No abstract

The hydroxide anion plays an essential role in many chemical and biochemical reactions. But a molecular-scale description of its hydration state, and hence also its transport, in water is currently controversial. A preeminent challenge in liquid-state physics is the understanding of aqueous phase chemical transformations on a molecular scale. Water undergoes limited autoprotolysis, which is enhanced in the presence of highly charged metal ions such as Be 2ϩ (1, 2). Understanding the hydration and transport of the autoprotolysis products, H ϩ and HO Ϫ , presents unique and interesting challenges for molecular-scale theories of solutions and for simulations. In this paper we focus on HO Ϫ (aq).Because H ϩ and HO Ϫ constitute the underlying aqueous matrix, it is not unreasonable to expect that their transport in water is different from the transport of other aqueous ions. This anomalous diffusion of the H ϩ (aq) and HO Ϫ (aq) has received extensive scrutiny over the years (for example, refs. 3-5), but recently ab initio molecular dynamics (AIMD) capabilities have evolved to provide new information on the solution condition and transport of these species. Over a similar period, the statistical mechanical theory of liquids (especially water) has also become more sophisticated (for example, ref. 6). These two approaches can be complementary, but in typical practice they remain imperfectly connected (but see refs. 2 and 7-11).In an initial AIMD study (12) Ϫ species was 2-3 ps, but statistical characterization was sketchy.Discussions of a transport mechanism for HO Ϫ (aq) typically focus on Agmon's (14, 15) extraction of an activation energy for hydroxide transport from the temperature dependence of the experimental mobilities. Near room temperature that empirical parameter is about 3 kcal͞mol, but it increases by roughly a factor of 2 for slightly lower temperatures. As a mechanical barrier this value, about 5-6 k B T, may be low enough to require some subtlety of interpretation (16); the observed temperature sensitivity of the activation energy, and particularly its increase with decreasing temperature, supports that possibility. We note that a standard inclusion of a tunneling correction would be expected to lead to a decrease of activation energy with decreasing temperature.Ref. 13 framed the consideration of HO Ϫ transport in terms of classical transition state theory and extracted an activation energy from the gas-phase study of Novoa et al. (17). Ref. 13 also considered the importance of tunneling in lowering the barrier for proton transfer by performing path integral calculations. Their combined value of 3

Stochastic dynamics simulations of vibrational relaxation rates are reported for a carbon monoxide molecule adsorbed on the (100) face of copper. A recently developed ‘‘molecular dynamics with electronic friction’’ scheme that self-consistently incorporates both phonon and nonadiabatic electron–hole (e-h) pair mechanisms of energy dissipation is employed. Lifetimes of the C–O stretch, the CO-surface stretch, the frustrated rotation, and the frustrated in-plane translation are examined as a function of temperature between 0 and 450 K. e-h pair dissipation plays a significant role for all modes above 150 K. For the C–O internal stretching mode, the e-h pair mechanism dominates and the lifetime depends weakly on temperature. The frustrated rotational (bending) mode is calculated to have the shortest lifetime at all temperatures, and the temperature dependence is weak. The e-h pair contribution is again largest although the phonon contribution is significant. The CO-surface stretch and the frustrated in-plane translation are the longest lived modes, but exhibit a dramatic decrease in lifetime with increasing temperature. The results suggest that e-h pair dissipation plays a significant role in adsorbate dynamics at metal surfaces when there are chemisorption interactions.

Proton conducting oxide ceramics have shown potential for use in fuel cell technologies. Understanding the energy pathways for proton conduction could help us design more efficient fuel cell materials. This paper describes how octahedral tilting affects the relative energies of proton binding sites, transition states, and conduction pathways in cubic and pseudo-cubic perovskites. First, the structure for cubic and pseudo-cubic forms of BaTiO(3), BaZrO(3), CaTiO(3), and CaZrO(3), is found. Even when cubic symmetry is enforced, CaTiO(3), and CaZrO(3) exhibit octahedral tilting distortions characteristic of orthorhombic phases while BaTiO(3) and BaZrO(3) remain undistorted. Octahedral tilting gives rise to proton binding sites facilitating inter- and intra-octahedral proton transfer while the proton binding sites of undistorted perovskites facilitate only intra-octahedral proton transfer. The nudged elastic band method is used to find minimum energy paths between the proton binding sites. As distortions increase, inter-octahedral proton transfer barriers decrease while intra-octahedral proton transfer barriers increase. Concurrently, rotational barriers from oxygens facilitating inter-octahedral proton transfer increase while rotational barriers from oxygens facilitating intra-octahedral proton transfer decrease. Intra-octahedral transfer is the rate-limiting step to the lowest energy extended proton conduction pathway in all the perovskites considered.

We develop a quasi-chemical theory for the study of packing thermodynamics in dense liquids. The situation of hard-core interactions is addressed by considering the binding of solvent molecules to a precisely defined 'cavity' in order to assess the probability that the 'cavity' is entirely evacuated. The primitive quasi-chemical approximation corresponds to a extension of the Poisson distribution used as a default model in an information theory approach. This primitive quasi-chemical theory is in good qualitative agreement with the observations for the hard sphere fluid of occupancy distributions that are central to quasi-chemical theories but begins to be quantitatively erroneous for the equation of state in the dense liquid regime of ρd 3 >0.6. How the quasi-chemical approach can be iterated to treat correlation effects is addressed. Consideration of neglected correlation effects leads to a simple model for the form of those contributions neglected by the primitive quasi-chemical approximation. These considerations, supported by simulation observations, identify a 'break away' phenomena that requires special thermodynamic consideration for the zero (0) occupancy case as distinct from the rest of the distribution. A empirical treatment leads to a one parameter model occupancy distribution that accurately fits the hard sphere equation of state and observed distributions.

When BaZrO(3) is doped with Y in 12.5% of Zr sites, density functional theory with the PBE functional predicts octahedral distortions within a cubic phase yielding a greater variety of proton binding sites than undoped BaZrO(3). Proton binding sites, transition states, and normal modes are found and used to calculate transition state theory rate constants. The binding sites are used to represent vertices in a graph. The rate constants connecting binding sites are used to provide weights for graph edges. Vertex and color coding are used to find proton conduction pathways in BaZr(0.875)Y(0.125)O(3). Many similarly probable proton conduction pathways which can be periodically replicated to yield long range proton conduction are found. The average limiting barriers at 600 K for seven step and eight step periodic pathways are 0.29 and 0.30 eV, respectively. Inclusion of a lattice reorganization barrier raises these to 0.42 and 0.33 eV, respectively. The majority of the seven step pathways have an interoctahedral rate limiting step while the majority of the eight step pathways have an intraoctahedral rate limiting step. While the average limiting barrier of the seven step periodic pathway including a lattice reorganization barrier is closer to experiment, how to appropriately weight different length periodic pathways is not clear. Likely, conduction is influenced by combinations of different length pathways. Vertex and color coding provide useful ways of finding the wide variety of long range proton conduction pathways that contribute to long range proton conduction. They complement more traditional serial methods such as molecular dynamics and kinetic Monte Carlo.

Quasi-chemical theory and electronic structure results on inner-sphere H(H 2 O) 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 H 5 O 2 + as the principal chemical structure contributing to this hydration free energy. The specific participation of an Eigen cation H 9 O 4 + 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.

The HO − (aq) ion participates in myriad aqueous phase chemical processes of biological and chemical interest. A molecularly valid description of its hydration state, currently poorly understood, is a natural prerequisite to modeling chemical transformations involving HO − (aq). Here it is shown that the statistical mechanical quasichemical theory of solutions predicts that− is the dominant inner shell coordination structure for HO − (aq) under standard conditions. Experimental observations and other theoretical calculations are adduced to support this conclusion. Hydration free energies of neutral combinations of simple cations with HO − (aq) are evaluated and agree well with experimental values.