Dynamic dichroism spectra

DIRLD data used for the 2D correlation analysis was obtained by applying a 23-Hz dynamic strain with an amplitude of 0.1 Xi to a thin solution-cast film of atactic polystyrene (M 150,000) at room temperature [39]. The resulting dynamic dichro-ism spectra have already been shown in Figures 1-5 and 1-6. The time-dependent reorientations of transition dipoles associated with the molecular vibrations of backbone and side groups are all observable in the original dynamic dichroism spectra. However, much more detailed features are recognized in the 2D correlation spectrum. 

To illustrate the type of information that can be obtained in experiments of this type, let us examine the DIRLD spectroscopy of poly(p-phenylene vinylene) (PPV) that was reported by Shah et al. [9]. This paper showed both static and dynamic dichroism spectra, with the dynamic data either in the form of in-phase, IP(v), and quadrature, Q v), spectra or as magnitude, S V), and phase, 4>(, spectra. As noted by Budevska et al. [10], the two outputs are simply different ways of displaying the same data, since... 

Figure 46 shows the temperature dependence of the in-phase dynamic dichroism spectrum of polystyrene in the semicircle-stretching vibration region. In order to eliminate the contributions from other modes of molecular vibrations to this spectral region, such as methylene deformations, the... 

The wavenumber-dependent terms, A/4 (v) and AA" v), are known respectively as the in-phase spectrum and quadrature spectrum of the dynamic dichroism of the system. They represent the real (storage) and imaginary (loss) components of the time-dependent fluctuations of dichroism. Figure 1-6 shows an example of the in-phase and quadrature spectral pair extracted from the continuous time-resolved spectrum shown in Figure 1-5. These two ways of representing a DIRLD spectrum contain equivalent information about the reorientation dynamics of transition dipoles. However, the orthogonal representation of the time-resolved spectrum using the in-phase and quadrature spectra is obviously more compact and easier to interpret than the stacked-trace plot of the time-resolved spectrum. 

Figure 1-13 shows an example of a train of lock-in amplifiers used to demodulate DIRLD signals obtained with a spectrometer described in Figure 1-12. In order to obtain the time-dependent dynamic absorbance and DIRLD responses, quadrature lock-in amplifiers are used. These devices monitor signals both in phase and 90° out of phase (quadrature) with the sinusoidal strain reference signal. The monochromator is scanned one wavelength at a time through the spectrum. Data are collected on six separate channels (e.g., in-phase and quadrature dynamic dichroism, in-phase and quadrature dynamic absorbance, static dichroism, and normal IR absorbance)...

Angell and Burgess described the synthesis of j8-tum mimics by intramolecular copper(I)-catalyzed cycloaddition of dipeptide 72 in Scheme 10.22. However, they noted the prevalence to form dimeric cycloaddition products 74 instead of the desired macrocycle 73, and yields were low due to poor solubility of both products. Molecular dynamics and NMR were used to identify plausible type I and 11 jS-tum conformations for 73, however, the circular dichroism spectrum did not correspond to typical type 1 and 11 jS-tum structures and this discrepancy was not discussed further. Burke Jr. el al. reported the preparation... 

The terms AA (v) and AA"(v) are referred to as the in-phase spectrum and quadrature spectrum of DIRLD. The two orthogonal components of the dynamic dichroism are related to the amplitude A A(v) and loss angle P v) by... 

The overall reorientability (mobility) of dipole transition moments is given by the power spectrum P(v) of the dynamic dichroism... 

Figure 36 The in-phase and quadrature spectrum of dynamic dichroism and the regular IR absorbance spectrum. Noda, l.Appl. Spectrosc. 1990, 44(4), 550. ...

Figure 39 In-phase and quadrature dynamic IR dichroism spectrum of atactic polystyrene in the CH-stretching region. Regular absorbance spectrum is also provided. Noda, I. Dowrey, A. E. Marcott, C. Appl. Spectrpsc. 1988, 42 2), 203. " ...

As the time scale of the Raman scattering event ( 3.3 x 10 14 s for a vibration with a Stokes wave number shift of 1000 cm 1 excited in the visible) is much shorter than that of the fastest conformational fluctuations, an ROA spectrum is a superposition of snapshot spectra from all the distinct conformations present in a sample at equilibrium. Since ROA observables depend on absolute chirality, there is a cancellation of contributions from enantiomeric structures arising as a mobile structure explores the range of accessible conformations. Therefore, ROA exhibits an enhanced sensitivity to the dynamic aspects of biomolecular structure. In contrast, conventional Raman band intensities are blind to chirality and so are generally additive and therefore less sensitive to conformational mobility. Ultraviolet circular dichroism (UVCD) also demonstrates an enhanced sensitivity to the dynamics of chiral structures ... 

A dynamic linear infrared dichroism study was carried out as a function of temperature on isotropic and cold drawn HDPE using a rapid-scan setup. The dynamic in-phase dichroic spectrum and its components (parallel and perpendicular polarised spectra) were analysed. 15 refs. 

Figure 1-5. A time-resoived dynamic IR dichroism (DIRLD) spectrum of an atactic poiystyrene him under a small-amplitude (ca. 0.1%) sinusoidal (23-Hz) dynamic strain at room temperature.

Visualizing the detailed dynamics of polymers subjected to a modulated strain from plots such as the one shown in Figures 21.8 to 21.13 can be quite difficult. The fact that the bands in the in-phase and quadrature spectra have such different shapes indicates that the time dependence of the dichroism for each band in the spectrum is quite different, since the rate of reorientation of each functional group in the molecule when subjected to the effect of a modulated strain may well be determined by their local environment. The fact that these rates vary can be used to differentiate between highly overlapped bands, as can be seen in the N—H stretching region of the in-phase and quadrature spectra of nylon-11 shown in Figure 21.6. 

Dynamic mechanical techniques are important in the characterization of the rheological properties of polymers. In dynamic mechanical analysis, a small-amplitude oscillatory strain is applied to a sample, and the resulting dynamic stress is measured as a function of time. The dynamic mechanical technique allows the simultaneous measurement of both the elastic and the viscous components of the stress response. Typically, the temperature and deformation frequencies are changed in order to determine the mechanical relaxation spectrum of the system. The molecular basis of changes in the dynamic mechanical properties can be investigated using dynamic IR dichroism. 

The in-phase and quadrature spectra represent orthogonal components of the dynamic optical anisotropy caused by the reorientation of electric dipole transition moments. The in-phase spectrum is proportional to the instantaneous extent of strain. The quadrature spectrum, on the other hand, represents the component of reorientation proportional to the rate of strain, which is n/2 out of phase with the extent of strain. Figure 36 shows an example of the in-phase and quadrature spectrum. These two component spectra contain the same amount of information content displayed in the set of time-resolved spectra shown in Figure 35. In other words, the entire time-dependence of the dichroism signals can be reconstructed from the linear combinations of the two orthogonal component spectra. 

---