Dichroic difference spectra

Figure 7 Relaxation of orientation measured simultaneously for both components in miscible PS/PVME blends following a rapid deformation (1 m/s) to a draw ratio of 2 at Tg +15°C. The time-resolved dichroic difference spectra were acquired using PM-IRLD. Reproduced with permission from Pellerin et al. [42]. Copyright 2000 American Chemical Society.

Figure 20.29 (a) Synchronous 2D-FTIR correlation map of poly DRl M-co-BEM. with a DRl M mole fraction of 0.23 during the photo-induced orientation process. In this figure, the negative peaks are indicated by a minus sign and the one-dimensional (ID) spectra are the dichroic difference spectra recorded after 5 min (dashed line) and 60 min (solid line) of irradiation (b) Asynchronous 2D-FTIR... 

Figure 12.4. Spectrum of an undeformed 300-pm PET film in the high-wavenumber region and the dichroic difference spectra recorded with a PEM at the end of the deformation and after 30 and 300 s of relaxation. (Reproduced from [3], by permission of the Society for Applied Spectroscopy copyright 2002.)...

The complete theory of how the output signals from the five lock-in amplifiers are treated in order to obtain static and dynamic spectra was originally given by Noda et al. [3] and Noda [4,5]. The inclusion of the full theory is beyond the scope of this book. Suffice it to say that the end result is calculation of the average static absorbance spectmm, the in-phase and quadrature components of the parallel and perpendicular absorbances, and the static and dynamic dichroic difference spectra. Of these, the in-phase and quadrature DIRLD spectra give the most important information about the rheological behavior of the polymer on a submolecular level. 

Time-resolved dichroic difference spectra of atactic polystyrene generated from the spectra shown in Figure 21.3 over a little more than one cycle of the stretcher are shown in Figure 21.4, along with the variation of the applied strain over the same period. From this curve, the variation in intensity and phase of each of the bands in the spectrum can be visualized. Under the same scan conditions, it would... 

A more complex but faster and more sensitive approach is polarization modulation (PM) IRLD. For such experiments, a photoelastic modulator is used to modulate the polarization state of the incident radiation at about 100 kHz. The detected signal is the sum of the low-frequency intensity modulation with a high-frequency modulation that depends on the orientation of the sample. After appropriate signal filtering, demodulation, and calibration [41], a dichroic difference spectrum can be directly obtained in a single scan. This improves the time resolution to 400 ms, prevents artifacts due to relaxation between measurements, and improves sensitivity for weakly oriented samples. However, structural information can be lost since individual polarized spectra are not recorded. Pezolet and coworkers have used this approach to study the deformation and relaxation in various homopolymers, copolymers, and polymer blends [15,42,43]. For instance, Figure 7 shows the relaxation curves determined in situ for miscible blends of PS and PVME [42]. The (P2) values were determined... 

Polarized FTIR spectroscopy has been used extensively to study polymer orientation (i.e., the dichroic ratio and dichroic difference are normally obtained from spectra recorded sequentially with the infrared radiation polarized parallel and perpendicular to a reference direction). To improve the sensitivity of this technique, and to follow accurately the dynamics of orientation, FTIR spectroscopy has been coupled with a polarization modulation (PM) technique whereby the dichroic difference spectrum is recorded directly, thus minimizing instrumental and sample fluctuations (this is discussed later in the chapter). 

Absorbance spectra measured with p- and 5-polarization are denoted as Ap(v) and A5(v), respectively. Ap(y) — A5(v) is known as the dichroic difference, and Ap v)/As v) is the dichroic ratio. Quantitative treatments of molecular orientation often require knowledge of the dichroic ratio. However, in regions where the sample absorption is very weak, calculation of the dichroic ratio involves taking the ratio of noise on the baseline of Ap(v) and A5(v) when both parameters are approximately zero. Thus, if qualitative or semiquantitative information is all that is required, the dichroic difference spectrum is more useful than the dichroic ratio. 

Two-step sequential and unistep superexchange primary electron transfer create primary radical pair states which differ in orientation of their dipole moments by cf 31°. The orientation is reflected in the angular dependence of the primary charge separation rate in an electric field and has been measured by the dichroic excitation spectrum of electric field modulated fluorescence yield at low temperatures. For reaction centers of Rb.sphaeroides R-26 embedded in polyvinyl alcohol films an orientation was obtained coinciding with the orientation of the dipole moment of within an angle of 5 . This result is only... 

Equations 1.32 and 1.17 are identical, which means that, at the point of elimination, the parameter c coincides with the dichroic ratio of the i-band. Consequently, in the reduced IR-LD spectrum obtained, all absorption maxima, generated by moments of transition that are parallel to those of the /-vibration are not present. This conclusion is mostly valid for the vibrations belonging to a given symmetry class and also for random colinearity of the moments of transition caused by the molecular or super molecular geometry (see Section 2.5 in Chapter 2). By varying the spectral subtraction factor some bands of the difference spectrum can be eliminated. At this moment the factor becomes equal to the dichroic ratio in the vanished bands. The orientation parameters of a given vibrational transition moment of the nth molecular direction is obtained from the dichroic ratio as [17] ... 

The circular dichroic spectrum of cobalt alkaline phosphatase (Fig. 16) shows more clearly the complexity of the visible absorption. Although it can not be ruled out that the spectrum of this Co (I I) enzyme represents two slightly different Co(II) sites, there are striking similarities with Co(II) carbonic anhydrase, which has only one metal-binding site. At high pH, cobalt carbonic anhydrase and cobalt alkaline phosphatase have several spectral features in common, and both may have a similar kind of irregular coordination. It should be noted, however, that the absorption coefficient for Co(II) alkaline phosphatase per equivalent of activity-linked metal ion is only half of the value for Co(II) carbonic anhydrase. 

A circular dichroic study (288) of these complexes has shown that the CD spectrum of bound auramine is different in binary compared to ternary complexes. This finding is consistent with the view that the coenzyme-induced conformational change alters some details of the substrate binding pocket (Section II,C,3,b). 

A glance at the circular dichroic spectra shown in Figures 1, 2, and 3 demonstrates a fundamental point starting from the dimer, all of these substances in neutral solution at low temperature exhibit the same kind of dichroic spectrum. While there are small differences in the positions and... 

A further indication of the nature of this neutral pH structure is provided by a contrast of Figures 2 and 4. In acidic solution, where poly A is known to form a two-strand helix, both poly A and the higher oligomers demonstrate a different circular dichroic spectrum, whereas the curves for poly HEA and the lower oligomers of A are unchanged (Figure 1) (25). 

Each of these bands will be of half-width comparable to that of the monomer band. Thus, while the difference in sign allows resolution in the circular dichroic spectrum, the bands cannot be resolved in the absorption spectrum. If we algebraically add two bands of appropriate width and intensity, the series of points shown in Figure 1 is obtained this accurately represents the observed circular dichroic spectrum of the dimer. A calculation (25) of the rotational strength to be observed in either half of the curve leads to values in good agreement with experiment, if Z is assumed to be 3-4 A., and a is between 30° and 45°. 

Some discussion should be devoted to the conditions under which two-strand helices can be formed. As has been shown, the configuration results in a circular dichroic spectrum differing markedly in intensity, form, and position on the wavelength scale from that of the single-chain helix. 

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