Solvent Dynamics In Ion Pair Dissociation

Having an accurate reaction coordinate provides mechanistic insight, facilitates rate constant calculations, and provides free energy landscapes that are kinetically meaningful. Reaction coordinates for reactions that involve the solvent are notoriously difficult to identify. The aqueous dissociation of NaCl would seem simple because there is only one solute coordinate, the ionic distance. However, many studies have shown that ionic distance is not a good coordinate. By using the aimless shooting version [1] of transition path sampling [2] and likelihood maximization [1], we have identified a reaction coordinate for this system. The coordinate combines a localized density of water molecules with the ionic distance.  We also investigate the relationship between the transmission coefficient and reaction coordinate error.  We discuss possible implications for simulations of charge recombination in photochemistry, ionic crystal growth by kink site insertion, and peptide aggregation.

 

1. B. Peters, G.T. Beckham, and B.L Trout, J. Chem. Phys. 127 034109 (2007)
2. P.G. Bolhuis, D. Chandler, C. Dellago, and P.G. Geissler, Ann. Rev. Phys. Chem. 53, 291 (2002)