Zundel Cation Research Articles

Solvation and Ionization Stages of HCl on Ice Nanocrystals

The study focuses on acid adsorption on cold ice particle surfaces. The investigation encompasses HCl, DCl, and HBr adsorbate spectroscopy, Monte Carlo simulations of molecular HCl adsorbate on a model ice particle, and ab initio studies of HCl solvation and ionization in mixed acid−water clusters. It is shown that ice nanocrystal surfaces offer a range of adsorption sites, in which HCl freezes in different recognizable solvation stages. These stages were identified spectroscopically and assigned, with the help of calculations, to weakly and strongly stretched HCl molecules, and to Zundel and hydronium ions that are products of proton transfer. At moderate submonolayer coverages in the 50−60 K range, the acid adsorbate is predominantly molecular. Heating promotes formation of contact hydronium − chloride ion pairs. Near 90 K, an ionization burst is observed, resulting in an ionic surface hydrate layer rich in Zundel cations.

Quasi-Chemical Theory and the Standard Free Energy of H+(aq)

Quasi-chemical theory and electronic structure results on inner-sphere H(H2O)n+ clusters are used to discuss the absolute hydration free energy of H+(aq). It is noted that this quantity is not thermodynamically measurable, and this leads to some relative misalignment of available tables of absolute hydration free energies of ions in water. The simplest quasi-chemical model produces a reasonable quantitative result in the range of −256 to −251 kcal/mol. The primitive concepts on which the model is based naturally identify the Zundel cation H5O2+ as the principal chemical structure contributing to this hydration free energy. The specific participation of an Eigen cation H9O4+ is not required in this model because the definition of that structure depends on outer-sphere arrangements, and a crude dielectric continuum model is here used for outer-sphere contributions.

Cluster size dependence of the H/D exchange reaction rate constant for protonated water clusters H +(H 2O) n ( n=2–11)

The collisions of size- and energy-selected H +(H 2O) n ( n=2–11) with D 2O have been investigated using guided-ion beam mass spectrometry. The H/D exchange reaction was observed by incorporation of D 2O into H +(H 2O) n . The measured rate constants of this reaction are found to depend strongly on the cluster size; the rate constants drastically decrease up to n=6, and then level off for 6⩽ n⩽11. This size dependence is discussed in terms of the structure of protonated water clusters: the core H 3O + and H 5O 2 + ions, so-called Eigen and Zundel cations respectively, are completely surrounded by the first water layer and cause the rate constants for 6⩽ n⩽11 essentially insensitive to n.

The computer simulation of proton transport in water

The dynamics and energetics of an excess proton in bulk phase water are examined computationally with a special emphasis on a quantum-dynamical treatment of the nuclear motion. The potential model used, the recently developed multistate empirical valence bond (MS-EVB) approach [U. W. Schmitt and G. A. Voth, J. Phys. Chem. B 102, 5547 (1998)], is also further refined and described in more detail. The MS-EVB model takes into account the interaction of an exchange charge distribution of the charge-transfer complex with the polar solvent, which qualitatively changes the nature of the solvated complex. Classical and quantum molecular dynamics simulations of the excess proton in bulk phase water reveal that quantization of the nuclear degrees of freedom results in an increased stabilization of the solvated H5O2+ (Zundel) cation relative to the H9O4+ (Eigen) cation, though the latter is still more stable, and that a species intermediate between the two also exists. The quantum proton transport rate, which is evaluated by the centroid molecular dynamics approach, is found to be on the order of two times faster compared to a purely classical treatment of the system and in good agreement with the experimental value. Calculation of the hydrogen-bonding lifetime beyond the first solvation shell of the excess proton reveals a similar quantum enhancement factor compared to the classical regime.

Quantum Properties of the Excess Proton in Liquid Water

AbstractThe classical and quantum equilibrium properties of an excess proton in bulk phase water are examined computationally with a special emphasis on the influence of an explicit quantum dynamical treatment of the nuclei on the calculated observables. The potential model used, our recently developed multistate empirical valence bond (MS‐EVB) approach is described. The MS‐EVB model takes into account the interaction of an exchange charge distribution of the charge‐transfer complex with the polar solvent, which qualitatively changes the nature of the solvated complex. The impact and importance of the exchange term on the stability of the solvated H5O2+ (Zundel) cation relative to the H9O4+ (Eigen) cation in the liquid phase is demonstrated. Classical and quantum path‐integral molecular dynamics (PIMD) simulations of an excess proton in bulk phase water reveal that quantization of the nuclear degrees of freedom results in an increased stabilization of the solvated Zundel cation relative to the Eigen cation, and that species intermediate between the two are also probable. Quantum effects lead to a significant broadening of the probability distributions used to characterize the two species, and a definite differentiation and sharp characterization of the species connected to the excess proton in liquid water is found to be difficult.