EVIDENCE FOR DIRECT INTERMOLECULAR INTERACTIONS AS AN ORIGIN OF THE HYDRATION SHIFTS OF THE C-H STRETCHING VIBRATIONS - 1,4-DIOXANE WATER-SYSTEM

Frequency shifts of the C-H stretching vibrations (upsilon-CH) of 1,4-dioxane (DX) and dimethylsulfoxide in aqueous solutions were observed as a function of concentration using a dual-beam difference Raman apparatus. A nonlinear positive increase of the shift was observed upon dilution for the upsilon-CH modes of DX, similar to the case of alcohol/water mixtures, while negative and zero shifts were observed for its C-O and C-C stretching modes, respectively. The shift of the upsilon-CH mode is not the secondary effect arising from a change in hydrogen bonding at oxygen. The DELTA-upsilon-AA versus chi-A curve of the upsilon-CH mode exhibited a deep minimum around chi-A = 0.08, indicating the formation of molecular assemblies of DX with the C-H groups inside in the concentration region of 0.08 < chi-A < 0.5 and its collapse into molecular dispersion at chi-A < 0.08. The transfer shift from H2O to D2O was significantly smaller for DX than for acetone and acetonitrile.