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 (–CHD₂), 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 T₁. 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.