The rate constant for the Fe2+–Fe3+ electron exchange is formulated as k23= ∫ 0∞g23(r) k̂23(r) 4πr2 dr, a form which also is used to analyze the data for the nuclear spin relaxation in Al3+ induced by collision with Ni2+. It is assumed that the equilibrium pair correlation function g23(r) is the same function of ionic composition and temperature in the two cases and that in the spin relaxation process the local rate constant k̂23(r) has the form that may be deduced from the Solomon–Bloembergen equations. In the case of the exchange reaction the theory of k̂23(r) is developed with respect to the contributions from slow inner shell or outer shell reorganization (activation) dynamics. It is concluded that in the present case these complications are not important and that the controlling dynamics is the crossing from the reactant to the product diabatic Born– Oppenheimer surface. Neither the exchange nor the spin relaxation data can be accounted for if the smallest metal–metal distance in collisions is that given by the closest approach of the envelopes of the M(H2O)6n+ complexes. However, allowing for overlap of the envelopes as one complex pokes into the interstices of the other reduces the distance of closest approach from 6.9 to 4.5 Å. Then one can find Gurney type models for the ion–ion forces in solution such that the model calculations are in good agreement with the experimental exchange and relaxation rate constants and their dependence on temperature and ionic strength, as far as the limited data for the last allow.

The molecular dynamics simulations of the Na+-Cl-ion pair in DMSO have been performed to obtain the potential of mean force (PMF) and the mean force between the Na+ and the C1-ions at room temperature.The reaction field method has been used to estimate the effect of the long-range interactions. The results obtained are compared with the reported potential of mean force of Na+-Cl-ion pair in water. The mean force potential in DMSO shows that the contact ion pair (CIP) is much stabler than the solvent-separated ion pair (SSIP), whereas in water, the SSIP is as stable as the CIP. The barrier for crossing from the contact ion pair to the solvent-separated ion pair in DMSO is much larger than the corresponding value in water, whereas the barriers for crossing from the SSIP to the CIP are similar in both these solvents. The density profiles of all the interaction sites around the ion pair at the first minimum in the PMF have also been presented.

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.

We correct an error in our work which does not affect the conclusions. A factor of! is missing from the second and third terms of Eq. (C3) and the error is propagated through several following equations. I The corrected equations read G(el,e Z ) = ~Q(A,A lei + !e)Q(A,B)e z + !ezQ(B,A)e l + !Q(B,B)eL