Exchange-modified DOSY experiments. the use of chiral solvating agents and lanthanide shift reagents as matrices

Abstract

Diffusion-ordered spectroscopy (DOSY) is a 2D NMR method that allows the separation of signals from different species of the same mixture if they have different translational self-diffusion coefficients, D^t. Concerning the achievable resolutions there are known limitations both in the chemical shift and diffusion dimensions. In the former signal overlap makes the reliable determination of the D^t values challenging. In the latter the similar diffusion coefficients often prohibit the separation of different components. Attempts were made to gain a deeper insight into the so-called matrix-assisted DOSY (MAD) approaches and point out their benefits and limitations. The scope was restricted to CSA and LSR matrices (chiral solvating agents and lanthanide shift reagents) that promise resolution improvement in both dimensions. The focus was on the modulation effect of chemical exchange both on the diffusion and chemical shift dimensions of the DOSY maps and the utility of the diffusion coefficient–molecular mass correlations. As ligands long-known commercially available small (<350 Da) organic molecules were selected. The dependence of resolution on the agent/ligand molar and molecular size ratios, and solvent properties was also studied. The results clearly indicate the non-negligible effects of the exchange processes on the obtained diffusion coefficients and therefore on the predicted molecular masses. In the case of LSR matrices the DOSY experiments of mixtures benefit from the improved resolution in the chemical shift dimension; however, the improvement of the diffusion resolution is often negligible both for the CSA and the LSR matrices.