✍ Scribed by Donghui Wu; Aidi Chen; Charles S. Johnson; Jr.
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Diffusion-ordered 2D NMR spectroscopy (DOSY) was tively shielded gradient coils (6, 7) and specially designed developed to aid in the analysis of complex mixtures (1). pulse sequences (8,9). Of course, the use of higher-field The idea was to provide, at each chemical shift, a spectrum spectrometers also helps by expanding the chemical-shift of diffusion coefficients of the various components obtained range (10), and more remarkable improvements can be obthrough approximate inverse Laplace transforms (ILT) of tained through heteronuclear ( 13 C) detected DOSY with copulsed-field-gradient NMR (PFG NMR) data sets. This herence transfer (11). scheme has been quite successful for components having
In this Communication, we describe 3D DOSY experiwidely separated diffusion coefficients (2) and for polydisments in which PFG NMR diffusion measurements are comperse samples with broad distributions of diffusion coeffibined with conventional 2D NMR to give the third dimencients (3). However, there are fundamental limitations on sion ( 11). This provides a major reduction in the probability the ability of numerical ILTs to resolve exponential compoof overlap since the cross peaks are spread over the full 2D nents in the presence of noise. Even with low noise and plane instead of along a single axis. We emphasize that the comparable intensities, one typically cannot resolve two acronym DOSY implies that the third dimension consists of components in a diffusion spectrum having diffusion coeffidiffusion spectra of peaks and not simple signal attenuation. cients that differ by less than a factor of two. Therefore, the Thus, 3D DOSY permits a plane with the characteristic 2D resolution of molecules of comparable sizes is quite impossi-NMR spectrum to be assigned to the layer of peaks for each ble at chemical shifts where resonance lines of the compodiffusing species. The creation of a 3D DOSY experiment nents overlap, and the only alternative is to avoid overlap requires (a) the design of a combined pulse sequence, and in the chemical-shift dimension.
(b) the development of algorithms for grouping data points In the 2D DOSY experiment on a mixture of discrete into clusters representing cross peaks, inverting the cluster components, one first obtains a set of FT PFG NMR spectra intensity versus q 2 data sets, and constructing the 3D specwith different values of the magnetic field gradient g. It is trum. Any current 2D NMR experiment can be used in this then assumed that this data set can be represented by a combination. Here, we illustrate the combination of a 2D weighted sum of Stejskal-Tanner factors (4) proton-proton correlation spectrum (COSY) with DOSY analysis to produce 3D COSY-DOSY. The combination of DOSY with inverse-detected heteronuclear correlation ex-
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