Molecular Mechanism of HCl Acid Ionization in Water:  Ab Initio Potential Energy Surfaces and Monte Carlo Simulations

Abstract

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-−H3O+, 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 H2O to 3 for H3O+ 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.

Reorientional dynamics of water molecules in anionic hydration shells

“…reoriented 2 I frame , which is determined to be 10 ps from our 3 M simulations (Fig. 3B) and which agrees with the 9.6-to 10-ps fsIRS result (9,10).…”

“…NMR determines the reorientation component proportional to the salt concentration. Because of the librational component, this time integral is not 2 I , which is defined in terms of an exponential decay after a brief librational transient (15); it is instead from Eq. 7 (1 Ϫ C lib )/(1/ lib ϩ 1/ 2 I ) ϩ C lib 2 I , which is 1.5 ps in the 1 M solution from our model (see Table 1), in reasonable agreement with the experimental 1.7 (13) to 2.3 (12) ps value.…”

“…hydrogen bonding ͉ ions ͉ orientational dynamics I on hydration shell dynamics are critical for aqueous chemical reaction mechanisms (1,2). They are also key for transport of ionic solutes in water; ionic mobility, for example, cannot be simply related to the ion size because of the important role of hydration shell structure and lability (3,4).…”

Electrode Dynamics from First Principles

“…23) To confirm the generality of the simulated dynamics, we introduced a second hydronium ion and restarted the MD run for approximately 2 ps. We found that a similar hydrogen-bonded hydronium ion is again formed, which plays the same role as that prior to the Volmer step.…”

Molecules in Motion: Chemical Reaction and Allied Dynamics in Solution and Elsewhere

“…The key issue had to do with the molecular-level mechanism of the heterogeneous HCl + ClONO 2 → Cl 2 + HNO 3 reaction of hydrochloric acid with chlorine nitrate that Susan Solomon (136) had proposed as the culprit for ultimately producing ozone-destroying chlorine. With (now) postdoc Gertner, we proposed a way for HCl-our old friend from our solution PT work (110,111)-to acid dissociate at the surfaces of polar stratospheric cloud ice particles (137,138). As for the net Cl 2 -producing reaction itself, one suggestion was that it occurred in two steps, the first being the hydrolysis of chlorine nitrate, ClONO 2 + H 2 O → HOCl + HNO 3 , followed by HCl+ HOCl → H 2 O + Cl 2 .…”