Ab initio molecular dynamics and quasichemical study of H+(aq)

Abstract

The excess proton in water, H(+)(aq), plays a fundamental role in aqueous solution chemistry. Its solution thermodynamic properties are essential to molecular descriptions of that chemistry and for validation of dynamical calculations. Within the quasichemical theory of solutions those thermodynamic properties are conditional on recognizing underlying solution structures. The quasichemical treatment identifies H(3)O(+) and H(2)O(5)(+) as natural inner-shell complexes, corresponding to the cases of n = 1, 2 water molecule ligands, respectively, of a distinguished H(+) ion. A quantum-mechanical treatment of the inner-shell complex with both a dielectric continuum and a classical molecular dynamics treatment of the outer-shell contribution identifies the latter case (the Zundel complex) as the more numerous species. Ab initio molecular dynamics simulations, with two different electron density functionals, suggest a preponderance of Zundel-like structures, but a symmetrical ideal Zundel cation is not observed.

Fig. 1.

Bonding arrangement about the excess proton H*.

Fig. 2.

Data from the PW91 run. g(H*O) is radial distribution of oxygens around H*. g(O*O) is radial distribution of oxygens around oxygen O*. The trajectory was sampled every 20 fs, and the data were binned in 0.05-Å intervals. The darker lines show decomposition according to the distance-order of the contributing atoms, i.e., the solid line is the contribution from the nearest-neighbor O atom in each case, the dark dashed curve is the contribution of the second nearest neighbor, and so on.

Fig. 3.

Data, as in Fig. 2, are shown from the rPBE run.

Fig. 4.

Exchange events involving H⁺ and HO^– motion in water. The vertical bars flag hydrogen exchange events, those times at which the identity of the excess proton and the hydroxide oxygen changes. (Upper) The rPBE run. (Lower) The PW91 run. The data for the hydroxide exchange are from an earlier paper (1), and note particularly the previous hydroxide PW91 calculation was for the hygrogenated, not deuterated, case. The hydroxide PW91 and rPBE production runs were for 8.2 and 5.9 ps, respectively. For a similar length of time, HO^–(aq) switches identity less frequently than H⁺(aq).