STRUCTURE AND DYNAMICS OF NA+-NA+, NA+-CL-, AND CL--CL- ION-PAIRS IN DMSO

The potentials of mean force (PMFs) and the dynamics of Na+-Na+, Na+-Cl-, and Cl--Cl- ion pairs in DMSO have been obtained by using the technique of constrained molecular dynamics simulation. The long range potentials are estimated by using the reaction field method. The PMFs of these ion pairs in DMSO are compared with the reported potentials of mean force of these ion pairs in water. The Na+-Na+ and the Cl--Cl- PMFs are repulsive throughout. In the case of the Na+-Na+ ion pair, the PMF shows a shallow minimum at 3.6 Angstrom while the Cl--Cl- PMF decays monotonically. The Na+-Cl- mean force potential shows a deep minimum corresponding to a contact ion pair (CIP) at 2.6 Angstrom and a much shallower solvent separated ion pair (SSIP) at 7.2 Angstrom. The trajectories of the ion pairs in DMSO are initiated in the regions of the minima and the maxima of the PMFs. It is found that the Na+-Cl- and Na+-Na+ ion pairs are very long lived and do not readily separate from their first minima at 2.6 and 3.6 Angstrom, respectively. The trajectories initiated at the local maximum of the PMF of the Na+-Cl- ion pair tend to move from the SSIP to the CIP region. The Na+-Na+ ion pair trajectories initiated at the local maximum at 4.9 Angstrom move outward in spite of the local minimum at 3.6 Angstrom being 2 kcal/mol below the local maximum