HCL ACID IONIZATION IN WATER - A THEORETICAL MOLECULAR MODELING

The results of the examination of the acid ionization of HCl in water via a combination of ab initio electronic structure calculations and M onte Carlo simulations are described. The following key aspects are ta ken into account: the electronic structure change of the solute reacti on system induced by the solvent polarization, the quantized nature of the proton nuclear motion, the solvent fluctuation and reorganization along with the solvent effects on the proton potential, and a Grotthu ss mechanism of the proton transfer in water. The mechanism is found t o involve stepwise transfers. The first is an almost activationless mo tion in a solvent coordinate, adiabatically followed by the quantum pr oton to produce a contact ion pair Cl--H3O+, which is stabilized by si milar to 7 kcal/mol. The second is a motion in the solvent with a smal l activation barrier, as the adiabatic proton transfer produces a solv ent-separated ion pair from the contact ion pair in an almost thermone utral process. Motion of a neighbouring water molecule, accomodating a primary coordination number change of a proton-accepting water, is id entified as a key in the reaction promoting solvent reorganization.