STUDY OF SLOW MOLECULAR MOTIONS IN SOLUTION USING OFF-RESONANCE IRRADIATION IN HOMONUCLEAR NMR .2. FAST CHEMICAL-EXCHANGE PROCESSES

The contribution due to fast-chemical exchange processes in off-resona nce relaxation measurement is studied both theoretically and experimen tally. The relaxation rates depend explicitly on the effective field a mplitude in the rotating frame, and therefore on the rf field strength and offset. This gives access to a larger domain of slow internal mot ion (characteristic times between 0.1 mu s and 10 mu s) in comparison to on-resonance relaxation measurement, avoiding problems of HOHAHA co herence transfer or angular dispersion. Different methods to detect an d measure fast-chemical exchange processes using off-resonance ROESY a nd local effective correlation time measurement are described. They ar e essentially based on a variation of the rf field amplitude at consta nt angle 0 between the effective field and the static magnetic field. As an illustration, the chemical shift difference between the two conf ormations of cyclohexane at room temperature is measured.