MOLECULAR MECHANISM OF HCL ACID IONIZATION IN WATER - AB-INITIO POTENTIAL-ENERGY SURFACES AND MONTE-CARLO SIMULATIONS

The acid ionization of HCl in water is examined via a combination of e lectronic structure calculations with ab initio molecular orbital meth ods and Monte Carlo computer simulations, The following key features a re taken into account in the modeling: the polarization of the electro nic structure of the solute reaction system by the solvent, the quantu m character of the proton nuclear motion, the solvent fluctuation and reorganization along with the solvent polarization effects on the prot on potential, and a Grotthuss mechanism of the aqueous proton transfer , The mechanism is found to involve the following: first, a nearly act ivationless motion in a solvent coordinate, which is adiabatically fol lowed by the quantum proton rather than tunneling, to produce a contac t ion pair Cl--H3O+, which is stabilized by similar to 7 kcal/mol; sec ond, motion in the sol vent with a small activation barrier, as a seco nd adiabatic proton transfer produces a solvent-separated ion pair fro m the contact ion pair in a nearly thermoneutral process, Motion of a neighboring water molecule-to accommodate the change of the primary co ordination number from 4 for H2O to 3 for H3O+ of a proton-accepting w ater molecule-is indicated as a key feature in the necessary solvent r eorganizations. It is estimated, via a separate argument, that the rem ainder of the process to produce the completely separated ions involve s a free energy change of less than 1 kcal/mol. It is argued that the reorganization of the heavy atoms between which the proton transfers p lays an essential role in assisting the adiabatic (nontunneling) and s tepwise transfer mechanism and that the concerted pathway of the multi ple proton transfers in water is unfavorable.