Hydration of the CH groups in dimethyl sulfoxide probed by NMR and IR

H-1 and C-13 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, delta (H2O), 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 degreesC, indicating the presence of anomalously polarized water molecules, so-called hydrophobic hydration. The frequencies of the CH stretching vibration bands for (DMSO)/D2O mixtures, v(CH), increase with increasing X-D2O, implying the progressive depolarization and contraction of the CH bonds, v(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 v(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 v(CH) can be related to the increase in the polarization of the water molecules probed by delta (H2O). The redshifts in v(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.