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 measu res longitudinal relaxation rates with slow chemical exchange in the p resence of a saturating RF field. First, the coefficients of the compl ete set of Bloch equations, obtained after a base transformation in th e rotating frame, have been calculated for a two-site chemical-exchang e spin system that is continuously irradiated. Then, with approximatio ns, these results were used to analyze the usual case having one speci es on resonance with a large resonance offset for the other one. This led to the expression of the components parallel to the respective eff ective magnetic fields of the two exchanging magnetizations as a funct ion of time. The analysis of the time courses of the magnetizations du ring the inversion-recovery sequence has been completed. The conclusio n is drawn that the relaxation delay can be reduced to its minimum val ue, annulling the transverse magnetizations without affecting the dete rmination of the effective longitudinal relaxation time. The theoretic al analysis has been confirmed by measurements relative to the exchang ing protons of the N,N-dimethylacetamide molecule undergoing hindered rotation around the amide bond at 40 degrees C. (C) 1994 Academic Pres s, Inc.