Solvation Structure and Dynamics of the Thiocyanate Anion in mixed N,N-Dimethylformamide-Water Solvents: A Molecular Dynamics Approach.

The solvation structure and dynamics of the thiocyanate anion at infinite dilution in mixed N, N-Dimethylformamide (DMF)-water liquid solvents was studied using classical molecular dynamics simulation techniques. The results obtained have indicated a preferential solvation of the thiocyanate anions by the water molecules, due to strong hydrogen bonding interactions between the anion and water molecules. A first hydration shell at short intermolecular distances is formed around the SCN anion consisting mainly by water molecules, followed by a second shell consisting by both DMF and water molecules. The strong interactions between the thiocyanate anion and water molecules are further reflected upon the calculated intermittent residence lifetimes of water and DMF in the first and second solvation shells. The dependence of the reorientational relaxation times of the thiocyanate anion upon the mole fraction of DMF in the mixtures has been found to be in good agreement with experiment, revealing strong concentration effects upon these relaxation phenomena. An appreciable solvent composition effect upon the low frequency intermolecular vibrations, due to the anion-water interactions, has also been revealed by calculating the atomic velocity correlation functions and corresponding spectral densities of the anion.