Factors influencing proton relaxation in the dideuteriomethyl group

Abstract

Reduction of geminal proton relaxation within methyl groups is made possible by use of the dideuteriomethyl group (-CH DJ, so that residual dipolar relaxation can reflect the steric environment of the methyl group. To search for any relationship between spin-lattice relaxation and steric environment, we have prepared a series of so-labelled molecules that are rigid except for methyl rotation. The importance of dipolar relaxation was demonstrated by nuclear Overhauser enhancements and by the temperature dependence of Tl. The extent of motional anisotropy was assessed by measuring the 13C relaxation times as a function of position. Proton relaxation times were adjusted for molecular size by multiplying by the molecular weight. Comparison of the size-adjusted proton relaxation times showed qualitative trends with steric environment. The major factor vitiating a more quantitative relationship is probably the contribution from spin-rotation relaxation, with a minor contribution from anisotropic motion.