Study of slow molecular motions in solution using off-resonance irradiation in homonuclear NMR

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Abstract
The contribution due to fast-chemical exchange processes in off-resonance relaxation measurement is studied both theoretically and experimentally. The relaxation rates depend explicitly on the effective field amplitude in the rotating frame, and therefore on the rf field strength and offset. This gives access to a larger domain of slow internal motion (characteristic times between 0·1 µs and 10 μs) in comparison to on-resonance relaxation measurement, avoiding problems of HOHAHA coherence transfer or angular dispersion. Different methods to detect and measure fast-chemical exchange processes using off-resonance ROESY and local effective correlation time measurement are described. They are essentially based on a variation of the rf field amplitude at constant angle 0 between the effective field and the static magnetic field. As an illustration, the chemical shift difference between the two conformations of cyclohexane at room temperature is measured.

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