Hydration of Sulfo and Methyl Groups in Dimethyl Sulfoxide Is Accompanied by the Formation of Red-Shifted Hydrogen Bonds and Improper Blue-Shifted Hydrogen Bonds:  An ab Initio Quantum Chemical Study

In this study, the hydration of dimethyl sulfoxide was investigated by means of molecular dynamics (MD) simulations and quantum chemical correlated ab initio calculations. MD simulations show the hydration sites when the systems are exposed to 1, 3, 6, 16, and 32 water molecules. Various DMSO···(H2O)n (n = 1−3) complexes where waters hydrate sulfo and methyl groups were then reoptimized at the ab initio level. The hydration of DMSO leads to an elongation of the SO bond and a contraction of methyl C−H bonds. Whereas the elongation of the SO bond is accompanied by a red shift of the respective stretch frequency, the contraction of the C−H bonds gives a blue shift to the C−H stretch frequencies. The former effect is easily explained by the transfer of electron density to the antibonding orbitals of the SO bond, yielding its weakening. Various mechanisms leading to the contraction of the methyl CH bonds were suggested. They were based on secondary geometry changes originating from the significant elongation of the SO bond and also on the changes of the electron density in DMSO upon complexation, resulting in a rehybridization of the CH bonds. The influence of the electrostatic field of hydrating waters was also considered. Predicted frequency shifts fully agree with the observed data. Also, the observed blue shift increase occurring as a consequence of progressive hydration was interpreted theoretically, and the mechanisms of this phenomenon are suggested.