Rotating frame spin-lattice relaxation in the presence of an off-resonance radio frequency field. Investigation of intermediate molecular motions

Abstract

An NMR relaxation technique is described that should prove valuable in studies of intermediate molecular motions, i.e., these motions having correlation times greater than the inverse of the Larmor frequency. The technique is applicable to multiline spectra, the experiment being carried out on a commmercial Fourier transform NMR spectrometer. The experiment consists of measuring resonance peak intensities in the absence and presence of a radio frequency field applied far off-resonance as a function of the off-resonance frequency. Although it is not necessary to determine relaxation times in order to examine molecular motions, a value for the nuclear spin-lattice relaxation time in the rotating frame with off-resonance rf irradiation, T 1 Q off , may be determined if a value for T 1, the nuclear Zeeman spin-lattice relaxation time, is known. This technique is thus designed to supplement or supercede spin-spin relaxation time ( T 2) and normal on-resonance rotating frame spin-lattice relaxation time ( T 1 Q ) measurements. The utility of the off-resonance T 1 Q technique for investigating molecular motions is illustrated with a viscous liquid, glycerol, at low temperatures.