EXTERNAL RANDOM-FIELD MODEL FOR INTERMOLECULAR SPIN INTERACTIONS - ACCURACY EVALUATION FOR THE COUPLED RELAXATION OF 2-SPIN MOLECULES IN A PULSE RESPONSIVE SOLVENT

The errors that may arise from the use of the external random field (E RF) approximation in the spin relaxation theory are estimated for the case of a binary solution. The experiments by M. Ishiwata and Y. Ishii (J. Phys. Sec. Jpn. 60 (1991) 1379, ibid. 60 (1991) 1743) have been a nalyzed. Coupled spin-lattice relaxation of solute molecules has been measured in the experiments where the solvent spin system was exposed to the effect of nonselective pulses. Going beyond the ERF model we as sume that the behavior of a solvent spin system can be characterized b y a single polarization. This approach involves the consideration of c oupled solvent-solute relaxation (CSSR). It has been found that the pa rameters of molecular motion, extracted with the CSSR approach, differ noticeably from those obtained within the scope of the ERF model. We conclude that if the solvent spin system is perturbed during the exper iment and its spin-lattice relaxation time is not too short, the CSSR approach should be recommended for quantitative study of the motion of solute molecules.