Hydration of the CH Groups in Dimethyl Sulfoxide Probed by NMR and IR

Abstract

1H and 13C NMR of dimethyl sulfoxide (DMSO)/H2O mixtures were measured, together with the IR of DMSO/D2O mixtures, to study the effect of the polar SO group on hydration of the CH groups. Chemical shifts were determined by the external double reference method, which provides the in situ volume magnetic susceptibility indispensable to the correction of the chemical shifts. The chemical shift of the water protons as the measure of the polarization of the water in the mixtures, δ H 2O, increases from 3.6 ppm at the water mole fraction X H2O = 0.05 to 4.8 ppm, the value for pure water, at X H2O = 0.80. It exceeds 4.8 ppm in the region of X H2O > 0.80 at 23.3 °C, indicating the presence of anomalously polarized water molecules, so-called hydrophobic hydration. The frequencies of the CH stretching vibration bands for (DMSO)/D2O mixtures, ν(CH), increase with increasing X D2O, implying the progressive depolarization and contraction of the CH bonds. ν(CH) values take maxima at X D2O = 0.96. The chemical shift of the CH proton increases very slightly with increasing X H2O, whereas that of the CH carbon decreases, suggesting the polarization of the CH bonds contrary to the depolarization in them as shown by the blueshifts of the ν(CH) values. The pushball hydration model previously presented is applied to interpret the results; the electron of CH hydrogen is pushed toward the carbon atom due to dispersion interaction with the electrons of water oxygen. The pushing effect probed by the blueshifts of ν(CH) can be related to the increase in the polarization of the water molecules probed by δ H 2O. The redshifts in ν(CH) in the water rich extreme may be ascribed to a partial polarization of the CH bond resulting from hydrogen bonding interaction with highly polarized water molecules, in addition to the dispersion interaction. The role of the SO group in the hydration of the CH groups is discussed in comparison with the roles of the hydrophilic groups of acetone and tert-butyl alcohol.

Systematic study of the thermal diffusion in associated mixtures

“…They observed that the water clusters disappear between x s = 0.19 and 0.25, which is the concentration range, in which the Soret coefficient changes it's sign. The structural change is also supported by the NMR-study by Mizuno et al 34 which has been discussed in Sec. IV A.…”

“…There are numerous nuclear magnetic resonance (NMR) 34 , mass spectrometry 38 , dielectric spectroscopy 39 and x-ray scattering 40 studies, which investigate the change from the tetrahedral structure of water to other structures such as chain-like conformations with increasing solvent concentration. Takamuku et al 40 investigated alcohol/water solutions by X-ray scattering and found that the structure changes with increasing alcohol concentration in the order of n-propyl ≈ iso-propyl > ethyl > methyl at alcohol mole fractions of 0.1, 0.1, 0.2, and 0.3, respectively.…”

Experimental investigation of the Soret effect in acetone/water and dimethylsulfoxide/water mixtures

“…Table I) correlates with the concentration x ± solvent , where the Soret coefficient changes sign. [38][39][40] This observation has also been made for the Soret coefficient of poly(ethylenoxide) in the solvent mixture ethanol/water, which changes sign at x ethanol = 0.08. 29 This indicates that thermal diffusion is quite sensitive to changes in the fluid structure.…”

Structures of Ionic Liquids with Different Anions Studied by Infrared Vibration Spectroscopy

“…External double-reference method was used with a 530 µL external reference tube filled with tetramethylsilane (TMS). 26 Figure 2 shows the ATR-IR spectra of [ 4 ] have been studied, and the peaks were assigned in previous report. 9 For [BMIM][I], although the IR spectrum has not been reported as yet, the Raman spectroscopic study of [BMIM][Cl] could be consulted for peak assignment.…”