Dichroism perpendicular

In Reference 146 three methods for playback of data are presented, which are superior to the usual method based on the contrast in light scattering readout of dichroism, perpendicular readout mode, and readout of birefringence. Some critical comments also are given about global or selective erasure of data. 

Data taken from [88], S strong, M medium, W weak, V very, sh shoulder, br broad, parallel dichroism, perpendicular dichroism. b s stretch, as antisymmetric stretch, ss symmetric stretch, b bend, ib in-plane bend, ob out-of-plane bend, ab antisymmetric bend, sb symmetric bend, r rock, d deformation, t torsion. Contributions >10. 

It may readily be shown that for the electric vector perpendicular to the symmetry axis (perpendicular dichroism) the absorption Ax is given by... 

The classical scheme for dichroism measurements implies measuring absorbances (optical densities) for light electric vector parallel and perpendicular to the orientation of director of a planarly oriented nematic or smectic sample. This approach requires high quality polarizers and planarly oriented samples. The alternative technique [50, 53] utilizes a comparison of the absorbance in the isotropic phase (Dj) with that of a homeotropically oriented smectic phase (Dh). In this case, the apparent order parameter for each vibrational oscillator of interest S (related to a certain molecular fragment) may be calculated as S = l-(Dh/Di) (l/f), where / is the thermal correction factor. The angles of orientation of vibrational oscillators (0) with respect to the normal to the smectic layers may be determined according to the equation... 

IR dichroism has also been particularly helpful in this regard. Of predominant interest is the orientation factor S=( 1/2)(3—1) (see Chapter 8), which can be obtained experimentally from the ratio of absorbances of a chosen peak parallel and perpendicular to the direction in which an elastomer is stretched [5,249]. One representation of such results is the effect of network chain length on the reduced orientation factor [S]=S/(72—2 1), where X is the elongation. A comparison is made among typical theoretical results in which the affine model assumes the chain dimensions to change linearly with the imposed macroscopic strain, and the phantom model allows for junction fluctuations that make the relationship nonlinear. The experimental results were found to be close to the phantom relationship. Combined techniques, such as Fourier-transform infrared (FTIR) spectroscopy combined with rheometry (see Chapter 8), are also of increasing interest [250]. 

The orientation dependence of the stress alignment effect is consistent with the trigonal symmetry of the B—H complex. Stress along the [110] direction lifts the orientational degeneracy of the four BC sites about the boron while stress along the [100] direction does not. (A [111] stress also leads to a dichroism of the expected magnitude.) The sites perpendicular to... 

Figure 9.13 Dichroism of fra/ry-[Co(en)2Cl2] after the results of Yamada et al. on /rrtA ,s-[Co(en)2Cl2]CI HCl 2H20. The full line shows the spectrum with light polarized parallel (or nearly parallel) to the Cl—Co—Cl axis, and the dashed line shows the spectrum with light polarized perpendicular to the Cl—Co—Cl axis.

Using this approach, dichroism is found in the spectra which enable one to distinguish modes where the vectors are parallel or perpendicular to the fibre axes. The type of dichroism typical in these experiments is illustrated in Fig. 4. If one accepts the principle that the Raman band will be maximized in intensity when the electric vector of the source and the movement vectors of the atoms in the normal mode are parallel, it is possible to assign the Raman spectrum of polyethylene to fundamental... 

If peptide chains can be oriented in a regular fashion, it may be useful to measure infrared linear dichroism.24 25 Absorption spectra are recorded by passing plane polarized light through the protein in two mutually perpendicular directions, with the electric vector either parallel to the peptide chains or perpendicular to the chains. Such a pair of spectra is shown in Fig. 23-3A for oriented fibrils of insulin. In this instance, the insulin molecules are thought to assume a P conformation and to be stacked in such a way that they extend transverse to the fibril axis (a cross-P structure). When the electric vector is parallel to the fibril axis, it is perpendicular to the peptide chains. Since the amide I band is dominated by a carbonyl stretching motion that is perpendicular to the... 

Figure 23-3 Infrared absorbance spectra of the amide regions of proteins. (A) Spectra of insulin fibrils illustrating dichroism. Solid line, electric vector parallel to fibril axis broken line, electric vector perpendicular to fibril axis. From Burke and Rougvie.24 Courtesy of Malcolm Rougvie. See also Box 29-E. (B) Fourier transform infrared (FTIR) spectra of two soluble proteins in aqueous solution obtained after subtraction of the background H20 absorption. The spectrum of myoglobin, a predominantly a-helical protein, is shown as a continuous line. That of concanavalin A, a predominantly (3-sheet containing protein, is shown as a broken line. From Haris and Chapman.14 Courtesy of Dennis Chapman.

In contrast to n-n transitions, the n-n transitions of heterocyclic compounds and carbonyl-containing rings are often polarized in a direction perpendicular to the plane of the ring. Linear dichroism of cytosine, adenine, and other nucleic acid bases has been measured on single crystals and in partially oriented polymer films.71 Magnetically induced linear dichroism provides a new tool for study of metalloproteins.72... 

Infrared dichroism of mutually perpendicular normal modes in oriented high polymers. J. chem. Phys. 27, 1437—1438 (1957). 

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