Multiple internal rotations

In order to understand the large amplitude motion, the allowed conformations brought about the multiple internal rotations were calculated for a single... 

D. AxeIson We published a paper (g. C. Levy, D. E. Axelson, R. Schwartz and J. Hochmann, J. Am. Chem. Soc., 100, ilO (19T8)) late last year on methacrylates in which we tried to interpret the side chains in terms of the effect of the backbone distribution. The side chain was governed by a combination of the distribution and the multiple internal rotations. The problem we had is that when you get to the end of the chain you still can t account for the frequency dependence. We have not proceeded to the stage of doing dilute solution studies because as you mentioned, there may be an Intermolecular effect with other chains. 

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. 

Howartht17b)has used the theory of Internal Librational Motion to successfully predict the field dependent relaxation behavior of the 1,2-decanediol (DD), PBMA, and PHMA systems (using our published experimental data). We have utilized together multiple internal rotations (MIR) and distributions of correlation times. These methods individually have been successful in predicting relaxation behavior at one field. However, only the distribution theory predicts the observed field dependence for the carbons at or near sites of motional restriction, yet still having apparent correlation times <10 <-)sec. Our interest in the study of concerted motions along these alkyl chains has led us to combine the two approaches in the treatment of 13C relaxation parameters. 

While use of multiple internal rotations does allow prediction of the Tp s at any one field better than a single correlation time theory, it still does not predict a Tp field dependence. 

Table XII. Calculated Carbon-13 T] s and NOEF s Using the Theory of Multiple Internal Rotations Both With and Without a Distribution of Correlation Times for Overall Motion.

Stone applied the theory of Longuet-Higgins to deduce the character tables for the multiple internal rotation in neopentane and in octahedral hexa-ammonium metallic complexes [6]. Dalton examined the use of the permutation-inversion groups for determining statistical weights and selection rules for radiative processes in non-rigid systems [7]. Many applications of the Molecular Symmetry Groups have been reviewed later by Bunker [8,9]. 

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. 

---