IR linear dichroism

More recently, the PCM has been amply extended to the treatment of vibrational spectroscopies, by taking into account not only solvent-induced vibrational frequency shifts, but also vibrational intensities in a unified and coherent formulation. Thus, models to treat IR [8], Raman [9], IR linear dichroism [10], VCD [11] and VROA [12] have been proposed and tested, by including in the formulation local field effects, as well as an incomplete solute-solvent regime (nonequilibrium) and, when necessary, by extending the model to the treatment of specific solute-solvent (or solute-solute) effects. 

In addition to the energies and intensities we have also included the transition moment directions in Table IX. These quantities, obtained in UV or infrared (IR) linear dichroism experiments, have been used... 

No.21, 19th Oct.1999, p.7147-55 STATIC AND DYNAMIC FT-IR LINEAR DICHROISM STUDIES OF PLASTICIZATION EFFECTS IN A POLYURETHANE ELASTOMER... 

Figure 1-3 shows a schematic diagram of a dynamic IR linear dichroism (DIRLD) experiment [20-25] which provided the foundation for the 2D IR analysis of polymers. In DIRLD spectroscopy, a small-amplitude oscillatory strain (ca. 0.1% of the sample dimension) with an acoustic-range frequency is applied to a thin polymer film. The submolecular-level response of individual chemical constituents induced by the applied dynamic strain is then monitored by using a polarized IR probe as a function of deformation frequency and other variables such as temperature. The macroscopic stress response of the system may also be measured simultaneously. In short, a DIRLD experiment may be regarded as a combination of two well-established characterization techniques already used extensively for polymers dynamic mechanical analysis (DMA) [26, 27] and infrared dichroism (IRD) spectroscopy [10, 11]. 

The optical anisotropy, as characterized by the difference between the absorption of IR light polarized in the directions parallel and perpendicular to the reference axis (i.e., the direction of applied strain), is known as the IR linear dichroism of the system. For a uniaxially oriented polymer system [10, 28-30], the dichroic difference, A/4(v) = y4 (v) - Ax v), is proportional to the average orientation, i.e., the second moment of the orientation distribution function, of transition dipoles (or electric-dipole transition moments) associated with the molecular vibration occurring at frequency v. If the average orientation of the transition dipoles absorbing light at frequency is in the direction parallel to the applied strain, the dichroic difference AA takes a positive value on the other hand, the IR dichroism becomes negative if the transition dipoles are perpendicularly oriented. 

The IR linear dichroism (i.e., the anisotropic absorption) is characterized by the integrated absorbances A and Ax measured at the band under inspection with light polarized parallel and perpendicular to the fixed reference direction, respectively. Commonly, the optical anisotropy of uniaxially oriented samples is characterized by the dichroic ratio Rdkhro = A /Ax, the dichroic difference AA = A — Ax, or by the... 

Measurement of IR linear dichroism requires light polarized both parallel and perpendicular to a fixed reference direction of the sample. For parallel polarized light, the absorbance is termed A, and the absorbance with perpendicular polarized light is termed A . The dichroic ratio, R, is defined as... 

Here the terms AA v) and AA v) are referred to as the in-phase spectrum and the quadrature spectrum of the dynamic IR linear dichroism, respectively. The two orthogonal spectra are related to the amplitude, AA(v), and loss angle, /S(v) by... 

Dynamic IR linear dichroism (DIRLD) studies have been made using a rapid-scan interferometer system. For dynamic strain frequencies achievable are between 0.1 Hz and 10 kHz step-scan interferometers are useful. 

FIGURE 1. IR linear dichroism spectra of air-dried LMH and LM reaction centers reconstituted in phosphatidylcholine vesicles. 

The theoretical model of the IR linear dichroism suggests a single-axis orientation of a molecular assembly with axial (cylindrical) synunetry. This means that the molecules are located with their longest axis in the direction, which induces the anisotropy, determined by the director n, and the side groups are randomly fixed or rotate along the length of this axis. Under these conditions, the dipole moments of transition caused by the normal vibrations form conical surfaces with director n (Figure 1.2) [2,3,6-8]. 

For an oriented system, the directional sensitivity of IR absorption at the submolecular level is manifested by an optical anisotropy called linear dichroism. The term linear is used here to distinguish it from circular dichroism, which is another type of optical anisotropy related to the chirality of the system. IR linear dichroism is measured with light polarized in planes parallel ( ) and perpendicular ( ) to a fixed reference direction of the sample, as shown in Figure 28. Such a measurement yields the directional absorbances, namely, A (v) and Aj (v), of the system. If the system is isotropic, the intensity of directional absorbance is constant regardless of the polarization direction of light. For an anisotropic system, on the other hand, directional absorbance takes different values depending on the relative direction of polarization of light with respect to the optical axis of the system. 

---