An increase in the resolving power in 2D NMR spectra is obtained by collapsing 2D signals with multiplet structure into 2D singlets. This resolution gain is achieved by combining 2D experiments with pure shift techniques and covariance processing (see picture). The method should be of value in both manual and automated structure determination.

High resolution diffusion-ordered NMR spectroscopy allows the separation of signals from different species based on their diffusion coefficients. In general this requires that the NMR spectra of the components do not have overlapping signals, and that the diffusion coefficients are significantly different. Modifying the solvent matrix in which a sample is dissolved can change the diffusion coefficients observed, allowing resolution ("matrix-assisted DOSY"). We show here that dissolving the two naturally-occurring epimers of naringin in an aqueous solution of β-cyclodextrin causes both shift and diffusion changes, allowing the signals of the epimers to be distinguished. Chiral matrix-assisted DOSY has the potential to allow simple resolution and assignment of the spectra of epimers and enantiomers, without the need for derivatisation or for titration with a shift reagent.

The structural similarity of flavonoids, often present in natural product mixtures, makes their analysis by NMR methods less than straightforward. This similarity is a dual problem for one of the most powerful NMR methods for mixture analysis, diffusion-ordered spectroscopy (DOSY), which relies both on well-resolved peaks and on differences in hydrodynamic radii for separating the signals from different components in a mixture. To overcome these limitations, we use a matrix-assisted DOSY approach that exploits differential chemical interactions with a slow diffusion matrix (here micellar sodium dodecyl sulfate) to resolve flavonoid mixtures in mixed solvents.

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