Abstract

In diffusion-ordered spectroscopy (DOSY) the aim is to separate signals from different molecular species according to their different diffusion coefficients. Each species has its individual diffusion coefficient (that may accidentally coincide with that of another species, e.g. if they are of very similar size). In exchanging systems, however, there is a serious complication in that the apparent diffusion coefficient of an exchanging signal will be a compromise that depends, among other factors, on the diffusion coefficients of the exchange partners and the rate of exchange between them. The DOSY spectrum will be much harder to interpret and can often give the appearance of extra (spurious) components in the mixture. Here a new and surprisingly simple experiment is described that suppresses the effects of exchange on apparent diffusion coefficients, restoring the simplicity of interpretation enjoyed by non-exchanging systems.

Highlights

  • Diffusion-ordered spectroscopy (DOSY) is a family of NMR experiments used in mixture analysis to allow signals belonging to a given species to be correlated through their rate of diffusion

  • If diffusion weighting is added, for example by including field gradient pulses in each echo as in the PROJECTED (PROJECT Extended to diffusion-ordered spectroscopy (DOSY)) sequences of Fig. 2, spin echo DOSY spectra may be obtained free of both exchange effects and J modulation if sk, sJ

  • At first sight there appear to be two impurities present. These signals are the flavone hydroxyl resonances, but their diffusion coefficients are increased by exchange with the small amount of water present in the sample

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Summary

Introduction

Diffusion-ordered spectroscopy (DOSY) is a family of NMR experiments used in mixture analysis to allow signals belonging to a given species to be correlated through their rate of diffusion. A simple pulsed field gradient spin echo experiment would be expected to yield correct diffusion coefficients for species with different frequencies, even in the presence of exchange; the effects of the latter will only survive for chemical shift differences between exchange partners of the order of the inverse of the echo time or less. If diffusion weighting is added, for example by including field gradient pulses in each echo as in the PROJECTED (PROJECT Extended to DOSY) sequences of Fig. 2, spin echo DOSY spectra may be obtained free of both exchange effects and J modulation if sk, sJ

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