Interpretation of Pulsed-Field-Gradient NMR in Terms of Molecular Displacements
Abstract
The relationship for Pulsed-Field-Gradient NMR between the amplitude I(t) of the spin echo and the molecular displacement X(t) is examined. I(t) of a single species in a simple solution is determined by the mean-square displacement X(t)2. With polydisperse species, or molecular probes in complex fluids showing memory effects, I(t) in general includes large contributions from all higher even moments X(t)2n. Conditions under which the NMR signal is indeed determined by the molecular mean-square displacement are noted. A diagnostic that sometimes identifies when these conditions are not met is presented.
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