Highly Viscous Binary Solvents: DMSO-d6/Glycerol and DMSO-d6/Glycerol-d8 for Polar and Apolar Mixture Analysis by NMR
Résumé
The use of two new highly viscous binary solvents, DMSO-d6/glycerol (GL) and DMSO-d6/glycerol-d8 (GL-d8), is reported for the first time in order to give access to the individual NMR spectra of mixture components. Their high dissolution power offers a wide range of potential applications to mixture analysis, regardless of polarity. Under particular conditions of viscosity, the tumbling rate of small and medium-sized molecules slows down in solution, so that the longitudinal cross-relaxation regime favors the observation of spin diffusion. As a consequence, all the resonances of the 1H nuclei within the same molecule tend to correlate together in a 2D nuclear Overhauser effect spectroscopy (NOESY) spectrum, thus opening the way to mixture analysis. This work reports the analysis of a polar mixture composed of Leu-Val, Leu-Tyr, Gly-Tyr, and Ala-Tyr dissolved in DMSO-d6/GL (8:2, v/v) and of an apolar mixture made of β-ionone, (±)-citronellal, (+)-limonene, and flavone dissolved in DMSO-d6/GL-d8 (5:5, v/v) by means of spin diffusion in homonuclear selective 1D NOESY, selective 2D NOESY, NOESY-correlation spectroscopy (COSY), NOESY-total correlation spectroscopy (TOCSY) experiments, and 2D heteronuclear single-quantum correlation spectroscopy (HSQC)-NOESY. DMSO-d6/GL must be preferred to DMSO-d6/GL-d8 for the study of biological active compounds in which labile protons must not be exchanged by deuterium nuclei. DMSO-d6/GL-d8 is more appropriate for organic compounds in which labile protons are not essential to the structure elucidation. DMSO-d6/GL and DMSO-d6/GL-d8 binary mixture solvents seem to be so far the most efficient viscous solvents described in the literature for the resolution of both polar and apolar complex mixtures components by NMR spin diffusion.