecular weight, functional groups, Raman frequencies and Laffort parameters (see ( )) The Laffort parameters are alpha (an apolar factor proportional to molvolume), rho (a proton receptor factor), epsilon (an electron factor) and pi (a proton donor factor). 

Raman Spectroscopy provides information comparable to that obtained by FTIR. The sample is illuminated by a laser beam (visible light), and the light dispersed from the sample (Raman effect) [46] is analyzed. The frequency differences between the light dispersed and that of the initial laser beam reflect the various functional groups of surface and solution species. This method can also be used... 

Early experiments with bacteriorhodopsin (228) interpreted the Raman spectrum in terms of an unprotonated Schiff base, forming a charge-transfer complex with a protein functional group (210,212). This interpretation of the Raman data, essentially based on a comparison with the frequencies of model Schiff bases in solution, was criticized by Honig and Ebrey (48), who pointed out that consistency could also be obtained with a protonated Schiff base model. The latter hypothesis was subsequently confirmed by deuteration experiments similar to those described for rhodopsin (229,230), and by Raman spectra in denatured systems (231). In variance with the clear-cut similarity observed between the resonance-Raman spectra of rhodopsin and isorhodopsin, and those of the 11-cis and 9-cis model compounds, respectively,... 

The molecular structure is the nonplanar configuration from the electron diffraction study of Akishin et al. (8). A planar model (also point group < 2 ) was assumed by Hisatsune et al. (9) in their approximate normal coordinate analysis of the infrared and Raman spectra. The frequency assignments of these authors are listed above in the order for the planar model, although the vibrations for the nonplanar form will separate differently into the species 5A, 3A2, 3B and 4B2. Hisatsune et al. (9) estimated the N-O -N deformation frequency (8 cm ) from combination bands in the solid and gas phase spectra. The JANAF thermodynamic functions were obtained using these frequencies and assuming the two N0 groups to be hindered internal rotators. 

Consequently, vibrational modes get broader and may shift towards lower frequencies. Although water gives only a very weak Raman spectram, interactions between water and functional groups of the hydrogel affect the Raman spectrum as well (Kwak and Lafleur 2003). 

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