DIPOLAR SPECTRAL DENSITIES FROM OFF-RESONANCE H-1-NMR RELAXATION MEASUREMENTS

The use of an off-resonance rf magnetic field in cross-relaxation meas urements opens new ways for the study of dipolar proton-proton spectra l densities, basic indicators of local motions. The effective relaxati on rates in these experiments can be expressed as weighted averages of longitudinal and transverse terms. By varying the value of the spin-l ock frequency in ROESY-type pulse sequences one determines these terms accurately and thus has access to the spectral density function at th ree frequencies, without any assumption on the type of motion. Experim ental illustrations are given.