Dynamics Simulations of Ion Pair Association

The interaction between solvation dynamics and chemical reaction is also evident in ion pair association reactions. The dynamics of this process has been studied for two different systems Na+-Cl in water by McCammon, Rossky, and co-workersi23-i27 and a model association reaction in a diatomic 

The first simulations of the dynamics of the interconversion between contact and solvent-separated ion pairs were those by Karim and McCammon. 25,126 Jheir system consisted of the Na —Cl ion pair in 295 TIPS2 water molecules with rectangular box boundary conditions. The dynamics was begun with the ion pair released from a constraint at the transition state (the 

A similar conclusion has been reached by Ciccotti et al. s-iao jj, their studies of the model ion association reaction. Their system consisted of two equally massive ions, modeled as Lennard-Jones spheres with a positive or negative charge, in a solvent of dipolar molecules. Each solvent molecule was modeled as a Lennard-Jones sphere with a dipole moment of either 2.4 or 3.0 D and with a mass equal to that of the ionic mass. As with the simulations of Karim and McCammon, Ciccotti et al. started the dynamics at the transition state, as determined from the free energy calculations, and ran 104-144 trajectories to determine the transmission coefficient. The values of the transmission coefficient they found were 0.18 in the 2.4 D solvent and 0.16 in the 3.0 D solvent (which are surprisingly, and perhaps coincidentally, close to the results of Karim and McCammon e). Ciccotti et al. also calculated the frequency-dependent friction that the solvent exerted on the reaction coordinate in order to compare the simulation results with Grote-Hynes theory for the rates. The comparison with Grote-Hynes theory was quite close, although within the outer reaches of the calculated uncertainties in the molecular dynamics transmission coefficients. 

As with the simulations of Karim and McCammon,the small transmission coefficient in this system indicated that the interionic separation might not be the best choice for the reaction coordinate. Ciccotti et al. performed an 

A system that bears some similarity to the ion pair systems discussed in the previous section is that of the dissociation of t-butyl chloride in water. The free energy profile for the ion pair products was originally calculated by Jorgensen et using both Monte Carlo and integral equation methods. 

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