Water Molecules In Solvation Shell Research Articles

Switching of Redox Levels Leads to High Reductive Stability in Water-in-Salt Electrolytes.

Developing nonflammable electrolytes with wide electrochemical windows is of great importance for energy storage devices. Water-in-salt electrolytes (WiSE) have attracted great interests due to their widely opened electrochemical windows and high stability. Previous theoretical investigations have revealed changes in solvation shell of water molecules result in opening of HOMO-LUMO gaps of water, leading to the formation of an anion-derived solid-electrolyte-interphase (SEI) in WiSE. However, how solvation structures affect electrochemical windows at atomic level is still a puzzle, which hinders optimization and design of aqueous electrolytes. Herein, machine learning molecular dynamics and free energy calculation method are applied to compute redox potentials of anions of Li-salts and water of aqueous electrolytes at a range of salt concentrations. Furthermore, an analysis based on local solvation structures is employed to demonstrate the structure-property relations. Our calculation shows that the hydrogen evolution reaction is impeded in WiSE due to switching of the order of redox levels of the anion and H2O, leading to formation of SEI and high reductive stability. Level switching is caused by the special solvation environments of isolated water molecules. Our work provides new insight into the electrochemistry of aqueous electrolytes which would benefit the electrolyte design in energy storage devices.

Structure and rotational dynamics of water around hydrogen peroxide

The density functional theory-based molecular dynamics simulations were induced to study various properties of water around different solvation pockets around hydrogen peroxide(H2O2). Our study aims to understand water behavior in the vicinity of H2O2 molecule through analyzing structure, hydrogen bond statistics, orientational and non-diffusive jump profile. A water-like peroxide molecule can form four hydrogen bonds with nearby water molecules: two as a proton donor and two as a proton acceptor, which must disturb the local structure and dynamics of the surrounding water. The presence of two different peak maxima in radial distribution function (RDF) allows us to resolve the solvation environment into three solvation sites: solvation shell I, solvation shell II, and bulk. The probability distribution of the average distance of central water molecule from 4 nearest water molecules shows a shift in the peak for solvation shell I water molecules towards a ∼0.3Å higher value compared to two other solvation shells. This further affects tetrahedral order parameters as water molecules shift from the regular tetrahedron. For the first solvation layer, the water molecules lose their tetrahedrality to a significant extent as qtet probability distribution peaks at 0.5 and broaden in the higher values of qtet without any shoulder. For the second solvation layer, we find the qtet peak at 0.87 with a minor shoulder appearing at qtet = 0.55, a sign of slight loss of tetrahedrality in the unstructured water compared to undisturbed water. Loss of tetrahedrality hints at the change of orientation of H2O molecules in the vicinity of H2O2 and affects the water-water hydrogen bond (HB) formation. A noteworthy reduction in the HB donor/acceptor account and per water HB count shows interference of H2O2. The water molecules prefer H2O2 compared to neighboring water molecules for HB formation. The orientation autocorrelation function provides the rotational pace of water molecules in the different solvation layers. The reorientation time constant difference in the solvation shells I and II fluctuate by mere 0.20 ps, showcasing the trivial nature of hydrogen peroxide and water HBs. Using the HB descriptors, we also report the non-diffusive jump profile involving vicinal water molecules of H2O2. The jump angle profile of solvation shell water molecules is in line with bulk water.

Bridging the 12-6-4 Model and the Fluctuating Charge Model.

Metal ions play important roles in various biological systems. Molecular dynamics (MD) using classical force field has become a popular research tool to study biological systems at the atomic level. However, meaningful MD simulations require reliable models and parameters. Previously we showed that the 12-6 Lennard-Jones nonbonded model for ions could not reproduce the experimental hydration free energy (HFE) and ion-oxygen distance (IOD) values simultaneously when ion has a charge of +2 or higher. We discussed that this deficiency arises from the overlook of the ion-induced dipole interaction in the 12-6 model, and this term is proportional to 1/r 4 based on theory. Hence, we developed the 12-6-4 model and showed it could solve this deficiency in a physically meaningful way. However, our previous research also found that the 12-6-4 model overestimated the coordination numbers (CNs) for some highly charged metal ions. And we attributed this artifact to that the current 12-6-4 scheme lacks a correction for the interactions among the first solvation shell water molecules. In the present study, we considered the ion-included dipole interaction by using the 12-6 model with adjusting the atomic charges of the first solvation shell water molecules. This strategy not only considers the ion-induced dipole interaction between ion and the first solvation shell water molecules but also well accounts for the increased repulsion among these water molecules compared to the bulk water molecules. We showed this strategy could well reproduce the experimental HFE and IOD values for Mg2+, Zn2+, Al3+, Fe3+, and In3+ and solve the CN overestimation issue of the 12-6-4 model for Fe3+ and In3+. Moreover, our simulation results showed good agreement with previous ab initio MD simulations. In addition, we derived the physical relationship between the C 4 parameter and induced dipole moment, which agreed well with our simulation results. Finally, we discussed the implications of the present work for simulating metalloproteins. Due to the fluctuating charge model uses a similar concept to the 12-6 model with adjusting atomic charges, we believe the present study builds a bridge between the 12-6-4 model and the fluctuating charge model.

Transfer free energy of micro-hydrated ion clusters from water into acetonitrile solvent

The transfer free energies of micro-hydrated alkali ions (Li+, Na+, K+, and Cs+) and halide ions (F-, Cl-, Br-, and I-) clusters from bulk water to bulk acetonitrile are investigated within the framework of reaction density functional theory (RxDFT), in which the formation of micro-hydrated ion clusters containing one to four water molecules is treated as associated reaction. By analyzing the free energy of hydration and solvation, we show that the micro-hydrated ion clusters, [M(H2O)n]+, exist stably in both water and acetonitrile and a larger percentage of the total electrostatic interactions of ions is contributed by their first solvation shell water molecules. For the transfer free energy of bare ions, the calculated results agree well with the values of experimental observation. The transfer free energy significantly decreases as the size of bare ions and the number of water molecules in the cluster increase. This dependence is stronger for small ions than for large ions. Moreover, the lowest transfer free energy is correspond to the different coordinated number of water molecule (Nw). For Li+, Na+, F-, Cl-, Br-, and I- ion, the lowest transfer free energy are correspond to Nw = 4, while for K+ and Cs+ ion, Nw with the lowest transfer free energy are 3 and 2, respectively. Our findings provide not only the framework to predict transfer free energy but also theoretical guidance for solvent extraction, ion-selective electrodes, biological ion channels, and phase transfer catalysis.

Importance of nuclear quantum effects on the hydration of chloride ion

Path-integral ab initio molecular dynamics (PI-AIMD) calculations have been employed to probe the nature of chloride ion solvation in aqueous solution. Nuclear quantum effects (NQEs) are shown to weaken hydrogen bonding between the chloride anion and the solvation shell of water molecules. As a consequence, the disruptive effect of the anion on the solvent water structure is significantly reduced compared to what is found in the absence of NQEs. The chloride hydration structure obtained from PI-AIMD agrees well with information extracted from neutron scattering data. Inparticular, the observed satellite peak in the hydrogen-chloride-hydrogen triple angular distribution serves as a clear signature of NQEs. The present results suggest that NQEs are likely to play acrucial role in determining the structure of saline solutions.

Aqueous hydroxyl group as the vibrational probe to access the hydrophobicity of amide derivatives

First principles molecular dynamics (FPMD) simulations of relatively dilute aqueous solutions of N-methylformamide (NMF), N, N-dimethylacetamide (DMA), N-methylacetamide (NMA) were carried out to elucidate the effects of variation of hydrophobicity of amide molecules on structure, dynamics and spectral properties of solvating water molecules. A quantitative analysis of dangling OH groups of water molecules, one of the consequences of hydrophobicity, around the amide molecules was performed to explore the structure of surrounding water molecules. We observe that DMA has the most hydrophobic character among the three amide molecules; the lifetime of dangling OH mode of water molecules and the number of such modes are found to be more inside the solvation shell of DMA as compared to NMA and NMF molecules. Overall this lifetime is more inside the solvation shell of amides compared to the bulk water molecules for all aqueous solutions of amides. Rotational dynamics calculation suggests significant retardation of OH bonds of water near the amide oxygen atom (O) due to the OwHw⋯O strong hydrogen bonds. A moderate slowdown of reorientational dynamics is also observed for OH modes that are close to the hydrophobic surface of DMA in its aqueous solution. Vibrational density of states (VDOS) and frequency distribution calculations point out the higher average stretching frequency (~3600–3850 cm−1) of free OH groups. Hydrogen bond lifetime calculations conceive that DMA, having three methyl groups, makes stronger hydrogen bonds through the CO moiety. Vibrational spectral diffusion of bulk water and solvation shell water molecules were also calculated in combination with wavelet transform and frequency-frequency autocorrelation functions. The CO group affected OH stretching frequency bands for aqueous solutions of NMF and NMA molecules are more pronounced than that of DMA. However, NH affected OH frequency bands are less pronounced for NMA and DMA due to the presence of more number of hydrophobic methyl groups. Going from NMF to DMA, the OH frequency bands inside the amide solvation shells shift towards the higher value due to the enhancement of hydrophobicity. The vibrational spectral diffusion of OH modes around CO and NH (N for DMA) groups, as well as for bulk water molecules, is also investigated. Three time scales were found for these calculations for all the cases. The fast time scales in the range ~50–100 fs are due to the amide-water intact hydrogen bonding, and two slower time scales in the range ~0.6–3.90 ps and ~10–20 ps were found. The time scales in the range of ~0.6 to 3.90 ps can be attributed to carbonyl-water and N-H-water hydrogen bonding, and the very long time scales are due to the escape dynamics water molecules from the solvation shell of CO and NH (N for DMA) groups.

Vibration Spectral Dynamics of Weakly Coordinating Water Molecules near an Anion: FPMD Simulations of an Aqueous Solution of Tetrafluoroborate.

The extent to which the ions affect the nearby water molecules will decide the structure-making or breaking nature of those ions in aqueous solutions. The effects of a weakly coordinating anion on the structure, dynamics, and vibrational properties of water molecules are not so significant as compared to an anion capable of making strong ion-water hydrogen bonds. The present work deals with the first-principles molecular dynamics study of an aqueous solution of such a weakly coordinating anion, tetrafluoroborate (BF4-), using dispersion-corrected DFT-based first-principles molecular dynamics (FPMD) simulations. Various structural, dynamical, and spectral properties, such as radial distribution functions (RDFs), rotational dynamics, vibrational density of states (VDOS), hydrogen bond as well as dangling OH autocorrelation functions, and residence dynamics, were calculated to investigate the effects of the anion on nearby water molecules. The process of spectral diffusion was assessed through a time series wavelet transformation of trajectories obtained from FPMD simulations. The first ion-water solvation shell extends up to 5.5 Å, containing around 20 water molecules. The lifetime of the ion-water hydrogen bond is found to be 1.19 ps, whereas the water-water hydrogen bond lifetime is found to be 1.13 ps. Inside the solvation shell, the persistence time of dangling OH chromophores and the average frequency of OH modes inside the solvation shell are found to be more compared to bulk. Three time scales are found for solvation shell OH modes from the frequency-frequency correlation function. A very short time scale is found for the intact ion-water interaction; the short time scale is for the ion-water hydrogen bond, and the long time scale is for escape dynamics of water molecules from the ion solvation shell. From the mean squared displacement, it is found that solvation water molecules diffuse slower than the bulk. However, solvation shell water molecules show faster relaxation from the analysis of rotational anisotropy. Within the longer time scale of spectral diffusion, this process (which is related to various dynamics of the molecules) is not yet complete, as compared to fast anisotropic decay. This fact is similar to the experimental finding of spectral diffusion and anisotropy time scales in the aqueous solution of borohydride anion. The calculated results are also compared with available experimental data wherever possible.

Understanding the electrochemical double layer at the hematite/water interface: A first principles molecular dynamics study.

Using first principles molecular dynamics simulations, we probe the electrochemical double layer formed at the interface between the hematite surface and water. We consider two terminations of the (001) surface, viz., the fully hydroxylated (OH) and the stoichiometric (FeO3Fe) termination. We explicitly incorporate the counterions (Na+ and F-) in the solution, and model both specific and nonspecific adsorption of F- ions. We find that F- ions prefer to bind directly to the Fe ions (specific adsorption), with a substantial energy gain (0.75 eV/ion). We investigate the effect of the interface and the counterions on the dipole of individual water molecules. We find significant deviations of +0.2/-0.15 D for dipoles of the first solvation shell water molecules of F-/Na+ ions, respectively. Additionally, the hydration layers at the interface show an enhancement in the dipole moment resulting from stronger hydrogen bonding interactions between the water molecules and surface charged species. Furthermore, we analyze the electrostatic potential profile at the solid/liquid interface as a function of the kind of counterion present in the double layer and compute the capacitance of the compact (Helmholtz) layer. We find that our results (40.3 ± 3.5 μF/cm2 for the OH termination and 51 ± 5 μF/cm2 for the FeO3Fe termination) compare favorably with values reported by potentiometric titration based experimental studies (10-100 μF/cm2).

Ab Initio Molecular Dynamics Simulation of the Phosphate Ion in Water: Insights into Solvation Shell Structure, Dynamics, and Kosmotropic Activity.

The structure and dynamics of solvation shells of the phosphate ion in deuterated water are studied by means of Born-Oppenheimer molecular dynamics simulation. The total number of molecules in the first and second solvation shells is found to be close to the effective hydration number reported experimentally. The OD bonds that are hydrogen bonded to the phosphate ion are found to be red shifted as compared to bulk water, which is consistent with experimental results. However, the two OD bonds of the same water molecule in the first hydration shell are found to be vibrationally distinct, which can be attributed to different strengths of the ion-water and water-water hydrogen bonds near the ion. Also, the hydrogen bonds formed by the second solvation shell OD bonds are somewhat stronger than the bulk. This finding shows a long ranged effect of the phosphate ion on water and also gives insights into the water structure making property of this anion. The dynamics of water in the first solvation shell is found to be significantly slower than that of the bulk. We have investigated the origin of the orientational slowing down of the first solvation shell water molecules and made connections to similar results observed experimentally.

Rejection mechanisms for contaminants in polyamide reverse osmosis membranes

Despite the success of reverse osmosis (RO) for water purification, the molecular-level physico-chemical processes of contaminant rejection are not well understood. Here we carry out non-equilibrium molecular dynamics (NEMD) simulations on a model polyamide RO membrane to understand the mechanisms of transport and rejection of both ionic and inorganic contaminants in water. While it is commonly presumed that the contaminant rejection rate is correlated with the dehydrated solute size, this approximation does not hold for the organic solutes and ions studied. In particular, the rejection of urea (2.4Å radius) is higher than ethanol (2.6Å radius), and the rejections for organic solutes (2.2–2.8Å radius) are lower than Na+ (1.4Å radius) or Cl− (2.3Å radius). We show that this can be explained in terms of the solute accessible intermolecular volume in the membrane and the solute-water pair interaction energy. If the smallest open spaces in the membrane's molecular structure are all larger than the solute size including its hydration shell, then the solute-water pair interaction energy does not matter. However, when the open spaces in the polymeric structure are such that solutes have to shed at least one water molecule to pass through a portion of the membrane molecular structure, as occurs in RO membranes, the pair interaction energy governs solute rejection. The high pair interaction energy for water molecules in the solvation shell for ions makes the water molecules difficult to shed, thus enhancing the rejection of ions. On the other hand, the organic solute-water interaction energies are governed by the water molecules that are hydrogen bonded to the solute. While these hydrogen bonds have pair interaction energies that are much larger than that of the non-hydrogen bonded water molecules in the solute solvation shell, they are significantly less than the ion-water pair interaction energy. Thus, organic solutes more easily shed water molecules than ions to pass through the RO membrane. Since urea molecules have more hydrogen-bonding sites than alcohol molecules, urea forms more hydrogen bonds with the membrane polymer chains leading to a higher rejection of urea than occurs for ethanol, a molecule of similar size but with fewer hydrogen bonding sites. These findings underline the importance of the solute's solvation shell and solute-water-membrane chemistry in the context of reverse osmosis, thus providing new insights into solute transport and rejection in RO membranes.

Empirical force field for cisplatin based on quantum dynamics data: case study of new parameterization scheme for coordination compounds.

Parameterization of molecular complexes containing a metallic compound, such as cisplatin, is challenging due to the unconventional coordination nature of the bonds which involve platinum atoms. In this work, we develop a new methodology of parameterization for such compounds based on quantum dynamics (QD) calculations. We show that the coordination bonds and angles are more flexible than in normal covalent compounds. The influence of explicit solvent is also shown to be crucial to determine the flexibility of cisplatin in quantum dynamics simulations. Two empirical topologies of cisplatin were produced by fitting its atomic fluctuations against QD in vacuum and QD with explicit first solvation shell of water molecules respectively. A third topology built in a standard way from the static optimized structure was used for comparison. The later one leads to an excessively rigid molecule and exhibits much smaller fluctuations of the bonds and angles than QD reveals. It is shown that accounting for the high flexibility of cisplatin molecule is needed for adequate description of its first hydration shell. MD simulations with flexible QD-based topology also reveal a significant decrease of the barrier of passive diffusion of cisplatin accross the model lipid bilayer. These results confirm that flexibility of organometallic compounds is an important feature to be considered in classical molecular dynamics topologies. Proposed methodology based on QD simulations provides a systematic way of building such topologies.

Molecular dynamics simulation study of distribution and dynamics of aqueous solutions of uranyl ions: the effect of varying temperature and concentration.

Investigating the characteristics of actinyl ions has been of great interest due to their direct relevance in the nuclear fuel cycle. All-atom molecular dynamics simulations have been employed to study the orientational structure and dynamics of aqueous solutions of uranyl ions of various concentrations. The orientational structure of water around a uranyl ion has been thoroughly investigated by calculating different orientational probability distributions corresponding to different molecular axes of water. The orientational distribution of water molecules in the first coordination shell of a uranyl ion is found to be markedly different from that in bulk water. Analysis of counterion distribution around the uranyl ion reveals the presence of nitrate ions along with water molecules in the first solvation shell. From the comparison of the number of coordinated water and nitrate ions at various uranyl nitrate concentrations, it is evident that these two species compete for occupying the first solvation shell of the uranyl ion. Orientational dynamics of water molecules about different molecular axes of water in the vicinity of uranyl ions have also been investigated and decreasing orientational mobility of water with increasing uranyl concentration has been found. However, it is observed that the orientational dynamics remains more or less the same whether we consider all the water molecules in the aqueous solution or only the solvation shell water molecules. The effect of temperature on the translational and orientational characteristics of the aqueous uranyl solutions has also been studied in detail.

Optical transitions in the light-harvesting complexes of bacterial photosynthetic centers

The geometric parameters of the model structure of the bacterial photosynthetic center, which consists of a light-harvesting antenna complex, a reaction center, lipid layers, and the solvation shells of water molecules, were optimized using quantum mechanics-molecular mechanics (QM/MM). The optical absorption spectra of the bacteriochlorophyll were evaluated in the quantum subsystem. It was shown that the shift of the absorption bands towards longer wavelengths as compared to the spectrum of the individual bacteriochlorophyll molecules is characteristic of the cluster of pigment molecules that are found in the antennas.

Effects of Ions on the OH Stretching Band of Water as Revealed by ATR-IR Spectroscopy

The effects of various cations (Li+, Na+, K+, Rb+, Cs+, Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, and Ni2+) and anions (Cl−, Br−, I−, $$ {\text{NO}}_{3}^{ - } $$ , $$ {\text{ClO}}_{4}^{ - } $$ , $$ {\text{HCO}}_{3}^{ - } $$ , and $$ {\text{CO}}_{3}^{2 - } $$ ) on the molar absorptivity of water in the OH stretching band region (2,600–3,800 cm−1) were ascertained from attenuated total reflection infrared spectra of aqueous electrolyte solutions (22 in all). The OH stretching band mainly changes linearly with ion concentrations up to 2 mol·L−1, but several specific combinations of cations and anions (Cs2SO4, Li2SO4, and MgSO4) present different trends. That deviation is attributed to ion pair formation and cooperativity in ion hydration, which indicates that the extent of the ion–water interaction reflected by the OH stretching band of water is beyond the first solvation shell of water molecules directly surrounding the ion. The obtained dataset was then correlated with several quantitative parameters representing structural and dynamic properties of water molecules around ions: ΔG HB, the structural entropy (S str), the viscosity B-coefficient (B η ), and the ionic B-coefficient of NMR relaxation (B NMR). Results show that modification of the OH stretching band of water caused by ions has quasi-linear relations with all of these parameters. Vibrational spectroscopy can be a useful means for evaluating ion–water interaction in aqueous solutions.

Azide-water intermolecular coupling measured by 2-color 2D IR spectroscopy

We present 2-color two-dimensional infrared spectroscopy of intermolecular coupling between azide ions and their solvation shell water molecules. The cross-peak between azide asymmetric stretch vibration and the OD-stretch vibration is a result of low- probability uphill population transfer. Narrow bleach/excited state absorption peak shows selectivity to solvation shell water molecules only and the characteristics of the cross-peak suggest that the solvation shell hydrogen bond potential has similar anharmonic properties as the hydrogen bond in ice Ih. Population and depopulation of the excited state of the OD-stretch vibration happen on 150 fs and 1.7 ps timescales, respectively, with early manifesting heating effects that limit the selectivity to population times up to 1 ps.

Azide–water intermolecular coupling measured by two-color two-dimensional infrared spectroscopy

We utilize two-color two-dimensional infrared spectroscopy to measure the intermolecular coupling between azide ions and their surrounding water molecules in order to gain information about the nature of hydrogen bonding of water to ions. Our findings indicate that the main spectral contribution to the intermolecular cross-peak comes from population transfer between the asymmetric stretch vibration of azide and the OD-stretch vibration of D(2)O. The azide-bound D(2)O bleach/stimulated emission signal, which is spectrally much narrower than its linear absorption spectrum, shows that the experiment is selective to solvation shell water molecules for population times up to ~500 fs. The waters around the ion are present in an electrostatically better defined environment. Afterwards, ~1 ps, the sample thermalizes and selectivity is lost. On the other hand, the excited state absorption signal of the azide-bound D(2)O is much broader. The asymmetry in spectral width between bleach/stimulated emission versus excited absorption has been observed in very much the same way for isotope-diluted ice Ih, where it has been attributed to the anharmonicity of the OD potential.

Interfacial water on hydrophobic surfaces recognized by ions and molecules

Recent spectrophotometric and molecular dynamics simulation studies have shown that the physicochemical properties and structures of water in the vicinity of hydrophobic surfaces differ from those of the bulk water. However, the interfacial water acting as a separation medium on hydrophobic surfaces has never been detected and quantified experimentally. In this study, we show that small inorganic ions and organic molecules differentiate the interfacial water formed on the surfaces of octadecyl-bonded (C(18)) silica particles from the bulk water and the chemical separation of these solutes in aqueous media with hydrophobic materials can be interpreted with a consistent mechanism, partition between the bulk water phase and the interfacial water formed on the hydrophobic surface. Thermal transition behaviour of the interfacial water incorporated in the nanopores of the C(18) silica materials and the solubility parameter of the water calculated from the distribution coefficients of organic compounds have indicated that the interfacial water may have a structure of disrupted hydrogen bonding. The thickness of the interfacial water or the limit of distance from the hydrophobic surface at which molecules and ions can sense the surface was estimated to be 1.25 ± 0.13 nm from the volume of the interfacial water obtained by a liquid chromatographic method and the surface area, suggesting that the hydrophobic effect may extend beyond the first solvation shell of water molecules directly surrounding the surfaces.

Computational investigation of the speciation of uranyl gluconate complexes in aqueous solution

The geometries, relative energies and spectroscopic properties of a range of D-gluconate complexes of uranyl(VI) are studied computationally using density functional theory. The effect of pH is accommodated by varying the number of water and hydroxide ligands accompanying gluconate in the equatorial plane of the uranyl unit. For 1 : 1 complexes, the calculated uranyl ν(asym) stretching frequency decreases as pH increases, in agreement with previous experimental data. Three different gluconate chelating modes are studied. Their relative energies are found to be pH dependent, although the energetic differences between them are not sufficient to exclude the possibility of multiple speciation. (13)C NMR chemical shifts are calculated for the coordinated gluconate in the high pH mimics, and show good agreement with experimental data, supporting the experimental conclusion that the six-membered chelate ring is favoured at high pH. Attempts to improve the description of the aqueous environment via the addition of second solvation shell water molecules resulted in significantly worse agreement with experiment for ν(asym). The effect of increasing the gluconate concentration is modelled by calculating 1 : 2 and 1 : 3 uranyl : D-gluconate complexes.

Local Effects in the X-ray Absorption Spectrum of Salt Water

Both first-principles molecular dynamics and theoretical X-ray absorption spectroscopy have been used to investigate the aqueous solvation of cations in 0.5 M MgCl(2), CaCl(2), and NaCl solutions. We focus here on the species-specific effects that Mg(2+), Ca(2+), and Na(+) have on the X-ray absorption spectrum of the respective solutions. For the divalent cations, we find that the hydrogen-bonding characteristics of the more rigid magnesium first-shell water molecules differ from those in the more flexible solvation shell surrounding calcium. In particular, the first solvation shell water molecules of calcium are able to form acceptor hydrogen bonds, and this results in an enhancement of a post-edge peak near 540 eV. The absence of acceptor hydrogen bonds for magnesium first shell water molecules provides an explanation for the experimental and theoretical observation of a lack of enhancement at the post-main-edge peak. For the sodium monovalent cation we find that the broad tilt angle distribution results in a broadening of postedge features, despite populations in donor-and-acceptor configurations consistent with calcium. We also present the reaveraged spectra of the MgCl(2), CaCl(2), and NaCl solutions and show that trends apparent with increasing concentration (0.5, 2.0, 4.0 M) are consistent with experiment. Finally, we examine more closely both the effect that cation coordination number has on the hydrogen-bonding network and the relative perturbation strength of the cations on lone pair oxygen orbitals.

Influence of Explicit Hydration Waters in Calculating the Hydrolysis Constants for Geochemically Relevant Metals

The effect of including a second, explicit solvation shell of water molecules is examined on the DFT calculation of a selection of aquo-metal ions' pK(a) values (deprotonation constants). Our goal is to gauge the accuracy of how certain cluster approximations and implicit solvation models (PCM or COSMO) affect the results. The ions in this study include: Al(3+)((aq)); Fe(3+)((aq)); Cr(3+)((aq)); Mn(3+)((aq)); Co(3+)((aq)); Ga(3+)((aq)); Fe(2+)((aq)); Mg(2+)((aq)); and Be(2+)((aq)). Overall, we find that experimental pK(a) constants can be calculated successfully to within 1-2 pH units, provided a consistent hydration structure is maintained for products and reactants.