Effect of internal librational motions of the 13C nuclear magnetic resonance relaxation times of polymers and peptides

Abstract

A general theory of internal librations and rotations in molecules is developed and successfully applied to the published field, temperature and concentration dependence of the ¹³C T₁ values and nuclear Overhauser enhancements in 1-decanol, 1,2-decan-2-ol, poly-n-butyl methacrylate and poly-n-hexyl methacrylate, and also to new data on random coil poly-γ-benzyl-L-glutamate. Internal rotation is generally found to be slower than libration at a given temperature, even for methyl groups, and is not of importance in determining relaxation behaviour in linear alkyl chains. In concentrated solutions the librational rate but not the amplitude increases towards the free end of a linear alkyl chain. However, when the chain ends in a phenyl ring, the librational rate is almost independent of chain position. The temperature-dependence of most of the motions observed can be described by activation energies of 20–30 kJ mol^–1.