Contribution of relaxation and chemical shift results to the elucidation of the structure of the water-DMSO liquid system

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Abstract
Proton spin‐lattice relaxation times (T1) of water and dimethyl sulfoxide (DMSO) in H2O–DMSO (I), H2O–DMSO‐d6 (II), and D2O–DMSO (IV) systems at 9.8, 18.7, 29.9, and 41.4°C exhibited minima near 0.65 mole fraction of water (XH2O). The depth of the minimum, relative to T1 values at higher and lower XH2O, increased as the temperature was lowered. The intramolecular contribution to the relaxation of DMSO protons was always greater than the intermolecular DMSO–DMSO contribution and the former showed a minimum at XD2O≈0.6 as compared with the latter's minimum at XD2O≈0.4–0.6. There is almost no contribution to water proton T1 from the interaction of water protons‐DMSO protons in the DMSO‐rich mixtures, while this contribution is rather significant in the DMSO‐poor mixtures. The proton chemical shifts of water with reference to DMSO protons (over a temperature range of −60−45°C) moves to higher fields with rising temperatures, and this temperature dependence is affected considerably by the changes in composition. All these experimental results suggest the presence of strong intermolecular interactions between water and DMSO molecules that contribute to the ``structuring'' of this binary mixture. The structure of this liquid system is very sensitive to composition and temperature.