Multiple internal rotations model

A. Jones I don t feel the multiple internal rotations model is applicable for side chains any longer than or k carbons. As soon as it is longer than carbons, a crankshaft type of motion dominates Instead of multiple internal rotations. The viewpoint probably reflects some bias on my part. 

Yasukawa et aU and Wittebort and Szabo have considered theoretically the effect of multiple internal rotations in alkyl side-chains, the forma employing a Monte Carlo approach, and the latter a lattice jump model. 

The advances in time resolved techniques have fostered a reexamination of theories of the rotational motions of molecules in liquids. Models considered include the anisotropic motion of unsymmetrical fluorophores the internal motions of probes relative to the overall movement with respect to their surroundings, the restricted motion of molecules within membranes (e.g., wobbling within a cone), and the segmental motion of synthetic macromolecules [8]. Analyses of these models point to experimental situations in which the anisotropy can show both multi-exponential and none-exponential decay. Current experimental techniques are capable in principle of distinguishing between these different models. It should be emphasized, however, that to extract a single average rotational correlation time demands the same precision of data and analysis as fluorescence decay experiments which exhibit dual exponential decays. Multiple or non-exponential anisotropy experiments are thus near the limits of present capabilities, and generally demand favourable combinations of fluorescence and rotational diffusion times [48]. 

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