On the molecular basis of HO/DMSO eutectic mixtures by using phenol compounds as molecular sensors: a combined NMR and DFT study.

NMR and DFT studies of phenol compounds as molecular sensors were carried out to investigate HO/DMSO eutectic mixtures at a molecular level. The experimental H NMR chemical shifts of the OH groups, δ(OH), of phenol, paracoumaric acid, and vanillic acid show maximum deshielding and, thus, hydrogen bond interactions in the range of mole fractions 0.20 < χ(DMSO) < 0.33. In the mole fractions χ(DMSO) < 0.2, a progressive decrease in δ(OH) was observed which demonstrates a decrease in hydrogen bond interactions at infinite dilution in HO, despite the increase in the number of available hydrogen bond acceptor and donor sites. DFT calculated δ(OH) of minimum energy solvation clusters were shown to be in reasonable agreement with the pattern in experimental δ(OH) data. The chemical shift deshielding and, thus, increased hydrogen bond interactions in the natural product + DMSO + nHO (n = 2, 3) solvation clusters, relative to complexes in DMSO or HO solutions, cannot be attributed to a single structural parameter of the cooperative interactions between HO and DMSO molecules with the phenol OH groups of the natural products. The minimum energy conformers of phenol compounds + 2HO + DMSO complexes are in excellent agreement with a recent low temperature neutron diffraction experiment of 3DO + DMSO and demonstrate a general structural motif of solvation complexes. The combined use of H NMR and DFT studies with emphasis on δ(OH) of phenol compounds, as molecular sensors, can provide an effective method for the study of solute-solvent interactions at the atomic level.