Raman versus Infrared Spectroscopy

Although IR and Raman spectroscopies are similar in that both techniques provide information on vibrational frequencies, there are many advantages and disadvantages unique to each spectroscopy. Some of these are listed here. 

As stated in Section 1.7, selection rules are markedly different between IR and Raman spectroscopies. Thus, some vibrations are only Raman-active while others are only IR-active. Typical examples are found in molecules having a center of symmetry for which the mutual exclusion rule holds. In general, a vibration is IR-active, Raman-active, or active in both however, totally symmetric vibrations are always Raman-active. 

Measurements of depolarization ratios provide reliable information about the symmetry of a normal vibration in solution (Section 1.9). Such information can not be obtained from IR spectra of solutions where molecules are randomly orientated. 

Since the diameter of the laser beam is normally 1-2 mm, only a small sample area is needed to obtain Raman spectra. This is a great advantage over conventional IR spectroscopy when only a small quantity of the sample (such as isotopic chemicals) is available. 

Since water is a weak Raman scatterer, Raman spectra of samples in aqueous solution can be obtained without major interference from water vibrations. Thus, Raman spectroscopy is ideal for the studies of biological compounds in aqueous solution. In contrast, IR spectroscopy suffers from the strong absorption of water. 

---

Raman and infrared vibrations are mutually exclusive and consequently use of both techniques is required in order to obtain a set of vibrational bands for a molecule. The advent of powerful computer-controlled instrumentation has greatly enhanced the sensitivity of these vibrational spectroscopies by the use of Fourier transform (FT) techniques, whereby spectra are recorded at all frequencies simultaneously in the time domain and then Fourier transformed to give conventional plots of absorbance versus frequency. The wide range of applications of FT Raman spectroscopy is discussed by Almond et al. (1990). Specific examples of its use in metal speciation are the observation of the Co-C stretch at 500 cm-1 in methylcobalamin and the shift to lower frequency of the corrin vibrations when cyanide is replaced by the heavier adenosyl in going from cyanocobalamin to adenosylcobalamin (Nie et al., 1990). 

In this presentation, two examples of the use of vibrational spectroscopy to probe water-solid interactions in materials of interest to the food and pharmaceutical sciences are described. First, the interaction of water vapor with hydrophilic amorphous polymers has been investigated. Second, water accessibility in hydrated crystalline versus amorphous sugars has been probed using deuterium exchange. In both of these studies, Raman spectroscopy was used as the method of choice. Raman spectroscopy is especially useful of these types of studies as it is possible to control the environment of the sample more easily than with infrared spectroscopy. 

The first step in a polymer analysis is to identify the specific type of polymer in a given sample. This may be complicated in a formulated sample by the presence of additives. Infrared spectroscopy will usually provide information on both the base polymer(s) and the additive(s) present. The second step, if possible, is to determine details of the chemical and physical characteristics, which define the quality and properties of the polymer. The chemical properties that can be determined are stereo specificity, any irregularities in the addition of monomer (such as 1,2- versus 1,4-addition and head-to-head versus head-to-tail addition), chain branching, any residual unsaturation, and the relative eoncentration of monomers in the case of copolymers. Other important characteristics include specific additives in a formulated product, and the physical properties, which include molecular weight, molecular-weight dispersion, crystallinity, and chain orientation. Some properties such as molecular weight and molecular-weight dispersion are not determined directly by infrared and Raman spectroscopy, except in some special cases. 

Fig.4.5-TI (CH3NHCH2C00H)3 CaCl2. vq versus T. Vo is the phonon mode frequency. Triangles measured by millimeter spectroscopy. Brown circles measured from far-infrared spectra. Gray circles measured from electric-field-induced Raman spectra. In the paraelectric phase T > 0f), Vo decreases as the temperature decreases toward 0f, that is, the phonon mode softens...

---