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Potentials of Mean Force (PMF) for the Na+Cl− ion pair in water–dimethyl sulfoxide (DMSO) mixtures for three DMSO mole fractions have been computed using constrained Molecular Dynamics (MD) simulations and confirmed by dynamical trajectories and residence times of the ion pair at various inter-ionic separations. The three ion-ion direct potentials used are 12-6-1, exp-6-1 and exp-8-6-1. The physical picture that emerges is that there is a strong contact ion pair (CIP) and strong to moderate solvent separated ion pair (SSIP) in these solutions. Analysis of local ion clusters shows that ions are dominantly solvated by water molecules. The 12-6-1 potential model predicts running coordination numbers closest to experimental data.

Structure and dynamics of potassium chloride ion pair in mixtures of dimethyl sulfoxide (DMSO) and water have been investigated using constrained molecular dynamics. We have obtained the potentials of mean force (PMFs) between the K+ and Cl- ions at 298 K over a wide concentration range of these mixtures. The results obtained are compared for relative stabilities of contact ion pair (CIP), solvent assisted ion pair (SAIP) and solvent separated ion pair (SSIP) in the mixtures with varying compositions. The stability of the CIPs decreases significantly as the mole fraction of water increases. The study of running coordination numbers at the first solvation shell suggests preferential solvation of ions by water in all the compositions and there is hardly any participation by DMSO molecules in the first solvation shell of the Cl- ion. The extent of association of potassium chloride ion pair in water. DMSO and their mixtures is determined by calculating the association constants. We have calculated the diffusion constants of ions and solvent molecules in the first solvation shell around K+ and Cl- in all the compositions. The diffusion constants of the shell molecules are the smallest in the DMSO mole fraction ranging from 0.2 to 0.5. (C) 2013 Elsevier B.V. All rights reserved.

In a mixed solvent of water and dimethyl sulphoxide (DMSO) (with a mole fraction of DMSO = 0.21), the solute sodium chloride forms two distinct solvent separated ion pair (SSIP) species at interionic separations of 5.4 Angstrom and 7.1 Angstrom respectively. These SSIPs are separated by a free energy barrier at 6.6 Angstrom. Molecular Dynamics (MD) simulations are performed on the ion pair in the presence of the mixed solvent to study the solvation structure and the solvation dynamics through the friction kernels. The structural studies bring out the gradual evolution of the solvation of the individual ions as the interionic separation is increased and also the influence of both the components of the solvent mixture. The solvation shell of the ion pair has dominant contribution from water at all the three separations. Analysis of the computed normalised friction kernels of the ion pair at ion-ion separations of 5.4 Angstrom. (SSIP1), 6.6 Angstrom (TS) and at 7.1 Angstrom (SSIP2) reveals that the relative contribution to the net solvent friction by the individual solvents are different at these interionic distances. At all the ion-ion separations, the net dynamic friction experienced by the ion pair in the presence of the mixed solvent is similar to that in water. Intersolvent cross-correlated frictions on the ion pair are negative at short times and decay to zero within 0.7 ps. The dynamics of the barrier crossing reaction has also been studied using the Kramers and Grote-Hynes (GH) theories. The magnitude of the barrier frequency in the mixed solvent is larger than the corresponding barrier frequencies in water as well as in DMSO. The GH transmission coefficient approximately matches with the transmission coefficient obtained by the direct MD simulations. (C) 1998

The dynamics of association of Na+−Cl-, Na+−Na+, and Cl-−Cl- ion pairs in liquid dimethyl sulfoxide is studied by using the method of constrained molecular dynamics. Mean force potentials are employed to investigate the role of the solvent on the ion pairs. Friction kernels for the relative dynamics of the ion pairs have been evaluated at several interionic distances. Kramers and Grote−Hynes theories are applied to understand the passage of the ion pairs across the potential energy barrier existing between a contact ion pair and a solvent-separated ion pair. Transmission coefficients for the Na+−Cl- ion pair calculated from the above theories are in good agreement with the direct computer simulation results. The magnitudes of the squares of the nonadiabatic barrier frequencies are very large, and these confirm a polarization caging of the reactant ion pairs by the large solvent molecules.

Computer simulations are performed to study the structure of the coordination shells of Fe+2 and Fe+3 ions fixed at a very close reactive separation. The simulations show that it is possible for the two octahedral aquo complexes, i.e., Fe(H2O)6(2+) and Fe(H2O)6(3+), to come as close as 5 angstrom without disrupting their coordination shells. The reorientational dynamics within the hydration shells of these ions at this separation is examined by studying the time correlation functions (TCFs) of the unit vectors on the water molecules and along the iron-oxygen vector. The quantities related to the solvent polarization relaxation during a change in the charge, the dipole moment, and the quadrupole moment located at the solute ions are examined by studying the corresponding TCFs in the system. The TCFs exhibited a bimodal response, with a very fast initial relaxation due to inertial motions of the solvent, followed by a long tail corresponding to a diffusive component. The polarization fluctuations are also estimated via the cavity field time correlation function (CFTCF), which is useful in the theory of electron transfer processes. The memory kernel or the time dependent friction (TDF) of the solvent is also estimated from the force-force time correlation function. The cross correlations between the Coulombic and the non-Coulombic components of the forces at the ions contribute significantly to the TDF.