MOLECULAR-DYNAMICS SIMULATIONS OF ELEMENTARY CHEMICAL PROCESSES IN LIQUID WATER USING COMBINED DENSITY-FUNCTIONAL AND MOLECULAR MECHANICS POTENTIALS .1. PROTON-TRANSFER IN STRONGLY H-BONDED COMPLEXES

The first molecular dynamics (MD) simulation of a chemical process in solution with an abinitio description of the reactant species and a cl assical representation of the solvent is presented. We study the dynam ics of proton (deuterium) transfer in strongly hydrogen-bonded systems characterized by an energy surface presenting a double well separated by a low activation barrier. We have chosen the hydroxyl-water comple x in liquid water to analyze the coupling between the reactive system and the environment. The proton is transferred from one well to the ot her with a frequency close to 1 ps(-1) which is comparable to the low- frequency band associated to hindered translations, diffusional transl ation and reorientation of water molecules in water. The proton transf er takes place in 20-30 fs whereas the solvent response is delayed by about 50 fs. Therefore, the reaction occurs in an essentially frozen-s olvent configuration. In principle, this would produce a barrier incre ase with respect to the equilibrium reaction path. However, solvent fl uctuations play a substantial role by catalyzing the proton transfer. The solvent relaxation time after proton transfer has been evaluated. Since it falls in the same time scale than the reactive events (0.6 ps ) it substantially influences the proton dynamics. The present study i s intended to model charge transfer processes in polar media having a low activation barrier for which many reactive events may be predicted in a MD simulation. The case of reactions with large activation barri ers would require the use of special techniques to simulate rare event s. But still in that case, hybrid QM/MM simulations represent a suitab le tool to analyze reaction dynamics and non-equilibrium solvent effec ts in: solution chemistry. (C) 1997 American Institute of Physics.