Multiple-quantum NMR spectroscopy of S=3/2 spins in isotropic phase: A new probe for multiexponential relaxation

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Abstract
In nuclear magnetic resonance of quadrupolar spins with S≥3/2, it is shown that excitation and observation of multiple‐quantum coherence is possible in the absence of scalar, dipolar, or quadrupolar splittings, in contrast to the widely accepted view that nonvanishing couplings are a prerequisite for the creation of multiple‐quantum coherence. In the absence of splittings, multiple‐quantum coherence can be excited because the longitudinal (‘‘T1’’) or transverse (‘‘T2’’) relaxation is multiexponential, which occurs if the motional correlation times τc are comparable to or larger than the inverse of the Larmor frequency (violation of the extreme narrowing approximation). Two experiments are described, which combine multiple‐quantum filtration with conventional spin‐echo and inversion‐recovery sequences. For isotropic motion each experiment allows one to determine the motional correlation time without knowledge of the magnitude of the quadrupolar coupling constant.

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