Dynamics Simulations of Proton Transfer Reactions in Solution

Another class of reactions whose understanding may require the inclusion of quantum effects consists of proton transfer reactions. The light mass of the proton indicates that such quantum effects might be quite important, but there have been attempts to simulate this process purely classically (primarily in the gas phase). An interesting method that lies in between gas phase calculations and full solution phase molecular dynamics is the supermolecule method used by Nagaoka et al. to calculate the dynamics of formamidine in water solvent. This system is quite interesting from the perspective of solution reaction dynamics because the transition state for this reaction incorporates a water molecule from the solvent. The overall process consists of two proton transfers, one from the formamidine molecule to the solvent water molecule and another one from the other end of the solvent water molecule back to the formamidine. 

Nagaoka et al. therefore treated the system consisting of the formamidine molecule and a single water molecule as a supermolecule in which the reaction dynamics takes place. They have suggested that this technique may be valuable in cases where one or two solvent molecules have a strong electronic effect on the reaction system, as is certainly the case in the formamidine system. Nagaoka et al. used ab initio calculations to determine the reaction coordinate in the supermolecule and have used this reaction coordinate as a 

The quantum mechanical nature of the proton transfer reaction has been dealt with in analytic theory and simulations by Warshel and Chu and by Borgis and Hynes.i s-i o xhe methods they describe explicitly take into account the solvent effect on the proton tunneling process. In that sense, they are perhaps more closely related to simulations of electron transfer reactions than to the simulations we have described so far. For that reason, we shall refer the reader to the original papers for further description of these techniques. 

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