Dipolar spectral densities from off-resonance 1H NMR relaxation measurements

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