Comparison of contribution of OH⋯OS hydrogen bond and CH⋯Ow interaction to the methyl blueshift in hydration of dimethyl sulfoxide

The methyl stretching frequencies of dimethyl sulfoxide in aqueous solutions have been studied with attenuated total reflection infrared spectroscopy and quantum chemical calculations. The blueshifts of CH3 stretching frequencies are more prominent in infrared spectra when the mole fraction of water is greater than 0.5. The contributions of OH⋯OS hydrogen bond and CH⋯Ow interaction to the blueshift were compared by means of quantum chemical calculations at the MP2/6-31++G(d,p) level. When the mole fraction of water is less than 0.5, the blueshift is attributed to a combined result of the OH⋯OS hydrogen bond and CH⋯Ow interaction, whereas the CH⋯Ow interaction plays an increasingly important role in the νCH blueshift when the mole fraction of water is greater than 0.5. The calculated frequency shifts are in good agreement with experimental results. The cooperativity between two types of hydrogen bond was also considered in the blueshift of the methyl stretching frequency.