Excess proton solvation and delocalization in a hydrophilic pocket of the proton conducting polymer membrane nafion

Solvation properties of the hydrated excess proton are studied in a hydrophilic pocket of Nafion 117 through a series of molecular dynamics simulations. The multistate empirical valence bond (MS-EVB) methodology, which enables the delocalization of the excess proton through the Grotthuss hopping mechanism, was employed for one of the excess protons in the simulation cell. Simulations were performed such that "classical" nondissociable hydronium cations and a single excess proton treated with the MS-EVB methodology were at a concentration ratio of 39:1. Two degrees of hydration of the Nafion polymer electrolyte membrane were simulated, each displaying the same marked difference between the solvation structures of the classical versus MS-EVB treated (Grotthuss shuttling) excess proton species. These differences are attributed to the solvent dynamics needed to transfer the cation between the solvent separated and contact pair positions about the sulfonic acid counterion. The results demonstrate that it is generally impossible to describe the low pH conditions in the hydrophilic domains of Nafion without the explicit treatment of Grotthuss delocalization in the underlying molecular dynamics model for the excess protons.     

Modelling of morphology and proton transport in PFSA membranes

Computational modelling studies of the structure of perfluorosulfonic acid (PFSA) ionomer membranes consistently exhibit a nanoscopic phase-separated morphology in which the ionic side chains and aqueous counterions segregate from the fluorocarbon backbone to form clusters or channels. Although these investigations do not unambiguously predict the size or shape of the clusters, and whether or not the channels percolate the matrix or if the connections between them are more transient, the sequence of co-monomers along the main chain appears strongly to influence the domain size of the ionic regions, with more blocky sequences giving rise to larger domain sizes. The fundamental insight that substantial rearrangement of the sulfonic acid terminated side chains and fluorocarbon backbone takes place during swelling or shrinkage is borne out by both molecular and mesoscale simulations of model PFSA polymers, along with ab initio electronic structure calculations of minimally hydrated oligomeric fragments. Molecular-level modelling of proton transport in PFSA membranes attests to the complexity of the underlying mechanisms and the need to examine the chemical and physical processes at several distinct time and length scales. These investigations have revealed that the conformation of the fluorocarbon backbone, flexibility of the sidechains, and degree of aggregation and association of the sulfonic acid groups under minimally hydrated conditions collectively control the dissociation of the protons and the formation of Zundel and Eigen cations. The former appear to be the dominant charge carriers when the limiting water content allows only for the formation of a contact ion pair with the tethered sulfonate anion. As the water content increases, solvent-separated Eigen ions begin to appear, indicating that the dominant mechanism for diffusion of protons occurs over a region approximately 4 A away from the sulfonate groups. Finally, both the vehicular and Grotthuss shuttling mechanisms contribute to the mobility of the protons but, surprisingly, they are not always correlated, resulting in a lower overall diffusion coefficient. In summary, as the preceding observations indicate, the state of computational modelling of PFSA membranes has progressed sufficiently over the last decade to enable its use as a powerful predictive tool with which to guide the process of designing novel membrane materials for fuel cell applications.     

A theoretical study on the frequency-dependent electric conductivity of electrolyte solutions

The theory on the ultrasonic absorption of electrolyte solutions we have proposed previously [T. Yamaguchi et al., J. Chem. Phys. 126, 144505 (2007)] is extended to calculate the frequency-dependent electric conductivity of the solution. The ionic contribution of the dielectric relaxation spectrum is obtained at the same time. The theory is able to handle the contributions of both the ion-pair dynamics and the relaxation of ionic atmosphere, as is the case of ultrasonic absorption. The effect of the barrier height between the contact and solvent-separated ion pairs is investigated in detail. It is clarified that the competition between the dissociation and reorientational relaxation rates of the contact ion pair is an important factor for the ion pair to be regarded as the ion pair in terms of ionic conductivity.     

Hydrated excess proton at water-hydrophobic interfaces

The behavior of the hydrated excess proton is investigated at the water-vapor, water-hydrophobic wall, and water-carbon tetrachloride interfaces through molecular dynamics simulations using the third-generation multistate empirical valence bond model (MS-EVB3). The MS-EVB3 simulations show a surface preference of the excess proton at the water-vapor interface, consistent with the discovery of this effect using an earlier version of the MS-EVB model (Petersen et al. J. Phys. Chem. B 2004, 108, 14804) and with the experimental results. The preference of the hydrated excess proton for other water-hydrophobic interfaces is also analyzed for the first time. The hydrated proton structures and charge defect delocalization effects at these interfaces are discussed in detail. By decomposing the free energy profiles into the internal energy and entropic contributions, the thermodynamic (free energy) driving forces for the surface preference of the excess proton are also elaborated. These results indicate that the "rigid" hydrated proton structures at all the interfaces are energetically (as opposed to entropically) stabilized due to the "amphiphilic" nature of the hydrated excess proton, resulting in its overall interfacial concentration enhancement. The effects of acid concentration and nuclear quantization are also explored.     

Proton solvation and transport in aqueous and biomolecular systems: insights from computer simulations

The excess proton in aqueous media plays a pivotal role in many fundamental chemical (e.g., acid-base chemistry) and biological (e.g., bioenergetics and enzyme catalysis) processes. Understanding the hydrated proton is, therefore, crucial for chemistry, biology, and materials sciences. Although well studied for over 200 years, excess proton solvation and transport remains to this day mysterious, surprising, and perhaps even misunderstood. In this feature article, various efforts to address this problem through computer modeling and simulation will be described. Applications of computer simulations to a number of important and interesting systems will be presented, highlighting the roles of charge delocalization and Grotthuss shuttling, a phenomenon unique in many ways to the excess proton in water.     

Molecular dynamics simulations of LiCl association and NaCl association in water by means of ABEEM/MM

Constrained molecular dynamics simulations have been used to investigate the LiCl and NaCl ionic association in water in terms of atom-bond electronegativity equalization method fused into molecular mechanics (ABEEM/MM). The simulations make use of the seven-site fluctuating charge and flexible ABEEM-7P water model, based on which an ion-water interaction potential has been constructed. The mean force and the potential of mean force for LiCl and NaCl in water, the charge distributions, as well as the structural and dynamical properties of contact ion pair dissociation have been investigated. The results are reasonable and informative. For LiCl ion pair in water, the solvent-separated ion pair configurations are more stable than contact ion pair configurations. The calculated PMF for NaCl in water indicates that contact ion pair and solvent-separated ion pair configurations are of comparable stability.     

Interaction of water with LiCl, LiBr, and LiI in the deeply supercooled region

The hydration mechanism of lithium halides was studied using time-of-flight secondary ion mass spectrometry as a function of temperature. The lithium halides embedded in thin films of amorphous solid water segregate to the surface at temperatures higher than 135-140 K, with efficiency increasing in the order of LiCl, LiBr, and LiI. A monolayer of LiCl and LiI adsorbed on the surface of amorphous solid water tends to diffuse into the bulk at 160 K. The infrared absorption band revealed that the aqueous lithium-halide solutions and crystals are formed simultaneously at 160 K; these phenomena are explicable as a consequence of the evolution of supercooled liquid water. The strong surfactant effect is inferred to arise from hydration of a contact ion pair having hydrophilic (lithium) and hydrophobic (halide) moieties. Furthermore, bulk diffusion of lithium halides might result from the formation of a solvent-separated ion pair in supercooled liquid water. The presence of two liquid phases of water with different local structures is probably responsible for the formation of these two hydrates, consistent with the calculated result reported by Jungwirth and Tobias[J. Phys. Chem. B 106, 6361 (2002)].     

Molecular modeling of proton transport in the short-side-chain perfluorosulfonic acid ionomer

An explanation for the superior proton conductivity of low equivalent weight (EW) short-side-chain (SSC) perfluorosulfonic acid membranes is pursued through the determination of hydrated morphology and hydronium ion diffusion coefficients using classical molecular dynamics (MD) simulations. A unique force field set for the SSC ionomer was derived from torsion profiles determined from ab initio electronic structure calculations of an oligomeric fragment consisting of two side chains. MD simulations were performed on a system consisting of a single macromolecule of the polymer (EW of 580) with the general formula F3C-[CF(OCF2CF2SO3H)-(CF2)7]40-CF3 at hydration levels corresponding to 3, 6, and 13 water molecules per sulfonic acid group. Examination of the hydrated morphology indicates the formation of hydrogen bond "bridges" between distant sulfonate groups without significant bending of the polytetrafluoroethylene backbone. Pair correlation functions of the system identify the presence of ion cages consisting of hydronium ions hydrogen-bonded to three sulfonate groups at the lowest water content. Such structures exhibit very low S-OH3+ separations, well below 4 A and severely inhibit vehicular diffusion of the protons. The number of sulfonate groups in the first solvation shell of a given hydronium ion correlates well with the differences between Nafion and the SSC polymer (Hyflon). The calculated hydronium ion diffusion coefficients of 2.84 x 10-7, 1.36 x 10-6, and 3.47 x 10-6 cm2/s for water contents of 3, 6, and 13, respectively, show only good agreement to experimentally measured values at the lowest water content, underscoring the increasing contribution of proton shuttling or hopping at the higher hydration levels. At the highest water content, the vehicular diffusion accounts for only about 1/5 of the total proton transport similar to that observed in Nafion.     

Rate constants for ion pair formation and dissociation in water

The reactive flux time correlation function for an associating ion pair in water is obtained by computer simulation. The transmission coefficient is found directly from the reactive flux and also through the method of absorbing barriers. Rate constants for the transitions between the contact and solvent-separated states of the ion pair are calculated.     

Mechanistic study of samarium diiodide-HMPA initiated 5-exo-trig Ketyl-Olefin coupling: the role of HMPA in post-electron transfer steps

The mechanistic importance of HMPA and proton donors (methanol, 2-methyl-2-propanol, and 2,2,2-trifluoroethanol) on SmI2-initiated 5-exo-trig ketyl-olefin cyclizations has been examined using stopped-flow spectrophotometric studies. In the presence of HMPA, the rate order of proton donors was zero and product studies showed that they had no impact on the diastereoselectivity of the reaction. Conversely, reactions were first-order in HMPA, and the additive displayed saturation kinetics at high concentrations. These results were consistent with HMPA being involved in a rate-limiting step before cyclization, where coordination of the intermediate ketyl to the sterically congested Sm(III)HMPA both stabilizes the intermediate and inhibits cyclization. Liberation of the contact ion pair through displacement by an equivalent of HMPA provides a solvent-separated ion pair releasing the steric constraint to ketyl-olefin cyclization. The mechanism derived from rate studies shows that HMPA is important not only in increasing the reduction potential of Sm(II) but also in enhancing the inherent reactivity of the radical anion intermediate formed after electron transfer through conversion of a sterically congested contact ion pair to a solvent-separated ion pair. The mechanistic complexity of the SmI2-HMPA-initiated ketyl-olefin cyclization is driven by the high affinity of HMPA for Sm(III), and these results suggest that simple empirical models describing the role of HMPA in more complex systems are likely to be fraught with a high degree of uncertainty.     

Parallel proton transfer pathways in aqueous acid-base reactions

We study the mechanism of proton transfer (PT) between the photoacid 8-hydroxy-1,3, 6-pyrenetrisulfonic acid (HPTS) and the base chloroacetate in aqueous solution. We investigate both proton and deuteron transfer reactions in solutions with base concentrations ranging from 0.25 M to 4 M. Using femtosecond midinfrared spectroscopy, we probe the vibrational responses of HPTS, its conjugate photobase, the hydrated proton/deuteron, and chloroacetate. The measurement of these four resonances allows us to follow the sequence of proton departure from the acid, its uptake by the water solvent, and its arrival at the base. In recent studies it was shown that proton transfer to carboxylate bases proceeds via Grotthuss conduction through a water wire connecting the acid and the base [Mohammed et al., Science 310, 83 (2005);Agnew. Chem. Int. Ed. 46, 1458 (2007);Siwick and Bakker, J. Am. Chem. Soc. 129, 13412 (2007); J. Phys. Chem. B 112, 378 (2008)]. Here we show that, for the weaker base chloroacetate, an alternative channel for proton transfer arises. In this channel the proton is first transferred to the water solvent and only later taken up from the water by the base. We study the base concentration dependence of the two competing channels.     

High Temperature Stability of Hydrated Ion Pairs Na<Superscript>+</Superscript>Cl<Superscript>–</Superscript>(H<Subscript>2</Subscript>O)<Subscript><Emphasis Type="Italic">N</Emphasis></Subscript> under Conditions of a Flat Nanopore

The high-temperature stability of hydrated ion pairs under conditions of a nanoscopic flat pore with hydrophobic structureless walls is studied by computer simulations. The limited space of the nanopore stimulates dissociation of the contact ion pair (CIP) with its transition to the state of the solvent-separated ion pair (SSIP); moreover, the ion pair demonstrates a high degree of stability on heating. The inverse temperature effect where the heating renders a moderate consolidating effect on the state of a hydrated contact ion pair is observed: when heated to the electrolyte boiling point, the free energy barrier that separates the CIP and SSIP states shifts by 2 molecules towards the larger hydration shells. On the pressure scale, the boundary between CIP and SSIP states shifts at the same rate as the saturating pressure with the increase in the temperature.     

Hydrogen bonding mediated ion pairs of some aprotic ionic liquids and their structural transition in aqueous solution

Ion pair speciation of ionic liquids(ILs) has an important effect on the physical and chemical properties of ILs and recognition of the structure of ion pairs in solution is essential. It has been reported that ion pairs of some ILs can be formed by hydrogen bonding interactions between cations and anions of them. Considering the fact that far-IR(FIR) spectroscopy is a powerful tool in indicating the intermolecular and intramolecular hydrogen bonding, in this work, this spectroscopic technique has been combined with molecular dynamic(MD) simulation and nuclear magnetic resonance hydrogen spectroscopy(~1H NMR) to investigate ion pairs of aprotic ILs [Bmim][NO_3], [BuPy][NO_3], [Pyr_(14)][NO_3], [PP_(14)][NO_3] and [Bu-choline][NO_3] in aqueous IL mixtures. The FIR spectra have been assigned with the aid of density functional theory(DFT) calculations, and the results are used to understand the effect of cationic nature on the structure of ion pairs. It is found that contact ion pairs formed in the neat aprotic ILs by hydrogen bonding interactions between cation and anion, were still maintained in aqueous solutions up to high water mole fraction(say 0.80 for [BuPy][NO3]). When water content was increased to a critical mole fraction of water(say 0.83 for [BuPy][NO3]), the contact ion pairs could be transformed into solvent-separated ion pairs due to the formation of the hydrogen bonding between ions and water. With the further dilution of the aqueous ILs solution, the solvent-separated ion pairs was finally turned into free cations and free anions(fully hydrated cations or anions). The concentrations of the ILs at which the contact ion pairs were transformed into solvent-separated ion pairs and solvent-separated ion pairs were transformed into free ions(fully hydrated ion) were dependent on the cationic structures. These information provides direct spectral evidence for ion pair structures of the aprotic ILs in aqueous solution. MD simulation and ~1H NMR results support the conclusion drawn from FIR spectra investigations.     

The influence of polar solvents on ions and ion pairs in the anionic polymerization of styrene

In the rather polar organic solvents dimethoxyethane, tetrahydrofuran and 3-methyltetrahydrofuran, the behaviour of contact ion pairs, solvent-separated ion pairs and free carbanions of polystyryl sodium has been investigated by kinetic and conductivity measurements. Both the equilibrium between the two kinds of ion pairs and the dissociation of solvent-separated ion pairs to free anions are followed over a wide range of temperature. Thereby, conditions can be found under which the polymerization takes place almost exclusively via one of the two types of ion pairs.The thermodynamic parameters of the equilibria and the Arrhenius parameters of the propagation rate constants of the different kinds of propagating chain ends are reported. The equilibria between these species are strongly influenced by the solvent whereas the individual propagation rate constants are scarcely affected by the solvent. The “effective charge distance” in the solvent-separated ion pair can be estimated from the corresponding dissociation constant. The mobility of the polymer carbanion is discussed.     

Influence of chemical composition and sequence length on the transport properties of proton exchange membranes

One of the integral parts of the fuel cell is the proton exchange membrane. Our research group has been engaged in the past few years in the synthesis of several sulfonated poly(arylene ether) random copolymers. The copolymers were varied in both the bisphenol structure as well as in the functional groups in the backbone such as sulfone and ketones. To compare the effect of sequence length, multiblock copolymers based on poly(arylene ether sulfone)s were synthesized. This paper aims to describe our investigation of the effect of chemical composition, morphology, and ion exchange capacity (IEC) on the transport properties of proton conducting membranes. The key properties examined were proton conductivity, methanol permeability, and water self diffusion coefficient in the membranes. It was observed that under fully hydrated conditions, proton conductivity for both random and block copolymers was a function of IEC and water uptake. However, under partially hydrated conditions, the block copolymers showed improved proton conductivity over the random copolymers. The proton conductivity for the block copolymer series was found to increase with increasing block lengths under partially hydrated conditions. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2226–2239, 2006     

Polarization effects and charge separation in AgCl-water clusters

Structural, energetic, vibrational, and electronic properties of salt ion pairs (AgCl and NaCl) in water (W) clusters were investigated by density functional theory. In agreement with recent theoretical studies of NaCl-water clusters, structures where the salt ion pair is separated by solvent molecules or solvent separated ion pair (SSIP) were found in AgCl-W(6) and AgCl-W(8) aggregates. Our results indicate that for small AgCl-water clusters, contact ion pair (CIP) structures are energetically more stable than SSIP, whereas an opposite tendency was observed for NaCl-water clusters. In comparison with CIP, SSIP are characterized by extensive electronic density reorganization, reflecting enhanced polarization effects. A major difference between AgCl-water and NaCl-water CIP aggregates concerns charge transfer. In AgCl-water CIP clusters, charge is transferred from the solvent (water) to the ion pair. However, in NaCl-water CIP clusters charge is transferred from the ion pair to the water molecules. The electronic density reorganization in the aggregates was also discussed through the analysis of electronic density difference isosurfaces. Time dependent density functional theory calculations show that upon complexation of AgCl and NaCl with water molecules, excitation energies are significantly blueshifted relative to the isolated ion pairs ( approximately 2 eV for AgCl-W(8) SSIP). In keeping with results for NaI-water clusters [Peslherbe et al., J. Phys. Chem. A 104, 4533 (2000)], electronic oscillator strengths of transitions to excited states are weaker for SSIP than for CIP structures. However, our results also suggest that the difference between excitation energies and oscillator strengths of CIP and SSIP structures may decrease with increasing cluster size.     

Types,Symbolism of Representation of Steric Structure,and Conformations of Ion Pairs. Stereochemical Version of the Generalized Rule of Elimination

Analysis of published data from the standpoint of the generalized rule of elimination demonstrated that in addition to contact and solvent-separated ion pairs, in elimination a species of a third type is generated, called spatially separated ion pair. This is an intermediate formed on the pathway of transformation of a contact ion pair to a solvent-separated one. Each of these ion pairs preserves its initial configuration of the bond C-nucleofuge starting from its origination to transformation into an elimination product, demonstrating discrete and inherent regio- and stereoselectivity: a contact ion pair shows nucleophilically controlled syn reactivity, and spatially- and solvent-separated ion pairs, electrophilically controlled syn and anti reactivity, respectively. The generalized rule of elimination allows almost faultless prediction of regio- and stereo- selectivity, being applicable to interpretation of even those published data which appear surprising or abnormal from the standpoint of the modern theoretical views.     

Further insights in the ability of classical nonadditive potentials to model actinide ion–water interactions

Pursuing our efforts on the development of accurate classical models to simulate radionuclides in complex environments (Réal et al., J. Phys. Chem. A 2010, 114, 15913; Trumm et al. J. Chem. Phys. 2012, 136, 044509), this article places a large emphasis on the discussion of the influence of models/parameters uncertainties on the computed structural, dynamical, and temporal properties. Two actinide test cases, trivalent curium and tetravalent thorium, have been studied with three different potential energy functions, which allow us to account for the polarization and charge‐transfer effects occurring in hydrated actinide ion systems. The first type of models considers only an additive energy term for modeling ion/water charge‐transfer effects, whereas the other two treat cooperative charge‐transfer interactions with two different analytical expressions. Model parameters are assigned to reproduce high‐level ab initio data concerning only hydrated ion species in gas phase. For the two types of cooperative charge‐transfer models, we define two sets of parameters allowing or not to cancel out possible errors inherent to the force field used to model water/water interactions at the ion vicinity. We define thus five different models to characterize the solvation of each ion. For both ions, our cooperative charge‐transfer models lead to close results in terms of structure in solution: the coordination number is included within 8 and 9, and the mean ion/water oxygen distances are 2.45 and 2.49 Å, respectively, for Th(IV) and Cm(III). © 2012 Wiley Periodicals, Inc.     

A comparative ab initio study of the primary hydration and proton dissociation of various imide and sulfonic acid ionomers

We compare the role of neighboring group substitutions on proton dissociation of hydrated acidic moieties suitable for proton exchange membranes through electronic structure calculations. Three pairs of ionomers containing similar electron withdrawing groups within the pair were chosen for the study: two fully fluorinated sulfonyl imides (CF(3)SO(2)NHSO(2)CF(3) and CF(3)CF(2)SO(2)NHSO(2)CF(3)), two partially fluorinated sulfonyl imides (CH(3)SO(2)NHSO(2)CF(3) and C(6)H(5)SO(2)NHSO(2)CF(2)CF(3)), and two aromatic sulfonic acid based materials (CH(3)C(6)H(4)SO(3)H and CH(3)OC(6)H(3)OCH(3)C(6)H(4)SO(3)H). Fully optimized counterpoise (CP) corrected geometries were obtained for each ionomer fragment with the inclusion of water molecules at the B3LYP/6-311G** level of density functional theory. Spontaneous proton dissociation was observed upon addition of three water molecules in each system, and the transition to a solvent-separated ion pair occurred when four water molecules were introduced. No considerable quantitative or qualitative differences in proton dissociation, hydrogen bond networks formed, or water binding energies were found between systems containing similar electron withdrawing groups. Each of the sulfonyl imide ionomers exhibited qualitatively similar results regarding proton dissociation and separation. The fully fluorinated sulfonyl imides, however, showed a greater propensity to exist in dissociated and ion-pair separated states at low degrees of hydration than the partially fluorinated sulfonyl imides. This effect is due to the additional electron withdrawing groups providing charge stabilization as the dissociated proton migrates away from the imide anion.