Nuclear Relaxation Processes in Systems of Two Spins

Abstract

Nuclear relaxation in a system of molecules with two nonidentical spins (S and I) may originate from intra- or intermolecular interactions between like and unlike spins. An experiment is described which, in combination with the nuclear Overhauser effect, allows the determination of the contributions from the various interactions. The spins (S=½, I=½) are supposed to be coupled by a small scalar interaction and four longitudinal magnetizations S+, S—, I+, I—, can be measured. The decay of the total magnetizations (S++S—) or (I++I—) was discussed by Solomon and Bloembergen. The decay of the difference of the magnetizations (S+—S—) or (I+—I—) is a useful parameter as is shown in this paper. The relaxation time of (S+—S—) and (I+—I—) is related to the question whether the interactions are intra- or intermolecular. We have derived the appropriate equations for the time dependence of the difference magnetizations; the relaxation rate of the difference magnetization is larger for intermolecular dipolar interactions than in the intramolecular case when compared with the relaxation of the sum magnetizations. Experiments on CHFCl2 at −142°, −115°, and at +20°C are reported. At temperatures just above the melting point both hydrogen and fluorine spins relax mainly by interactions between unlike spins; the contributions of intramolecular interactions are predominant. At 20°C it appears that the fluorine spin is relaxed by spin rotation and hydrogen by intermolecular dipole interactions.