IONIC ASSOCIATION OF NA-CL-, NA+-NA+, AND CL--CL- IN METHANOL - MEAN FORCE POTENTIALS AND FRICTION KERNELS()

Solvent-averaged potentials for Na+-Cl-, Na+-Na+, and Cl--Cl- in metha nol have been evaluated using the constrained molecular dynamics simul ation technique. Solvent contributions to the total force on the solut e pairs have been analyzed carefully in all of the systems. In order t o get a deeper insight into the influence of the solvent on the ion pa irs, the most probable solvent distributions at the relevant points of the calculated mean force potentials have been studied. Our mean forc e potentials show important discrepancies with the ones obtained using the RISM (reference interaction site model) approximation. Friction k ernels for the relative dynamics of the ion pairs have also been evalu ated. The passage across the barrier existing in the Na+-Cl- mean-forc e potential has been analyzed in light of Kramers and Grote-Hynes theo ries. It turns out that the association-dissociation process takes pla ce in the polarization caging regime. A comparison between our results and the ones corresponding to the same ion pairs in water has been do ne.