Models for Molecular Rotation in Liquids

As noted above, the main object of studying halogen relaxation in liquids has been to obtain data on molecular reorientational correlation times which can be used to test the applicability of theoretical 

Numerous models for motion in liquids have been proposed and while it is beyond the scope of this article to give a detailed accoun of these it is pertinent for the discussion of the experimental data in the next section to briefly describe the more important models which have been considered in connection with halogen relaxation studies. 

Schematically, theories of rotational motion in liquids may be divided into two groups, which may be called classical reorientation and jump reorientation models. For the case that the rotation of a molecule in a liquid is regarded as a solid body moving in a fluid continuum the Debye-Stokes-Einstein relation [66] should apply. Thus for the reorientation of a spherical molecule 

Whereas the Debye-Stokes-Einstein equation may be applicable for macromolecules in a low molecular weight solvent it is apparently not a realistic model for molecular motion in a neat liquid. A modificatior of the Debye-Stokes-Einstein relation was proposed by Gierer and Wirtz [67] who tried to take into account the discontinuous nature of the liquid. For a spherical molecule they obtained 

Jump reorientation models may involve activation over barriers to rotation or the migration of lattice defects or holes. Reorientation is in both cases discontinuous and changes in orientation occur-ing in one step are assumed to be large. Both types of jump reorientation models have been discussed by O Reilly [68], In his quasilattice random flight model, for example, O Reilly [69 70] assumes that the liquid structure up to the first coordination shell may be approximated by a lattice. Some of the properties of the solid state such as vacancies and translational diffusion by vacancy migration are considered present. In general difficulties arise when these jump reorientation models are compared with experimental data because several parameters are needed in the analysis. Furthermore, it appears that O Reilly [71] employs results obtained by Huntress [55] which apply only in the limit of small-step reorientation to treat the case of 

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