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 electronic structure calculations with ab initio molecular orbital methods and Monte Carlo computer simulations. The following key features are taken into account in the modeling: the polarization of the electronic structure of the solute reaction system by the solvent, the quantum character of the proton nuclear motion, the solvent fluctuation and reorganization along with the solvent polarization effects on the proton potential, and a Grotthuss mechanism of the aqueous proton transfer. The mechanism is found to involve the following: first, a nearly activationless motion in a solvent coordinate, which is adiabatically followed by the quantum proton rather than tunneling, to produce a contact ion pair Cl - −H 3 O +, which is stabilized by ∼7 kcal/mol; second, motion in the solvent with a small activation barrier, as a second adiabatic proton transfer produces a solvent-separated ion pair from the contact ion pair in a nearly thermoneutral process. Motion of a neighboring water molecule to accommodate the change of the primary coordination number from 4 for H 2 O to 3 for H 3 O + of a proton-accepting water molecule is indicated as a key feature in the necessary solvent reorganizations. It is estimated, via a separate argument, that the remainder of the process to produce the completely separated ions involves 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 plays an essential role in assisting the adiabatic (nontunneling) and stepwise transfer mechanism and that the concerted pathway of the multiple proton transfers in water is unfavorable.