Eigen versus Zundel complexes in HCl-water mixtures

There is an ongoing debate on the nature of hydration of the hydrogen ion, H+ in solution, and the extent to which Eigen or Zundel complexes occur. Here, our previous neutron diffraction data on a solution of 1:9 HCl in water are reanalyzed using a new starting hypothesis for the Monte Carlo simulation of the data. Either bare H+ ions, all H3O+ ions, or all H5O2 + ions are allowed in the simulation box together with the water and chlorine ions. All three simulations give a satisfactory fit to the experimental data. From the simulation with simple H+ ions, it is found that all H+ ions form one strong and very short hydrogen bond with water molecules and that on average 75% of them also engage in a second, slightly longer hydrogen bond. This result can be interpreted alternatively either in terms of the formation of a high percentage of asymmetric Zundel complexes or in terms of the formation of distorted H3O+ ions, which in turn form two or three hydrogen bonds, respectively, with neighboring molecules opposite their unbonded hydrogen sites (thus forming Eigen complexes). Therefore the new analysis is not inconsistent with our previous conclusion that the solution consists primarily of Eigen complexes, but does highlight the difficulty of making a clear distinction between Eigen and Zundel complexes due to the continuous random network of hydrogen bonds formed between water and hydrated protons. The role of hydrogen ion to chloride counterion contacts is also discussed in these solutions.     

Ions in water: the microscopic structure of concentrated NaOH solutions

A neutron diffraction experiment with isotopic H/D substitution on four concentrated NaOH/H(2)O solutions is presented. The full set of partial structure factors is extracted, by combining the diffraction data with a Monte Carlo simulation. These allow to investigate both the changes of the water structure in the presence of ions and their solvation shells. It is found that the interaction with the solute affects the tetrahedral network of hydrogen bonded water molecules in a manner similar to the application of high pressure to pure water. The solvation shell of the OH(-) ions has an almost concentration independent structure, although with concentration dependent coordination numbers. The hydrogen site coordinates a water molecule through a weak bond, while the oxygen site forms strong hydrogen bonds with a number of molecules that is on the average very close to four at the higher water concentrations and decreases to about three at the lowest one. The competition between hydrogen bond interaction and Coulomb forces in determining the orientation of water molecules within the cation solvation shell is visible in the behavior of the g(NaHw)(r) function     

Be2+ hydration in concentrated aqueous solutions of BeCl2

Neutron diffraction experiments were carried out on concentrated aqueous solutions of beryllium chloride at three concentrations: 1.5, 3, and 6 molal. By working with a specific ("null") mixture of heavy water (D2O) and water (H2O), information on the local structure around Be2+ ions was extracted directly. For all three BeCl2 solutions, the results show that the Be2+ ion has a well-defined 4-fold coordination shell that is dominated by oxygen atoms. There is also a relatively small probability (10-15%) that there are direct contacts between Be2+ and Cl- at a distance of approximately 2.2 angstroms. The oxygen atoms of the highly structured Be2+ first hydration shell are found to be situated at 2.6 angstroms apart, and form a pyramidal structure, in agreement with recent MD simulation results. The Cl- ions have approximately seven oxygen atoms (water molecules) in their hydration shells sited at 3.2 angstroms.     

The electronic structure of the hydrated proton: a comparative X-ray absorption study of aqueous HCl and NaCl solutions

The oxygen K edge X-ray absorption spectra of aqueous HCl and NaCl solutions reveal distinct perturbations of the local water molecules by the respective solutes. While the addition of NaCl leads to large spectral changes, the effect of HCl on the observed X-ray absorption spectrum is surprisingly small. Density functional theory calculations suggest that this difference primarily reflects a strong blue shift of the hydrated proton (in either the Eigen (H9O4+) or Zundel (H2O5+) forms) spectrum relative to that of H2O, indicating the tighter binding of electrons in H3O+. This spectral shift counteracts the spectral changes that arise from direct electrostatic perturbation of water molecules in the first solvation shell of Cl-. Consequently, the observed spectral changes effected by HCl addition are minimal compared to those engendered by NaCl. Additionally, these results indicate that the effect of monovalent cations on the nature of the unoccupied orbitals of water molecules in the first solvation shell is negligible, in contrast to the large effects of monovalent anions.     

Ions in water: the microscopic structure of concentrated hydroxide solutions

Neutron-diffraction data on aqueous solutions of hydroxides, at solute concentrations ranging from 1 solute per 12 water molecules to 1 solute per 3 water molecules, are analyzed by means of a Monte Carlo simulation (empirical potential structure refinement), in order to determine the hydration shell of the OH- in the presence of the smaller alkali metal ions. It is demonstrated that the symmetry argument between H+ and OH- cannot be used, at least in the liquid phase at such high concentrations, for determining the hydroxide hydration shell. Water molecules in the hydration shell of K+ orient their dipole moment at about 45 degrees from the K+-water oxygen director, instead of radially as in the case of the Li+ and Na+ hydration shells. The K+-water oxygen radial distribution function shows a shallower first minimum compared to the other cation-water oxygen functions. The influence of the solutes on the water-water radial distribution functions is shown to have an effect on the water structure equivalent to an increase in the pressure of the water, depending on both ion concentration and ionic radius. The changes of the water structure in the presence of charged solutes and the differences among the hydration shells of the different cations are used to present a qualitative explanation of the observed cation mobility.     

An ab initio quantum mechanical charge field molecular dynamics simulation of a dilute aqueous HCl solution

An ab initio quantum mechanical charge field (QMCF) molecular dynamics simulation has been performed to study the structural and dynamical properties of a dilute aqueous HCl solution. The solute molecule HCl and its surrounding water molecules were treated at Hartree‐Fock level in conjunction with Dunning double‐ζ plus polarization function basis sets. The simulation predicts an average H? Cl bond distance of 1.28 Å, which is in good agreement with the experimental value. The H_HCl···O_w and Cl_HCl···H_w distances of 1.84 and 3.51 Å were found for the first hydration shell. At the hydrogen site of HCl, a single water molecule is the most preferred coordination, whereas an average coordination number of 12 water molecules of the full first shell was observed for the chloride site. The hydrogen bonding at the hydrogen site of HCl is weakened by proton transfer reactions and an associated lability of ligand binding. Two proton transfer processes were observed in the QMCF MD simulation, demonstrating acid dissociation of HCl. A weak structure‐making/breaking effect of HCl in water is recognized from the mean residence times of 2.1 and 0.8 ps for ligands in the neighborhood of Cl and H sites of HCl, respectively. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

The arrangement of first- and second-shell water molecules in trivalent aluminum complexes: results from density functional theory and structural crystallography

The structural and energetic features of a variety of gas-phase aluminum ion hydrates containing up to 18 water molecules have been studied computationally using density functional theory. Comparisons are made with experimental data from neutron diffraction studies of aluminum-containing crystal structures listed in the Cambridge Structural Database. Computational studies indicate that the hexahydrated structure Al[H(2)O](6)(3+) (with symmetry T(h)()), in which all six water molecules are located in the innermost coordination shell, is lower in energy than that of Al[H(2)O](5)(3+).[H(2)O], where only five water molecules are in the inner shell and one water molecule is in the second shell. The analogous complex with four water molecules in the inner shell and two in the outer shell undergoes spontaneous proton transfer during the optimization to give [Al[H(2)O](2)[OH](2)](+).[H(3)O(+)](2), which is lower in energy than Al[H(2)O](6)(3+); this finding of H(3)O(+) is consistent with the acidity of concentrated Al(3+) solutions. Since, however, Al[H(2)O](6)(3+) is detected in solutions of Al(3+), additional water molecules are presumed to stabilize the hexa-aquo Al(3+) cation. Three models of a trivalent aluminum ion complex surrounded by a total of 18 water molecules arranged in a first shell containing 6 water molecules and a second shell of 12 water molecules are discussed. We find that a model with S(6) symmetry for which the Al[H(2)O](6)(3+) unit remains essentially octahedral and participates in an integrated hydrogen bonded network with the 12 outer-shell water molecules is lowest in energy. Interactions between the 12 second-shell water molecules and the trivalent aluminum ion in Al[H(2)O](6)(3+) do not appear to be sufficiently strong to orient the dipole moments of these second-shell water molecules toward the Al(3+) ion.     

Ab initio QM/MM dynamics of H3O+ in water

A molecular dynamics (MD) simulation based on a combined ab initio quantum mechanics/molecular mechanics (QM/MM) method has been performed to investigate the solvation structure and dynamics of H3O+ in water. The QM region is a sphere around the central H3O+ ion, and contains about 6-8 water molecules. It is treated at the Hartree-Fock (HF) level, while the rest of the system is described by means of classical pair potentials. The Eigen complex (H9O4+) is found to be the most prevalent species in the aqueous solution, partly due to the selection scheme of the center of the QM region. The QM/MM results show that the Eigen complex frequently converts back and forth into the Zundel (H5O2+) structure. Besides the three nearest-neighbor water molecules directly hydrogen-bonded to H3O+, other neighbor waters, such as a fourth water molecule which interacts preferentially with the oxygen atom of the hydronium ion, are found occasionally near the ion. Analyses of the water exchange processes and the mean residence times of water molecules in the ion's hydration shell indicate that such next-nearest neighbor water molecules participate in the rearrangement of the hydrogen bond network during fluctuative formation of the Zundel ion and, thus, contribute to the Grotthuss transport of the proton.     

Aqueous solutions of divalent chlorides: ions hydration shell and water structure

By combining neutron diffraction and Monte Carlo simulations, we have determined the microscopic structure of the hydration ions shell in aqueous solutions of MgCl(2) and CaCl(2), along with the radial distribution functions of the solvent. In particular the hydration shell of the cations, show cation specific symmetry, due to the strong and directional interaction of ions and water oxygens. The ions and their hydration shells likely form molecular moieties and bring clear signatures in the water-water radial distribution functions. Apart from these signatures, the influence of divalent salts on the microscopic structure of water is similar to that of previously investigated monovalent solutes, and it is visible as a shift of the second peak of the oxygen-oxygen radial distribution function, caused by distortion of the hydrogen bond network of water.     

Collisions of DCl with pure and salty glycerol: enhancement of interfacial D --> H exchange by dissolved NaI

The fate of DCl molecules striking pure glycerol and a 2.6 M NaI-glycerol solution is investigated using scattering, uptake, and residence time measurements. We find that dissolved Na+ and I- ions alter every gas-liquid pathway from the moment of contact of DCl with the surface to its eventual emergence as HCl. In particular, the salt enhances both trapping-desorption of DCl and interfacial DCl --> HCl exchange at the expense of DCl entry into the bulk solution. The reduced entry and enhanced desorption of thermalized DCl molecules are interpreted by assuming that Na+ and I- ions bind to interfacial OH groups and tie up surface sites that would otherwise capture incoming DCl molecules. These ion-glycerol interactions may also be responsible for enhancing interfacial D --> H exchange by disrupting the interfacial hydrogen bond network that carries the newly formed H+ ion away from its Cl- pair. This disruption may increase the fraction of interfacial Cl- and H+ that recombine and desorb immediately as HCl before the ions separate and diffuse deeply into the bulk.     

Fluctuating hydration structure around nanometer-size hydrophobic solutes. I. Caging and drying around C60 and C60H60 spheres

The hydration structure around nanometer-size hydrophobic solutes is studied with molecular dynamics simulation by taking aqueous solutions of C60 and C60H60 as examples. In the hydration shell around a single C60 or C60H60, dipoles of simulated water molecules tend to be aligned to form the vortexlike coherent pattern which lasts for 100 ps, while individual water molecules stay within the hydration shell for about 10 ps. This structural pattern organized by fluctuating and diffusively moving molecules should be called a "fluctuating cage". In the narrow region between a pair of C60 molecules or a pair of C60H60 molecules, water density strongly fluctuates and is correlated to the mean force between solutes. The fluctuating caging and drying between solutes affect the hydrophobic interaction and dynamical behaviors of solutes.     

An interaction model for OH + H2O-mixed and pure H2O overlayers adsorbed on Pt(111)

A model potential for the adsorbate-adsorbate interaction among OH and H2O molecules adsorbed on a Pt(111) surface has been developed solely based on first-principle calculations. By combining this directional-dependent model potential for the lateral interaction with a lattice model of Ising type, large length scale structure calculations can be made. The strength of different hydrogen bonds can be analyzed in detail from this model potential. It is found that the hydrogen bond between OH and H2O molecules is stronger than that between two H2O molecules (0.4 eV per pair as compared to 0.2 eV per pair, respectively). Via the computed chemical potential for water in mixed OH + H2O overlayers the water uptake as a function of oxygen precoverage on Pt(111) has been determined. The results compare very well with recent experiments.     

Microscopic probing of the size dependence in hydrophobic solvation

We report small angle x-ray scattering data demonstrating the direct experimental microscopic observation of the small-to-large crossover behavior of hydrophobic effects in hydrophobic solvation. By increasing the side chain length of amphiphilic tetraalkyl-ammonium (C(n)H(2n+1))(4)N(+) (R(4)N(+)) cations in aqueous solution we observe diffraction peaks indicating association between cations at a solute size between 4.4 and 5 A?, which show temperature dependence dominated by hydrophobic attraction. Using O K-edge x-ray absorption we show that small solutes affect hydrogen bonding in water similar to a temperature decrease, while large solutes affect water similar to a temperature increase. Molecular dynamics simulations support, and provide further insight into, the origin of the experimental observations.     

Hydration of sodium, potassium, and chloride ions in solution and the concept of structure maker/breaker

Neutron diffraction data with hydrogen isotope substitution on aqueous solutions of NaCl and KCl at concentrations ranging from high dilution to near-saturation are analyzed using the Empirical Potential Structure Refinement technique. Information on both the ion hydration shells and the microscopic structure of the solvent is extracted. Apart from obvious effects due to the different radii of the three ions investigated, it is found that water molecules in the hydration shell of K+ are orientationally more disordered than those hydrating a Na+ ion and are inclined to orient their dipole moments tangentially to the hydration sphere. Cl- ions form instead hydrogen-bonded bridges with water molecules and are readily accommodated into the H-bond network of water. The results are used to show that concepts such as structure maker/breaker, largely based on thermodynamic data, are not helpful in understanding how these ions interact with water at the molecular level.     

Chlorine anion encapsulation by molecular capsules based on cucurbit[5]uril and decamethylcucurbit[5]uril

Three barrel-shaped artificial molecular capsules 1-3, based on normal cucurbit[5]uril (Q[5]) and decamethylcucurbit[5]uril (Me10Q[5]), were synthesized and structurally characterized by single-crystal X-ray diffraction. Encapsulation of a chlorine anion in the cavity of a Q[5] or Me10Q[5] to form closed a molecular capsule with the coordinated metal ions or coordinated metal ions and water molecules in the crystal structures of these compounds is common. The three complexes [Pr2(C30H30N20O10)Cl3(H2O)13]3+ 3 Cl- x 5 H2O (1), [Sr2(C40H50N20O10)(H2O)4Cl]3+ 3 Cl- x 2 (HCl) 19 H2O (2) and [K(C40H50N20O10)(H2O)Cl] x [Zn(H2O)2Cl2] x [ZnCl4]2- x 2 (H3O)+ x 8 H2O (3) all crystallize as isolated molecular capsules.     

Effect of ionic solutes on the hydrogen bond network dynamics of water: power spectral analysis of aqueous NaCl solutions

To understand the modifications of the hydrogen bond network of water by ionic solutes, power spectra as well as static distributions of the potential energies of tagged solvent molecules and solute ions have been computed from molecular dynamics simulations of aqueous NaCl solutions. The key power spectral features of interest are the presence of high-frequency peaks due to localized vibrational modes, the existence of a multiple time scale or 1/falpha frequency regime characteristic of networked liquids, and the frequency of crossover from 1/falpha type behavior to white noise. Hydrophilic solutes, such as the sodium cation and the chloride anion, are shown to mirror the multiple time scale behavior of the hydrogen bond network fluctuations, unlike hydrophobic solutes which display essentially white noise spectra. While the power spectra associated with tagged H2O molecules are not very sensitive to concentration in the intermediate frequency 1/falpha regime, the crossover to white noise is shifted to lower frequencies on going from pure solvent to aqueous alkali halide solutions. This suggests that new and relatively slow time scales enter the picture, possibly associated with processes such as migration of water molecules from the hydration shell to the bulk or conversion of contact ion pairs into solvent-separated ion pairs which translate into variations in equilibrium transport properties of salt solutions with concentration. For anions, cations, and solvent molecules, the trends in the alpha exponents of the multiple time scale region and the self-diffusivities are found to be strongly correlated.     

DFT:B3LYP ab initio molecular dynamics study of the Zundel and Eigen proton complexes, H5O2+ and H9O4+, in the triplet state in gas phase and solution

DFT:B3LYP ab initio molecular dynamics (MD) approach is used to elucidate the properties of the Zundel and Eigen, H5O2+ and H9O4+, proton complexes in the triplet state. The simulation considers the complexes in the gas phase (isolated complexes) and inside the clusters composed of 32, 64, and 128 water molecules, mimicking the behavior of aqueous solutions. MD simulations reveal three distinct periods. For the complex in solutions, the periods are smoothed out. The H5O2+ and H9O4+ complexes in the triplet state undergo structural rearrangements, which eventually result in hydrogen elimination. For the H5O2+, the hydrogen is eliminated from the center of the water cluster, whereas for the H9O4+ it is removed from a near-surface water molecule. The rate of hydrogen elimination decreases with increasing the number of water molecules surrounding the complex.     

Is proton cationization promoted by polyatomic primary ion bombardment during time-of-flight secondary ion mass spectrometry analysis of frozen aqueous solutions?

Ion bombardment of pure water ice by Au+ monoatomic and Au3 + and C60 + polyatomic projectiles results in the emission of two series of water cluster ions-(H2O)n + and (H2O)nH+-with n ranging from 1 to >40. The cluster ion yields are very significantly higher under polyatomic ion bombardment than when using an Au+ primary ion. The yield of the protonated water species (H2O)nH+ is found to be enhanced by increasing ion fluence. C60 + bombardment results in a very dramatic increase in the (H2O)nH+ yield and decrease in the yield of (H2O)n +. Au3 + also significantly increased the yield of protonated species relative to the non-protonated but to a lesser extent than C60 +. Bombardment by Au+ also increased the yield of protonated species but to a very much smaller extent. The hypothesis that the protonated species may enhance the yield of [M+H]+ from solute molecules in solution has been investigated using two amino acids, alanine and arginine, and a nucleic base, adenine. The data suggest that the protons produced by the sputtering of water ice are depleted in the presence of these solutes and concurrently the yields of solute-related [M+H]+ and immonium secondary ions are greatly enhanced. These yield enhancements are analysed in the light of other possible contributors such as increased rates of sputtering under polyatomic beams and increased secondary ion yields as a consequence of solute dispersion. It is concluded that enhanced proton attachment is occurring in polyatomic sputtered frozen aqueous solutions.     

Hydrogen Bonding in Liquid Water and in the Hydration Shell of Salts

A near‐IR spectral study on pure water and aqueous salt solutions is used to investigate stoichiometric concentrations of different types of hydrogen‐bonded water species in liquid water and in water comprising the hydration shell of salts. Analysis of the thermodynamics of hydrogen‐bond formation signifies that hydrogen‐bond making and breaking processes are dominated by enthalpy with non‐negligible heat capacity effects, as revealed by the temperature dependence of standard molar enthalpies of hydrogen‐bond formation and from analysis of the linear enthalpy–entropy compensation effects. A generalized method is proposed for the simultaneous calculation of the spectrum of water in the hydration shell and hydration number of solutes. Resolved spectra of water in the hydration shell of different salts clearly differentiate hydrogen bonding of water in the hydration shell around cations and anions. A comparison of resolved liquid water spectra and resolved hydration‐shell spectra of ions highlights that the ordering of absorption frequencies of different kinds of hydrogen‐bonded water species is also preserved in the bound state with significant changes in band position, band width, and band intensity because of the polarization of water molecules in the vicinity of ions.     

Hydration of the Calcium Dication: Direct Evidence for Second Shell Formation from Infrared Spectroscopy

Infrared laser action spectroscopy in a Fourier‐transform ion cyclotron resonance mass spectrometer is used in conjunction with ab initio calculations to investigate doubly charged, hydrated clusters of calcium formed by electrospray ionization. Six water molecules coordinate directly to the calcium dication, whereas the seventh water molecule is incorporated into a second solvation shell. Spectral features indicate the presence of multiple structures of Ca(H₂O)₇²⁺ in which outer‐shell water molecules accept either one (single acceptor) or two (double acceptor) hydrogen bonds from inner‐shell water molecules. Double‐acceptor water molecules are predominately observed in the second solvent shells of clusters containing eight or nine water molecules. Increased hydration results in spectroscopic signatures consistent with additional second‐shell water molecules, particularly the appearance of inner‐shell water molecules that donate two hydrogen bonds (double donor) to the second solvent shell. This is the first reported use of infrared spectroscopy to investigate shell structure of a hydrated multiply charged cation in the gas phase and illustrates the effectiveness of this method to probe the structures of hydrated ions.