Spectroscopy and molecular dynamics simulations of aqueous and nonaqueous mixtures

Binary mixtures of water, methanol, acetone, and acetonitrile have been studied experimentally and computationally in the far-infrared (far-IR) and low frequency infrared (IR) regions of the spectrum. The experimental spectra, which are indicative of intermolecular motions, were compared to the results of molecular dynamics (MD) simulations for all six binary combinations. The far-IR spectra of aqueous mixtures show a pronounced decrease in the absorption coefficient relative to the ideal value, and similar deviations from ideality were observed in the index of refraction. Debye time constants calculated from the spectra indicate a slowing of the dielectric relaxation in the aqueous mixtures. In contrast, the far-IR optical constants of the nonaqueous mixtures exhibit essentially ideal behavior. The behavior of the hydrogen librational band, which occurs at about 650 cm−1, is similar in both aqueous and methanolic mixtures and undergoes a large red-shift upon dilution with acetone or acetonitrile, whereas a slight blue shift occurs when water is diluted with methanol. MD simulations convincingly reproduce the composition dependence of the spectra. The single particle and angular velocity spectra suggest that the red-shift in methanolic mixtures is a manifestation of weaker hydrogen bonds between methanol and co-solvent molecules. In aqueous mixtures, the single particle dynamics are not appreciably affected: the red-shift therefore appears to reflect the collective dynamics of the hydrogen bonding network. The hydrogen bonding environment of each molecule was categorized and the hydrogen bonding statistics for each mixture were calculated. Hydrogen bonding statistics, and the radial and spatial distribution functions (RDFs and SDFs), reveal a loss of tetrahedral water structure on mixing. The RDFs and SDFs also show self-association of water and methanol molecules in the mixtures. Chains of methanol molecules occur in methanolic mixtures and were studied as a function of composition.