High-field spin-lattice relaxation of methyl groups: relation to neutron scattering

Abstract

The modern theory of high-field spin-lattice relaxation is applied to methyl group rotation. It is shown that the same time correlation functions are observed in nuclear magnetic resonance (NMR) relaxation experiments and in inelastic neutron scattering (INS) experiments. A linear relationship is derived between the spectral function S( omega ) observed in INS and the spin-lattice relaxation rate T1-1( omega ) as a function of the Larmor frequency. The similarities between the two methods are pointed out. For CD3, NMR experiments on systems with high tunneling frequencies yield the width of the quasi-elastic Ea to or from Eb line of INS. For CH3, the effect of intermolecular dipolar interactions on the spin-lattice relaxation time T1 is calculated using a series expansion in r/Ri, where r is the radius of the methyl group and Ri is the distance of the considered proton i from the centre of the CH3 group. It is shown that for systems with high tunneling frequencies T1 is mainly determined by intermolecular dipolar interactions, whereas these have a negligible effect on spin conversion. T1 experiments on diluted CH3-containing systems yield the width of the inelastic A to or from E line, also in the so-called 'quasi-classical' temperature regime.