FAST INVERSION-RECOVERY MEASUREMENTS IN THE PRESENCE OF A SATURATING FIELD FOR A 2-SPIN SYSTEM IN CHEMICAL-EXCHANGE

This work shows that a fast inversion-recovery sequence reliably measures longitudinal relaxation rates with slow chemical exchange in the presence of a saturating RF field. First, the coefficients of the complete set of Bloch equations, obtained after a base transformation in the rotating frame, have been calculated for a two-site chemical-exchange spin system that is continuously irradiated. Then, with approximations, these results were used to analyze the usual case having one species on resonance with a large resonance offset for the other one. This led to the expression of the components parallel to the respective effective magnetic fields of the two exchanging magnetizations as a function of time. The analysis of the time courses of the magnetizations during the inversion-recovery sequence has been completed. The conclusion is drawn that the relaxation delay can be reduced to its minimum value, annulling the transverse magnetizations without affecting the determination of the effective longitudinal relaxation time. The theoretical analysis has been confirmed by measurements relative to the exchanging protons of the N,N-dimethylacetamide molecule undergoing hindered rotation around the amide bond at 40 degrees C. (C) 1994 Academic Press, Inc.