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

In NMR of quadrupolar spins with S >= 3/2, 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 or transverse relaxation is multiexponential, which occurs if the motional correlation times are comparable to or larger than the inverse of the Larmor frequency (violation of the extreme narrowing approxn.). 2 Expts., which combine multiple-quantum filtration with conventional spin-echo and inversion-recovery sequences are described and were performed on BuLi. For isotropic motion each expt. allows one to det. the motional correlation time without knowledge of the magnitude of the quadrupolar coupling const. [on SciFinder (R)]