Polarized infrared radiation

Q are the absorbance and wavenumber, respectively, at the peak (center) of the band, p is the wavenumber, and y is the half width of the band at half height. Liquid band positions ate usually shifted slightly downward from vapor positions. Both band positions and widths of solute spectra are affected by solute—solvent interactions. Spectra of soHd-phase samples are similar to those of Hquids, but intermolecular interactions in soHds can be nonisotropic. In spectra of crystalline samples, vibrational bands tend to be sharper and may spHt in two, and new bands may also appear. If polarized infrared radiation is used, both crystalline samples and stressed amorphous samples (such as a stretched polymer film) show directional effects (28,29). 

A powerful characteristic of RAIR spectroscopy is that the technique can be used to determine the orientation of surface species. The reason for this is as follows. When parallel polarized infrared radiation is specularly reflected off of a substrate at a large angle of incidence, the incident and reflected waves combine to form a standing wave that has its electric field vector (E) perpendicular to the substrate surface. Since the intensity of an infrared absorption band is proportional to / ( M), where M is the transition moment , it can be seen that the intensity of a band is maximum when E and M are parallel (i.e., both perpendicular to the surface). / is a minimum when M is parallel to the surface (as stated above, E is always perpendicular to the surface in RAIR spectroscopy). 

A STUDY WITH PLANE-POLARIZED INFRARED RADIATION OF SOME CRYSTALLINE MODIFICATIONS OF CELLULOSE ... 

The study of oriented polymers with polarized infrared radiation is an equally important tool in the detailed analysis of the vibrational spectrum, since it permits us, within certain restrictions, to determine the orientation (with respect to the molecular structure) of the transition moment for a given normal mode. This makes it possible, as we shall see, not only to classify bands in the spectrum but to establish their origin. Although polarizers are available with commercial spectrometers, their use has not yet become as general as would be desirable. Some comments... 

We noted earlier (Section I. 1.) that the intensity of an absorption band is proportional to the square of the changing dipole moment in the molecule (i. e., transition moment) during the corresponding normal vibration. The intensity also depends upon the direction that the electric vector in the incident radiation makes with the transition moment. In particular, the intensity is proportional to the square of the scalar product of the transition moment and electric field vectors. This implies, for example, that if the electric field vector is perpendicular to the transition moment vector no absorption will occur. This fundamental relationship is the basis for the utilization of polarized infrared radiation as a powerful tool in the study of the spectra and structure of oriented polymers. We consider below some aspects of this technique. 

It has been mentioned by Baly279 that, although carbohydrates do not selectively absorb ultraviolet and visible light, they do show selective absorption of infrared radiation in the wavelength range of 1.5-6 /im. This observation prompted Sem-mens280-281 to examine the action of polarized infrared radiation upon starch. She realized that mild hydrolysis takes place in starch subjected to such treatment, as well as to irradiation by nonpolarized infrared radiation. The effects of such radiation are observed in samples both in vitro and in vivo (namely in plants). 

As described above, there are two forms of vibrational optical activity, one derived from infrared absorption and the other from Raman scattering. Both forms involve the differential response of a molecule to the modulation of polarization of the interacting radiation between right and left circularly polarized states. In the case of infrared absorption, VCD is defined as die differential absorbance for left minus that for right circularly polarized infrared radiation. This is expressed by the relation ... 

In VCD, the differential absorption of left and right circularly polarized infrared radiation by a vibrational transition of a chiral molecule is observed. Equations (1-5) hold equally well for electronic and vibrational transitions, but in all transition moments, the electronic wave functions need to be replaced by vibrational (or vibro-nic) wave functions. In vibrational CD, the ratio of differential absorption to the infrared absorption, defined as... 

The crystalline structure of cellulose has been characterized by X-ray diffraction analysis and by methods based on the absorption of polarized infrared radiation. The unit cell of native cellulose (cellulose I) consists of four glucose residues (Figs. 3-6 and 3-7). In the chain direction (c), the repeating unit is a cellobiose residue (1.03 nm), and every glucose residue is accordingly displaced 180° with respect to its neighbors, giving cellulose a... 

Another interesting method of investigating oriented films involves using polarized infrared radiation. Two different FTIR spectra are obtained of such samples if the electric field of the incident IR beam is polarized parallelly or perpendicularly to the primary axis of orientation of the sample. Such dichroic spectra may provide information concerning the orientation of subgroups in a biopolymen for example of the phosphate groups of nucleic acids, with respect to the helical axis (Sec. 4.7.1.3.4). 

Vibrational optical activity, in particular, vibrational circular dichroism (VCD), has become a topic of considerable interest since 1970 m. The phenomenon is related to the different absorption of left- and right-circularly polarized infrared radiation and, hence, involves excitations of molecules into particular vibrational states. 

Single crystal studies of solid hydrates are scarce. There are two experimental procedures possible (i) transmission spectra of thin crystal plates (see, for example. Refs. 16, 17) and (ii) reflection spectra of crystal faces . Using polarized infrared radiation, the species (symmetry) and other directional features of the water bands can be determined. In the case of reflection measurements, the true transverse and longitudinal optic phonon frequencies can be additionally computed by means of Kramers-Kronig analyses and oscillator fit methods, respectively. Both experimental techniques, however, are relatively difficult because of the lack of suitable monocrystals, the requirement of preparing sufficiently thin, i.e., <0.1 mm, crystal plates (except for studying overtone bands, see Sect. 4.2.6), and the efflorescence or absorption of water at the crystal surfaces. In favorable cases, thin sheets of orientated powdery material can be obtained . ... 

The definitions given above for An and Ca are in terms of the anisotropies in the optical properties over the wavelength range of visible light. Similar anisotropies can also exist or can be induced in other wavelength ranges. For example, infrared dichroism (the absorption of different amounts of polarized infrared radiation in the directions parallel and perpendicular to the direction of orientation) is useful in characterizing polymers by vibrational spectroscopy. 

Neither of the two earlier reviews on the infrared spectroscopy of carbohydrates " dealt with the uses of plane-polarized radiation. This is, no doubt, attributable to the fact that both reviews were principally concerned with crystalline sugars, for which few such spectra are available. However, the polarized infrared spectra of such polysaccharides as cellulose, chitin, and xylans, in the form of oriented films, have been measured " and have provided information that other techniques could not give. It is, therefore, desirable that a brief discussion should be here provided of both the experimental and the interpretational aspects (see p. 28) for a detailed discussion, the reader is referred elsewhere. The commonest method of obtaining polarized infrared radiation is with a transmission polarizer (rather than a reflection polarizer). Selenium film and silver chloride sheet have both been used of these, the latter is the more popular because it is the more robust. A stack of about six sheets, each about 50-100 m thick, is... 

Study of the crystal structure of polysaccharides, particularly of cellulose, has provided the main use for polarized infrared radiation in connection with carbohydrate spectra. Since this is another technique whereby band assignments can be made, the basic steps involved will be described in a simplified manner with reference to a polymer sample having uniaxial orientation. This is a common type of orientation, characteristic of fibers,... 

Suppose that we irradiate a single crystal of calcite with polarized infrared radiation whose electric vector vibrates along the c axis (z direction) in Fig. 1.42. Then the infrared spectrum shown by the solid curve of Fig. 1.48 is obtained [142]. According to Table 1.21, only the A2 vibrations are activated under such conditions. Thus, the three bands observed at 885(v), 357(0, and 106(r) cm are assigned to the species. The spectrum shown by the dotted curve is obtained if the direction of polarization is perpendicular to the c axis (x,y plane). In this case, only the vibrations should be infrared-active. Therefore the five bands observed at 1484(v), 706(v), 330(0, 182(0, and 106(r) cm are assigned to the species. Here, v, t, and r denote intramolecular, translatory lattice, and rotatory lattice modes, respectively. 

Using polarized infrared radiation, a team of workers at Courtaulds Ltd. Research Laboratory, Maidenhead, England, have paid particular attention to the orientation of the hydrogen-bonded link between the carbonyl and imino groups of the polypeptide main chain. In the a-form of keratin, myosin, and tropomyosin they find that the C=0 and N—H bonds are directed mainly along the fiber axis, while in sundry... 

Suppose that we irradiate a single crystal of calcite with polarized infrared radiation whose electric vector vibrates along the c axis (z direction) in Fig. 1-22. Then the infrared spectrum shown by the solid curve of Fig. 1-23 is obtained. According to Table 1-13, only the A u vibrations are activated... 

Another use of infrared spectroscopy employs linearly polarized infrared radiation to determine information about oriented samples, an experiment that has been termed infrared dichroism. The absorbance will be a maximum when the electric field vector and the dipole moment vector are in the same direction, and the absorbance will be zero when the two are perpendicular. Only uniaxial orientation will be considered because this situation is normally found in the literature. R, the dichroic ratio, is defined as follows ... 

Solid films have been examined frequently for infrared analysis in biochemical work, for example, in structural studies of proteins, polypeptides, and polysaccharides. Such films have been of particular value for studying polarization spectra of macromolecules in intact films and in oriented ones (stretched, rolled, or stroked), thereby permitting knowledge to be gained concerning spatial arrangements within the molecule and conformational effects among molecules. (See The Use of Polarized Infrared Radiation and the Measurement of Dichroism, p. 73, for a detailed discussion.) A few workers have discussed the film technique (Lecomte, 1948 Randall et al., 1949 Hacskaylo, 1954). 

Polarized infrared radiation is used to obtain information about the direction of transition moments of normal modes of vibration in solid oriented compounds. If one knows the molecular orientation in a solid, he can use polarization studies in making band assignments. (See Chapters 6 and 10 for such applications in carbohydrate and polypeptide chemistry.) The measured direction of the transition moment of the vibration producing a band must coincide with the direction deduced from the structure if the assignment is correct. On the other hand, knowing the band assignment but not the molecular orientation in the solid, one can deduce some knowledge of the molecular orientation. 

To obtain a beam of polarized infrared radiation one can use silver chloride (Newman and Halford, 1948) or selenium (Elliott et ai, 1948a) polarizers. Several plates of silver chloride or a thin film of selenium are tilted at the polarizing angle relative to the unpolarized infrared beam of radiation. Figure 3.28 (Colthup et ai, 1964) describes the optical effects in schematic fashion. 

The directions of the hydroxyl groups were determined by means of plane-polarized infrared radiation (Mann and Marrinan, 1958a,b Tsuboi, 1957) (See Specialized Techniques below). The OH band at 3309 cm was perpendicular, and it was suggested that some of the hydroxyl bonds lie along the chain direction and form intramolecular hydrogen bonds. 

For some samples, dipole moment changes may be in a fixed direction during a molecular vibration and, as such, can only be induced when the infrared radiation is polarized in that direction. Polarized infrared radiation can be produced by using a polarizer consisting of a fine grating of parallel metal wires. This approach is known as linear infrared dichroism [13]. 

Figure 6. Specular reflection FT-IR spectra of ultra-thin polymer films obtained at grazing angle using parallel polarized infrared radiation (26).

Thin films on reflecting substrates may be characterized by reflecting parallel-polarized infrared radiation off of the surface see Infrared spectroscopy RAIR. 

Infrared dichroism is based on the interaction between linearly polarized infrared radiation and the oriented material. The atoms of a polymer molecule vibrate in characteristic normal modes, each of which produces a change in dipole moment (the transition moment) that has a specific direction. Each mode absorbs infrared energy at a characteristic frequency, giving rise to peaks in the infrared spectrum. The peak intensity (i.e. the absorbance) depends on the angle between the transition moment and the electric field vector of the radiation, and it is this that provides information on the molecular orientation. The orientation is defined in terms of the second moment of the orientation function Pjlcos 0), where ... 

Noting that (tm x cos is proportional to the infrared absorbance, and that tm is the transition moment of the molecule (dipole) of interest and is the included angle. An illustration of the dipole electric field vector and the electric field vector of the s-polarized infrared radiation (with respect to the angle of polymer stretching) is illustrated in Figs 1 and 2. 

With the aid of polarized infrared radiation, it is possible to investigate the dichroism associated with a particular vibration in a fiber or film of a... 

Some spectra of oriented polypeptides obtained with polarized infrared radiation (Ambrose and Elliott, 1951a) are shown in Fig. 111. In the case of poly -benzyl-L-glutamate, the NH stretching mode at 3292 cm. and the C=0 stretching mode at 1653 cm. show parallel dichroism while the band associated with NH deformation at 1549 cm. shows perpendicular dichroism. The ester C=0 band at 1730 cm. does not show much dichroism. This is the behavior to be expected of the a form since the axes of the helices would be oriented preferentially in the direction of stretching, and the NH and CO bonds are essentially parallel to the axes of the helices. In some cases, the solvent used for casting films favors the extended /8 form, with the opposite dichroic behavior since the NH and CO bonds in the ]3 form are essentially normal to the backbone chain. Such is the case... 

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