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Rotational diffusion in liquid acetonitrile, dimethylsulphoxide (DMSO), water, and methanol is studied with molecular dynamics simulations. The effects of hydrogen bonding and local dipole-dipole correlations (Kirkwood g-factor) on the relationship between the single molecule and collective relaxation are examined. The first rank single molecule dipole moment autocorrelation functions (ACFs) are constructed in the molecule-fixed coordinate frame and the principal components of rotation diffusion tensor are reported. Higher rank orientational ACFs are computed. These ACFs, as a rule, are strongly nonexponential (at least not single exponential) at longer times and the decomposition of these functions into a series of single exponentials results in broad distributions of relaxation times, with the broadening being particularly prominent in the case of higher rank ACFs. The rank dependence of characteristic times calculated as weighted averages over the relaxation time distributions does not follow the pattern of small angle (Debye) diffusion model for all liquids studied in this work except methanol. In contradiction, the same rank dependence computed by direct integration of ACFs leads to good agreement with the Debye diffusion model in the case of acetonitrile, DMSO, and water (but not methanol). The linear-angular momentum cross correlation functions are also computed and the effect of rototranslational coupling on reorientaional relaxation at longer times (>1.0 ps) is found to be small.

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Microwave spectroscopy has been used to study the dielectric properties of aqueous electrolyte solutions over a wide range of concentrations, the frequency range (7 to 25 or 7 to 120) GHz, and the temperature range (283 to 313 or 288 to 308) K. The static dielectric permittivity, dielectric relaxation time, and activation enthalpy have been calculated for 46 binary systems. General relations between dielectric relaxation parameters and structural effects in aqueous salt solutions are considered. The increase in the order of complementary structural organization for water in the bulk and hydration shells of ions gives rise to the molecular-kinetic stabilization of water and to the structure-making effect. This explains the difference between dielectric relaxation characteristics for a wide variety of electrolyte solutions. We present systematic dielectric data for aqueous electrolytes with hydrophilic and hydrophobic hydration of cations and anions. The temperature effect is also examined. On the basis of this study, new criteria for the difference between hydrophilic and hydrophobic hydration of ions are suggested. † Part of the "Josef M. G. Barthel Festschrift".

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