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 3O ؉ and H2O5 ؉ as natural innershell 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.Eigen cation ͉ Zundel cation T he hydrated proton, H ϩ (aq), plays a fundamental role in aqueous phase chemistry. Here, we address two related issues of the molecular theory of H ϩ (aq) that haven't been resolved: the structural categorization of the local environment of H ϩ (aq) and hydration free energy of this ion under standard conditions.The difficulty of treating H ϩ (aq) on the basis of molecular theory is principally that the interactions are fundamentally chemical, and therefore complicated when viewed on a thermal energy scale. This is also true of other aqueous ions (1, 2), but H ϩ (aq) is an extreme example. In addressing this complexity, computations of thermodynamic properties of these solutions and comparisons with experimental results give the clearest assessment of theory and simulation work (3-8).Structural and thermodynamic issues are connected, of course, and particularly directly by the quasichemical theory (9-17). A primitive quasichemical approximation has been developed for the hydration free energy of H ϩ (aq) (4, 7). The conclusion of that work was that the H-centered Zundel complex ion H 5 O 2 ϩ provided a simple and valid structural description of the local environment of H ϩ (aq); that theory identifies the Eigen complex ion H 9 O 4 ϩ (i.e., hydrated oxonium) as a structural specification of outer-shell hydration of a distinguished H ϩ , and therefore assigns the Eigen cation less significance.A distinguishing feature of the Zundel complex is an OOO distance of 2.4 Å, shorter than OOO distances between water molecules, or within the hydrated oxonium ion. The latter case (the Eigen structure) might be taken as an O-center basis of a quasichemical model of the solution thermodynamics. But a subsequent observation obtained from ab initio molecular dynamics (AIMD) simulation of HO Ϫ (aq) then becomes relevant (1). AIMD results showed that the nearest coordinating O atom to a HO Ϫ oxygen has the 2.45-Å displacement distinctive of the H 3 O 2 Ϫ complex. This separation is analogous to the case o...

Cluster Model for the Ionic Product of Water: Accuracy and Limitations of Common Density Functional Methods

“…35 Recently, using CPMD combined with transition path sampling, [36][37][38] it was shown 1 that the transfer of a proton in the O-H‚‚‚O system represents a first step and that the dissociation of O-H bonds is driven by the concerted changes in the electric field and in the hydrogen-bond network. Despite extensive studies employing different theoretical approaches, 26,35,[39][40][41][42][43] the detailed mechanism of proton transfer is still debated.…”

Dynamical aspects of intermolecular proton transfer in liquid water and low-density amorphous ices

“…The temperature and, more specifically, the thermodynamical state of the system plays a central role both on structure and location of the proton species and also on dynamics of the proton transfer. Whereas plenty of information about PT in liquid water and in biomolecular systems at ambient conditions is available [9][10][11][12][13][14][15][16][17][18][19][20][21][22], its characteristics at low temperatures, such as in supercooled states, for the wide variety of ice * jordi.marti@upc.edu classes and for high-and low-density amorphous ices (LDA) is still largely unknown. Given the complexity of the phase diagram of water [23,24], studies of structure and dynamics of aqueous lone protons are usually reported for restricted regions of the diagram.…”

Proton transfer in liquid water confined inside graphene slabs

“…At ambient conditions, plenty of information about PT in liquid water and in biomolecular systems at ambient conditions is available. [2,6,[8][9][10][11][12][13][14][15][16][17][18][19][20][21] Far from ambient conditions, it has been observed that rates of PT and diffusion coefficients of the proton show marked variations from room temperature values, when a wide range of states (from supercooled to supercritical) have been explored. [22][23][24][25] When confined in constrained geometries, the microscopical properties of the proton also suffer drastic changes.…”