Eigen Cation Research Articles

MP-5 周囲の水分子の影響を考慮したプロトンホッピング現象の分子動力学的研究(ポスターセッション)

The purpose of this study is the development of proton(s) hopping rate by applying new model for Molecular Dynamics (MD) calculation with Empirical Valence Bond (EVB) method. The overview about this new model is an acceptor selection method based on energy prediction instead of the one based on distance. The effect of water molecules outside the Eigen cation was mainly considered in this new model. Then, energy prediction and comparison to presumed hopped states were performed to determine the acceptor. After energy prediction, the water for the lowest energy state was chosen as the proton acceptor. In this model, the microenvironment interaction to the formation of the cluster and proton hopping were reasonably expressed as an actual condition. This procedure was applied for proton hopping.

Structure, stability and spectral signatures of monoprotic carborane acid–water clusters (CBWn, where n = 1–6)

The gas phase structure, stability, spectra, and proton transfer properties of monoprotic carborane acid-water clusters [CB(11)F(m)H(11-m)(OH(2))(1)]-(H(2)O)(n) (where m = 0, 5, and 10; n = 1-6) have been calculated using density functional theory (DFT) with the Becke's three-parameter hybrid exchange functional and Lee-Yang-Parr correlation functional (B3LYP) using 6-31+G* basis set. Results reveal that Eigen cation defects are found in CBW(n) (where n = 2-6) clusters and these clusters are significantly more stable than the non-Eigen geometry. In addition to the conventional hydrogen bond (H-bond) the role of dihydrogen bond (DHB) and halogen bond (XB) in the stabilization of these clusters can be observed from the molecular graphs derived from the atoms in molecules (AIM) analysis. Spectral information shows the features of Eigen cation and proton oscillation involved in the proton transfer process. The dissociation of proton from the perfluoro derivatives with two water molecules is more favorable when compared to the other derivatives.

S053014 ホッピングを考慮したプロトン拡散現象の分子動力学的研究([S05301]次世代電池のナノ・マイクロフルイディクス(1))

The aim of this study is refinement of proton(s) hopping rate by applying new model for Molecular Dynamics (MD) calculation with Empirical Valence Bond (EVB) method. The overview about this new model is an acceptor selection method based on energy prediction instead of the one based on distance. The effect of water molecules outside the Eigen cation was mainly considered in this new model. Then, energy estimation and comparison to presumed hoping states were done before acceptor selection. After energy estimation, the water for lower energy state was chosen as the proton acceptor. In this model, the microenvironment interaction to the formation of the cluster and proton hopping were reasonably expressed as an actual condition. This procedure was applied for proton hopping. The total number of cluster formation and cancel were also analyzed.

Infrared Spectrum of the Hydrated Proton in Water.

Reactive molecular dynamics simulations have been utilized to calculate the infrared (IR) spectra of acidic HCl solutions of varying concentration with the goal of achieving a better understanding of the spectral features of the hydrated excess protons in bulk water. To incorporate the essential physics of the hydrated proton, we carried out the simulations using the specialized self-consistent iterative multistate empirical valence bond (SCI-MS-EVB) method, which is a form of multiconfigurational (reactive) molecular dynamics. After the pure water absorption background was removed, the calculated difference spectra are in good agreement with prior experimental results. The continuous broad absorption band in the acidic IR spectrum is, for the first time, interpreted based on the concept of a dynamically distorted Eigen cation, H9O4(+), which has been shown to provide the most accurate description for the charge defect character of the hydrated excess proton in liquid water.

Proton Transfer in Perfluorosulfonic Acid Functionalized Carbon Nanotubes

AbstractProton dissociation and transfer are investigated with ab initio molecular dynamics (AIMD) simulations of carbon nanotubes (CNT) functionalized with perfluorosulfonic acid (-CF2SO3H) groups with 3 H2O/–SO3H. The CNT systems were constructed both with and without fluorine atoms covalently bound to the inner walls to determine the effects of the presence of fluorine on proton dissociation, hydration, and stabilization. The results of the AIMD trajectories show that decreasing the separation of sulfonic acid groups increases the propensity for proton dissociation. The simulations also revealed that the dissociated proton was preferentially stabilized as a hydrated hydronium (H3O+) cation in the CNT systems with the fluorine. This feature is attributed to the fluorine atoms providing a localized negative charge that promotes hydrogen bonding of the water molecules coordinated to the central hydronium ion. The hydrated H3O+ ion differed from a traditional Eigen cation (H9O4+) as it donated hydrogen bonds to sulfonate oxygen atoms, as well as water molecules.

Stepwise Hydration of Protonated Carbonic Acid: A Theoretical Study

The gas-phase geometries, binding energies (BEs), and sequential binding energies (SBEs) of protonated carbonic acid (PCA)-water (W) clusters (PCAW(n), where n = 1-6) have been calculated using density functional theory (DFT) with Becke's three-parameter hybrid exchange functional and the Lee-Yang-Parr correlation functional (B3LYP) and M05-2X methods. The presence of wirelike structures of protonated water in PCAW(n)(x) clusters is evident from the results. The results indicate that a proton is transferred from PCA to its immediate water molecule in the linear and monohydroxy clusters of PCA. The involvement of the Eigen cation and Grotthuss type of mechanism in the proton transport is observed from the sequential hydration energies and from the calculated vibrational spectra. Although geometrical parameters clearly reveal the presence of the Eigen core, calculated lower-energy vibrational modes provide clues about the involvement of the sequence Eigen --> Zundel --> Eigen in the proton transfer.

Nitric Acid Dissociation at an Aqueous Surface: Occurrence and Mechanism

AbstractHere we briefly review some highlights of our recent work on the acid dissociation of nitric acid HNO3 at an aqueous surface, a proton transfer reaction of interest not only from a fundamental perspective, but also in connection with heterogeneous chemistry in a wide range of atmospheric contexts. Two types of studies of the potential acid dissociation are discussed, quantum chemical reaction path calculations to assess the reaction free energy and Car‐Parrinello molecular dynamics simulations to assess the reaction feasibility. We discuss the agreement and disagreement between the predictions of these two calculations as a function of the initial location of the HNO3 molecule, ranging from a positioning on top of the aqueous surface to one several water layers below the surface. Special attention is given to the four key water solvent motions found to be necessary for the proton transfer reaction to occur. Finally, an Eigen cation, rather than a Zundel cation, is in all cases found to be predominant next to the nitrate ion in contact ion pairs produced in the acid dissociation. This predominance remains, although diminished, for solvent‐separated ion pairs.

Depth-Dependent Dissociation of Nitric Acid at an Aqueous Surface: Car−Parrinello Molecular Dynamics

The acid dissociation of a nitric acid HNO(3) molecule located at various depths in a water slab is investigated via Car-Parrinello molecular dynamics simulations. HNO(3) is found to remain molecular when it is adsorbed on top of the surface with two hydrogen-bonds, and to dissociate--although not always--by transferring a proton to a water molecule within a few picoseconds when embedded at various depths within the aqueous layer. The acid dissociation events are analyzed and discussed in terms of the proton donor-acceptor O-O hydrogen bonding distance and the configurations of the nearest-neighbor solvent waters of an HNO(3).H(2)O pair. Four key structural features for the HNO(3) acid dissociation are identified and employed to analyze the trajectory results. Key solvent motions for the dissociation include the decrease of the proton donor-acceptor O-O hydrogen bonding distance and a 4 to 3 coordination number change for the proton-accepting water. The Eigen cation (H(3)O(+)), rather than the Zundel cation (H(5)O(2)(+)), is found to be predominant next to the NO(3)(-) ion in contact ion pairs in all cases.

Flexible-Boundary Quantum-Mechanical/Molecular-Mechanical Calculations: Partial Charge Transfer between the Quantum-Mechanical and Molecular-Mechanical Subsystems.

Based on the principle of electronic chemical potential equalization, we propose a flexible-boundary scheme to account for partial charge transfers between the quantum-mechanical (QM) and molecular-mechanical (MM) subsystems in combined QM/MM calculations. The QM subsystem is viewed as an open system with a fluctuating number of electrons and is described by a statistical mixture of ensemble that consists of states of integer number of electrons. The MM subsystem serves as a reservoir that exchanges electrons with the QM subsystem. The electronic chemical potential of the MM subsystem varies whenever charges flow in or out, until equilibrium is established for the electronic chemical potentials between the QM and MM subsystems. Our scheme is demonstrated by calculations of the partial atomic charges for 7 small model systems, each consisting of a singly charged ion and a water molecule, as well as for the Eigen cation, a model system for the solvated structure of hydronium ion in water. Encouraging results are obtained for the partial atomic charges, which are in reasonable agreement with full-QM calculations on those model systems. The averaged mean unsigned deviations between the QM/MM and full-QM calculations are 0.16 e for the partial atomic charges of the entire systems and 0.13 e for the amount of charge transferred between the QM and MM subsystems.

Time-resolved observation of the Eigen cation in liquid water

Experimental observation and time relaxation measurement of the hydrated proton Eigen form [H(3)O(+)(H(2)O)(3)] are presented here. Vibrational time-resolved spectroscopy is used with an original method of investigating the proton excess in water. The anharmonicity of the time-resolved spectra is characteristic of the Eigen-type proton geometry. Proton relaxation occurs in less than 200 fs. A calculation of the potential energy confirms the experimental result and the Eigen cation lifetime is in good agreement with previous molecular dynamics simulations.

Protons in supercritical water: a multistate empirical valence bond study.

Molecular dynamics simulations have been performed to analyze microscopic details related to aqueous solvation of excess protons along the supercritical T = 673 K isotherm, spanning a density interval from a typical liquid down to vapor environments. The simulation methodology relies on a multistate empirical valence bond Hamiltonian model that includes a proton translocation mechanism. Our results predict a gradual stabilization of the solvated Eigen cation [H(3)O.(H(2)O)(3)](+) at lower densities, in detriment of the symmetric Zundel dimer [H.(H(2)O)(2)](+). At all densities, the average solvation structure in the close vicinity of the hydronium is characterized by three hydrogen bond acceptor water molecules and presents minor changes in the solute water distances. Characteristic times for the proton translocation jumps have been computed using population relaxation time correlation functions. Compared to room temperature results, the rates at high densities are 4 times faster and become progressively slower in steamlike environments. Diffusion coefficients for the excess proton have also been computed. In agreement with conductometric data, our results show that contributions from the Grotthus mechanism to the overall proton transport diminish at lower densities and predict that in steamlike environments, the proton diffusion is almost 1 order of magnitude slower than that for pure water. Spectroscopic information for the solvated proton is accordant to the gradual prevalence of proton localization in Eigen-like structures at lower densities.

A Computer Simulation Study of the Hydrated Proton in a Synthetic Proton Channel

Classical molecular dynamics simulations using the multistate empirical valence bond model for aqueous proton transport were performed to characterize the hydration structure of an excess proton inside a leucine-serine synthetic ion channel, LS2. For such a nonuniform pore size ion channel, it is found that the Zundel ion (H 5O 2 +) solvation structure is generally more stable in narrow channel regions than in wider channel regions, which is in agreement with a recent study on idealized hydrophobic proton channels. However, considerable diversity in the relative stability of the Zundel to Eigen cation (H 9O 4 +) was observed. Three of the five wide channel regions, one located at the channel's center and the other two located near the channel mouths, are found to show extraordinary preference for the Eigen solvation structure. This implies that proton hopping is inhibited in these regions and therefore suggests that these regions may behave as barriers in the proton conducting pathway inside the channel. The proton solvation is also greatly influenced by the local molecular environment of the protein. In particular, the polar side chains of the Ser residues, which are intimately involved in the solvation structure, can greatly influence proton solvation. However, no preference of the influence by the various Ser side chains was found; they can either promote or prevent the formation of certain solvation structures.

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.

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.