It was recalled that nuclear magnetic resonance diffusion experiments, using pulsed field gradients, were well established as sensitive probes of the displacement of individual nuclear spins in a sample. Such measurements were generally used as a measure of translational diffusion, but it was demonstrated here that, under certain conditions, rotational motion could contribute very significantly to the experimental data. This situation occurred when at least one spatial dimension of the species under study exceeded the root-mean-square displacement which was associated with translational diffusion, and led to anomalously large apparent diffusion coefficients when conventional analytical procedures were used. It was shown that, in such situations, the effective diffusion coefficient was a function of the duration of the diffusion delay used. This dependence provided a means of characterizing the dimensions of the species under investigation.

Contribution of Rotational Diffusion to Pulsed Field Gradient Diffusion Measurements. A.J.Baldwin, J.Christodoulou, P.D.Barker, C.M.Dobson, G.Lippens: Journal of Chemical Physics, 2007, 127[11], 114505