Transverse Relaxation in Systems of Two Nonidentical Spins

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Abstract
Nuclear relaxation in molecules with two nonidentical spins may originate from intra- and intermolecular interactions between like and unlike spins. The various contributions to the relaxation may be determined from the longitudinal relaxation combined with the Overhauser effect. Transverse relaxation rates are likewise related to the contributions of the possible interactions. We have derived the appropriate equations for transverse relaxation in systems consisting of scalar-coupled nuclear spins. The expressions have been applied to CHFCl2, both neat and diluted in COS. The results on the neat liquid are in agreement with those obtained previously from longitudinal relaxation: at −142°C hydrogen and fluorine relax mainly by interactions between unlike spins. Dilution in COS barely affects the ratio of intra- to intermolecular interactions under conditions where the relaxation is of dipolar nature. This surprising finding must be related to nonviscous rotational behaviour of molecules in liquids of low viscosity like COS. The moment of inertia of the molecule rather than the viscosity of the liquid determines rotational relaxation.