Rigid dipoles independency

The most intuitively acceptable explanation for the breakdown of the theory relating IR dichroism and the uniaxial molecular orientation is that some of the molecular structural parameters, such as i and Aq, are indeed affected by dynamic orientation processes. In other words, molecules undergoing dynamic reorientation processes do not always rotate as rigid and independent entities. Changes in local molecular environments and molecular conformations induced by the macroscopic perturbations imposed on the system significantly affect the submolecular spatial relationship between the individual electric dipole transition moments arising from the vibrations of the molecular constituents and the principal orientation axis of the molecule. 

The motion of the absorption and emission dipoles in the molecular frame R is now assumed to be statistically independent of the motion of the R frame in the laboratory frame L. In a deformable molecule, the R frame may be attached to some small part of the molecule, which can be regarded as locally rigid. In this case, motion of the R frame occurs as a consequence of molecular deformation, as well as overall (uniform) rotation of the molecule. In such a case, statistical independence of the motion of a dipole in the R frame and the motion of the R frame itself is not guaranteed. However, with this assumption, Eq. (4.15) becomes... 

Figure 4.34. Time-independent part of the orientation parameter as a function of the field frequency for different values of the internal magnetic anisotropy of the particles. The ratio e = TdAb = 1CT4 curves correspond to a = 100 (1), a = 10 (2), o = 5 (3), a = 2 (4), a = 0.1 (5). Thin lines a and b resemble the limiting behavior predicted by the rigid and soft dipole models, respectively. Note that at this graph the lines a and 1 visually coincide.

Here we designate by pi and pn the permanent moments of the dipole molecule, which for an absolutely rigid molecule are of course independent... 

An alternative explanation, consistent with the data, is that dipole-dipole relaxation dominates for /1-carotene, but that molecular reorientation is anisotropic and occurs in a time range where the NOE is less than the maximum of 2.99 that is found in the extreme naiTOwing region. For a rigid isotropic rotor, the con-elation times are such that I and Tj will be independent of magnetic field strength and the NOE will... 

In polymer systems such a mutually independent dipole orientation is inapplicable because dipole orientation is highly correlated. The very essence of a pd3mer chain generally renders independent orientation of a main chain dipole component impos-rible and frequently, coupling between side chain and main chain modes are involved. For a rigid chain polymer in solution, dipole orientation requires rotatory diffusion of a macromolecule as a whole, and no component due to local modes is involved, but this situation is the exception. Flexible polymers permit polarization by local mode motions as well as rotatory diffusion as illustrated in Fig. 5. Equation (5) is also inapplicable for polymers because of dispersity in molecular weight, since if the relaxation involves molecular wel t dependent modes there will be a tead of relax-... 

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