Hydration Energy Research Articles

Monte Carlo simulations for free energies of hydration: Past to present.

A summary of the development of Monte Carlo statistical mechanics simulations for the computation of free energies of hydration of organic molecules is followed by presentation of results with the latest version of the optimized potentials for liquid simulations-all atom force field and the TIP4P water model. Scaling of the Lennard-Jones interactions between water, oxygen, and carbon atoms by a factor of 1.25 is found to improve the accuracy of free energies of hydration for 50 prototypical organic molecules from a mean unsigned error of 1.0-1.2 to 0.4 kcal/mol.

Conjoining the benefits of an additional phenyl ring in a simple benzophenone hydrazone-based platform @ discriminatory recognition of F− over CN− with quantitation of aqueous Cu2+: New tricks for an old dog

Herein, benzophenone hydrazone derived two π-conjugative networks viz.DMHMN and DMHMP have been judiciously designed for competitive recognition of F− (LOD: 6.8 ppm; Ka: 7.2 × 103 M−1) over CN− (LOD: ∼0.3 ppm; Ka: 20.3 × 104 M−1) along with Cu2+. Interestingly, the presence of an extra phenyl ring in DMHMN played a pivotal role for discriminative colorimetric recognition of F− over CN− despite having greater hydration energy of F−. Deliberate implantation of an extra phenyl ring leads to enhanced acidity of the benzophenone platform, rationalizing structure-performance synergies. While the presence of dangling OH group and azomethine functionality enables both the chemosensors to exhibit selective recognition toward Cu2+ in purely aqueous phase with the detection limit of ∼0.7 ppm and ∼0.16 ppm by DMHMP and DMHMN respectively, which are far below the safe limit set by World Health Organization (WHO). The sensing phenomenon by the redox non-innocent sensory receptors has been thoroughly investigated by UV–Vis, cyclic voltammetry and 1H NMR spectroscopic studies validated by density functional theory (DFT). Moreover, relay recognition of Al3+ by [DMHMN·F−] ensemble made it a suitable contender for complex logic circuitry fabrication like IMPLICATION/INHIBIT circuit. Additionally, the sensory probes have the capability of quantitative estimation of Cu2+ from unknown water samples, which was cross-examined with the help of the lab-based market available portable device. Lastly, smartphone assisted point-of-care testing (POCT) application has been performed for on-field detection of F− by DMHMN.

Size-Dependent Ab Initio Atomistic Thermodynamics from Cluster to Bulk: Application to Hydration of Titania Nanoparticles.

Ab initio atomistic thermodynamics (AIAT) has become an indispensable tool to estimate Gibbs free energy changes for solid surfaces interacting with gaseous species relative to pressure (p) and temperature (T). For such systems, AIAT assumes that solid vibrational contributions to Gibbs free energy differences cancel out. However, the validity of this assumption is unclear for nanoscale systems. Using hydrated titania nanoparticles (NPs) as an example, we estimate the vibrational contributions to the Gibbs free energy of hydration (ΔGhyd(T,p)) for arbitrary NP size and degree of hydration. Comparing ΔGhyd(T,p) phase diagrams for NPs when considering these contributions (AIATnano) relative to a standard AIAT approach reveals significant qualitative and quantitative differences, which only become negligible for large systems. By constructing a size-dependent ΔGhyd(T,p) phase diagram, we illustrate how our approach can provide deeper insights into how nanosytems interact with their environments, with many potential applications (e.g., catalytic nanoparticles, biological colloids, nanoparticulate pollutants).

Calculated Aqueous Reduction Potentials of Neutral and Anionic Halogen Diatomic Molecules.

The free energy of hydration, aqueous, and gas phase electron affinity and aqueous reduction potentials of F2, Cl2, Br2, I2, ClF, BrF, IF, BrCl, ICl, IBr, and their corresponding anions were calculated using an electronic structure approach previously developed for these properties for X• and XO•, where X is a halogen which yielded excellent results. The gas phase electron affinities were calculated at the Feller-Peterson-Dixon level based on complete basis set extrapolation of CCSD(T) results with additional corrections. The agreement with the available experimental data is excellent, and the calculations provide a complete set of reliable electron affinities for these diatomic halogens. The hybrid solvation approach uses single point implicit solvation calculations on gas phase optimized clusters with explicit solvent molecules. The gas phase energy calculations were performed using MP2 and CCSD(T)-F12b for tetramer clusters (four explicit waters) and MP2 for octamer clusters (eight explicit waters). The final redox potentials were obtained at the MP2/aug-cc-pVTZ (aT) with a self-consistent reaction field (SMD) level using the octamer clusters. The aqueous reduction potentials of the neutral diatomic halogens are predicted within 0.06 V of the experiment for diatomic neutrals. The same agreement of 0.06 V is predicted for the redox potential resulting from dissociation electron attachment of the diatomic halogen anions. The current work extends reduction potentials for multiple redox couples for which no experimental data is available, for example, those containing iodine and the interhalogen anions. F2•- is predicted to dissociate for its lowest energy structure in both the tetramer and octamer clusters to form solvated F-, HF, and OH•.

Enhancing chloride/fluoride selectivity with hydrophobic side-chain anion exchange membranes in electrodialysis

The selective separation of chloride ions (Cl−) from fluoride ions (F−) is a critical concern in various industries due to the potential risks of F− on public health and industrial operations. However, Cl−/F− separation is known to be technologically challenging, considering they have the same charge and valency. This study explores the design of anion exchange membranes (AEMs) in electrodialysis to achieve Cl−/F− separation based on the difference in their hydration energy. Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) polymers brominated at varying reaction temperatures were employed as the basic AEM material, enabling fine-tuning of the hydrophobic properties of the resulting membranes through selective bromination at the benzyl or aryl positions and the subsequent quaternization with tertiary amines of different chain lengths, ultimately enhancing their selectivity. The developed AEMs are tested in electrodialysis involving a binary Cl−/F− mixture. The obtained membranes exhibited remarkable selectivity, reaching as high as 12.5 ± 1.0, surpassing the performance of commercial monovalent selective AEMs. The results demonstrate the potential of Cl−/F− separation by electrodialysis and highlight the significance of tailored membrane design in achieving the separation of ions with the same valency.

Synthesis, Characterization, and Analysis of Probenecid and Pyridine Compound Salts

This study aimed to address the issue of the low solubility in the model drug probenecid (PRO) and its impact on bioavailability. Two salts of probenecid (PRO), 4-aminopyridine (4AMP), and 4-dimethylaminopyridine (4DAP) were synthesized and characterized by PXRD, DSC, TGA, FTIR, and SEM. The crystal structures of the two salts were determined by SCXRD, demonstrating that the two salts exhibited different hydrogen bond networks, stacking modes, and molecular conformations of PRO. The solubility of PRO and its salts in a phosphate-buffered solution (pH = 6.8) at 37 °C was determined, the results showed that the solubility of PRO salts increased to 142.83 and 7.75 times of the raw drug, respectively. Accelerated stability experiments (40 °C, 75% RH) showed that the salts had good phase stability over 8 weeks. Subsequently, Hirshfeld surface (HS), atom in molecules (AIM), and independent gradient model (IGM) were employed for the assessment of intermolecular interactions. The analyses of salt-forming sites and principles were conducted using molecular electrostatic potential surfaces (MEPs) and pKa rules. The lattice energy (EL) and hydration-free energy (EHF) of PRO and its salts were calculated, and the relationships between these parameters and melting points and the solubility changes were analyzed.

Cation interception and permeability characteristics of bentonite barriers exposed to NaCl and NH4Cl solutions.

High concentrations of Na+ and NH4+ in landfill leachate lead to deterioration of bentonite barrier and pose a threat to the environment. This study focused on the pollution interception and permeability characteristics of the bentonite barrier exposed to NaCl and NH4Cl solutions. Based on previous findings, salt solution concentrations were established at 74.80, 37.40, 18.70, and 9.4 mmol/L. The bentonite contents in the mixture were set at 0, 5, 10, and 15%. The results indicate that the samples exhibit better interception of NH4+ compared to Na+. This difference arises from the cation exchange sequence, the size of the hydration radius, and the hydrogen bonding of the two cations. Additionally, the difference in hydration enthalpy between the two cations leads to variations in the swelling of bentonite, resulting in a higher hydraulic conductivity coefficient in NH4Cl solution. This study shows that although bentonite barriers have better interception for NH4+, they exhibit greater hydraulic conductivity in NH4Cl solution, increasing the risk of leachate carrying other contaminants.

Correlation Between Ionization and Hydration Energies

Correlation Between Ionization and Hydration Energies

Significant role of interlayer in strontium adsorption on weathered biotite

Weathering of micaceous minerals play a significant role in the adsorption and migration of radioactive strontium. Weathering changes the structure of minerals and the capacity of adsorption sites on their surfaces. The regulatory mechanism of environmental factors on radioactive strontium on weathered micaceous minerals remains to be sorted out and clarified. In this work, we studied the adsorption behavior of Sr2+ on weathered micaceous minerals with different weathering degrees via batch experiments and spectroscopic approaches. The batch experiment results indicated that the adsorption capacity of Sr2+ increased from 5.36 mg/g (Na-Bio) to 19.89 mg/g (SCa-Bio) as the weathering degree of biotite increased. The outer-sphere complexation and electrostatic attraction/ion exchange mainly governed the adsorption of Sr2+ on weathered biotite. The type of cation co-existing in solution and the weathering degree of biotite regulate Sr2+ adsorption into the interlayers. Owing to their similar ionic properties (e.g., hydration energy and hydrated radius), Sr2+ competed more fiercely with Ca2+ for adsorption sites. Hydrated Na+ and Sr2+ could enter and adsorb on the interlayer, but the difference is that the hydrated Na+ collapsed the interlayer of weathered biotite (from 15.0 Å to 11.8 Å), while the hydrated Sr2+ expanded the interlayers (from 12.0 Å to 14.8 Å). Nevertheless, the hydrated Sr2+ could occupy most interlayers when Na+ and Sr2+ co-existed in the solution. It was noted that dehydrated K+ present in the interlayer of weathered biotite could collapse the expanded interlayer of biotite. The d-spacing of weathered biotite (e.g., SCa-Bio) would decrease from 15.0 Å to 10.4 Å, significantly limiting the interlaminar adsorption of Sr2+. Furthermore, this inhibitory effect was based on the high weathering degree of micaceous minerals and the noticeable interlayer expansion. XPS results demonstrated that the co-existing cations hardly influence the adsorption form of Sr2+ on weathered micaceous minerals. The above findings provide intrinsic insights into the understanding of radioactive strontium transport and fate in the environment.

Biomimetic Charge‐Neutral Anion Receptors for Reversible Binding and Release of Highly Hydrated Phosphate in Water

AbstractControl of phosphate capture and release is vital in environmental, biological, and pharmaceutical contexts. However, the binding of trivalent phosphate (PO43−) in water is exceptionally difficult due to its high hydration energy. Based on the anion coordination chemistry of phosphate, in this study, four charge‐neutral tripodal hexaurea receptors (L1–L4), which were equipped with morpholine and polyethylene glycol terminal groups to enhance their solubility in water, were synthesized to enable the pH‐triggered phosphate binding and release in aqueous solutions. Encouragingly, the receptors were found to bind PO43− anion in a 1 : 1 ratio via hydrogen bonds in 100 % water solutions, with L1 exhibiting the highest binding constant (1.2×103 M−1). These represent the first neutral anion ligands to bind phosphate in 100 % water and demonstrate the potential for phosphate capture and release in water through pH‐triggered mechanisms, mimicking native phosphate binding proteins. Furthermore, L1 can also bind multiple bioavailable phosphate species, which may serve as model systems for probing and modulating phosphate homeostasis in biological and biomedical researches.

Biomimetic Charge-Neutral Anion Receptors for Reversible Binding and Release of Highly Hydrated Phosphate in Water.

Control of phosphate capture and release is vital in environmental, biological, and pharmaceutical contexts. However, the binding of trivalent phosphate (PO4 3-) in water is exceptionally difficult due to its high hydration energy. Based on the anion coordination chemistry of phosphate, in this study, four charge-neutral tripodal hexaurea receptors (L1-L4), which were equipped with morpholine and polyethylene glycol terminal groups to enhance their solubility in water, were synthesized to enable the pH-triggered phosphate binding and release in aqueous solutions. Encouragingly, the receptors were found to bind PO4 3- anion in a 1 : 1 ratio via hydrogen bonds in 100 % water solutions, with L1 exhibiting the highest binding constant (1.2×103 M-1). These represent the first neutral anion ligands to bind phosphate in 100 % water and demonstrate the potential for phosphate capture and release in water through pH-triggered mechanisms, mimicking native phosphate binding proteins. Furthermore, L1 can also bind multiple bioavailable phosphate species, which may serve as model systems for probing and modulating phosphate homeostasis in biological and biomedical researches.

Multi-armed bandit algorithm for sequential experiments of molecular properties with dynamic feature selection.

Sequential optimization is one of the promising approaches in identifying the optimal candidate(s) (molecules, reactants, drugs, etc.) with desired properties (reaction yield, selectivity, efficacy, etc.) from a large set of potential candidates, while minimizing the number of experiments required. However, the high dimensionality of the feature space (e.g., molecular descriptors) makes it often difficult to utilize the relevant features during the process of updating the set of candidates to be examined. In this article, we developed a new sequential optimization algorithm for molecular problems based on reinforcement learning, multi-armed linear bandit framework, and online, dynamic feature selections in which relevant molecular descriptors are updated along with the experiments. We also designed a stopping condition aimed to guarantee the reliability of the chosen candidate from the dataset pool. The developed algorithm was examined by comparing with Bayesian optimization (BO), using two synthetic datasets and two real datasets in which one dataset includes hydration free energy of molecules and another one includes a free energy difference between enantiomer products in chemical reaction. We found that the dynamic feature selection in representing the desired properties along the experiments provides a better performance (e.g., time required to find the best candidate and stop the experiment) as the overall trend and that our multi-armed linear bandit approach with a dynamic feature selection scheme outperforms the standard BO with fixed feature variables. The comparison of our algorithm to BO with dynamic feature selection is also addressed.

Ion- and surface-sensitive interactions during oxygen evolution reaction in alkaline media

Abstract Clean and sustainable energy has turned towards electrochemical water splitting as a viable solution in minimizing carbon emissions. Electrolysis of water converts electrical energy to chemical energy, through the production of hydrogen and oxygen gases, which can be harnessed for potential applications without contributing to greenhouse emissions. While this energy storage process shows great potential, its efficiency is hindered by the sluggish kinetics of the oxygen evolution reaction (OER). As a result, its widespread application in green electrolytic technologies is limited hence investigations on improving OER kinetics are of utmost importance. Recent research breakthroughs indicate that alkali metal cations are more than passive observers. They play complex roles in the electric double layer (EDL), which positively influences the OER kinetics. The presence of numerous ions and their combinations presents a challenge of complexity. This study aims to delve into the impact of alkali metal cations on OER activity due to the variance in their hydration energies. Specific investigations focusing on different alkali metal cations in solution, such as Li+, Na+, and K+, was conducted on RuO2 to gain a deeper understanding of how these ions interact with both reactants and intermediate species in the reaction kinetics. Traditional electrochemical tests, including cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and accelerated degradation test (ADT) measurements were employed to elucidate critical aspects such as surface activation, electric double layer interactions, catalytic activity and stability, ohmic resistance, and mass and charge transport.

Evaluation of rational design and hydration ability of medium-entropy Mn-doped LSCF-based phases

The development of multi-doped oxides based on the La0.6Sr0.4Co0.2Fe0.8O3–δ (LSCF) and La1–xSrxMnO3+δ perovskites would help to achieve the improved electrochemical activity of air electrodes in protonic ceramic cells due to the formation of the single-phase triple-conducting material. The search for prospective medium-entropy and high-entropy oxides based on (La,Sr)(Co,Fe,Mn)O3–δ could provide the electrode material with advanced hydration ability. In the present work, the formation and stability of new La0.6Sr0.4CoxFe1–x–yMnyO3–δ phases are estimated by thermodynamic calculations and the structure factors. Experimentally obtained La0.6Sr0.4CoxFe1–x–yMnyO3–δ powder samples are characterized by X-ray and neutron powder diffraction. The refined crystal structure parameters of the medium-entropy single-phase La0.6Sr0.4Co0.33Fe0.33Mn0.33O3–δ (LSCFM333) oxide with the trigonal structure (space group R3‾c) and the cell parameters of a = 5.4642(1) Å and c = 13.284(4) Å in hexagonal axes are used to design of a supercell simulation for density functional theory (DFT) calculations. The first-principles results are proposed to evaluate the thermodynamic parameters of proton uptake to assess future applications of Mn-doped LSCF in proton-conducting ceramic cells. The calculated values of the hydration enthalpy ΔHhydr for the LSCF and LSCFM333 derivatives are equal to 4.5 kJ mol−1 and –3.5 kJ mol−1, respectively, which are supported by the thermogravimetric analysis (TGA). The present study shows that the modeling techniques, including the thermodynamic and DFT calculations, can be successfully applied to the design of the related oxide materials for applications in solid oxide cells.

Hydration energies of calcium hydroxide cation, CaOH+(H2O)x (x = 1–6), studied using guided ion beam tandem mass spectrometry

Hydration energies of CaOH+(H2O)x, x = 1–6, were obtained using threshold collision-induced dissociation (TCID) with xenon (Xe) as conducted with a guided ion beam tandem mass spectrometer (GIBMS). The primary reaction pathway observed for all complexes is the loss of one water ligand, followed by the loss of additional water molecules at higher collision energies for x > 1. The kinetic-energy-dependent cross sections for dissociation of CaOH+(H2O)x complexes were modeled to obtain 0 K binding energies after accounting for lifetime effects, energy distributions, and pressure effects. Except for x = 5, experimental threshold energies measured through TCID agree well with theoretical hydration energies determined using B3LYP/6-311+G(d,p) structure geometries followed by single point energies calculated at B3LYP, B3P86, M06, and MP2(full) levels of theory with a 6-311+G(2d,2p) basis set. B3LYP-GD3BJ and ωB97XD calculations were also used, with the former yielding results having the best agreement with the threshold energies extracted from the analysis of the TCID cross sections.

Thermodynamic parameters of L-carnosine dissolution and solvation in binary aqueous-organic mixtures at T = 298.15 K.

Isothermal calorimetry was used to measure the enthalpies of L-carnosine digestion in aqueous solutions of acetonitrile (AN) 1,4-dioxane (DO), acetone (AC), dimethyl sulfoxide (DMSO), ethanol EtOH), 1-propanol (1-PrOH), and 2-propanol (2-PrOH). The values of ΔsolH° obtained were combined with the standard enthalpies of L-carnosine sublimation to obtain the corresponding enthalpies of solvation ΔsolvH°. In addition, the enthalpic coefficients of pairwise interactions (hxy) of L-carnosine with organic solvent molecules and the enthalpy of transfer (ΔtrH°) from water in their mixture with the latter were calculated. It is established that the solvation of L-carnosine is weakened in a series of (H2O + EtOH) > (H2O + 1-PrOH) > (H2O + 2-PrOH). This may be due to an increase in the hydrophobicity of alcohols of the same order as well as the presence of CH3-groups within the alcohol molecule. The weakening of the solvation energy of L-carnosine by co-solvent molecules in a series of mixed solvents: (H2O + AN) < (H2O + AC) < (H2O + DO) < (H2O + DMSO) is associated both with an increase in the hydration energy of co-solvents in the same order and with their physicochemical properties. The enthalpic coefficients of water-organic solvent pairwise interactions were discovered to be linearly related to the corresponding coefficients of L-carnosine-organic solvent interactions. An increase in the polarity and acidity of an organic solvent increases the energy of pairwise interactions between L-carnosine and the organic solvent. Increasing the cohesion energy density, molar volume, and basicity of the organic solvent reduces the energy of the pairwise interactions between L-carnosine and the organic solvent.

Should we still be using HCl in the synthesis of SBA-15 and Al-SBA-15?

HCl is not always the best option for synthesizing SBA-15 and Al-SBA-15, and using different acids can bring beneficial structural and surface properties. While SBA-15-HCl and SBA-15-H2SO4 exhibited similar structural and textural characteristics, SBA-15-H2SO4 showed significantly lower contamination of the acid counterion in the final material, making it advantageous in catalytic applications where Cl− poisoning is a concern. On the other hand, SBA-15-HBr demonstrated a narrower pore size distribution. Notably, no residual Br− was detected in the final material for SBA-15-HBr. SBA-15-HNO3 and SBA-15-HBr displayed the highest concentration of 29Si MAS-NMR Q2 species, which can benefit grafting applications. The high degree of silica condensation in SBA-15-HCl and SBA-15-H2SO4 provides superior hydrothermal stability for these samples. From a fundamental perspective, a linear correlation was also identified between the Gibbs Free Energy of Hydration (ΔGhyd) of the anions and the SBA-15 mesopore sizes. Additionally, the study extended to preparing Al-SBA-15, and a facile methodology based on the pH-adjusting method was proposed. The samples displayed a Brønsted/Lewis ratio higher than typically reported, suggesting an efficient incorporation of Al as Brønsted acid sites.

Inclusion of Water Multipoles into the Implicit Solvation Framework Leads to Accuracy Gains.

The current practical "workhorses" of the atomistic implicit solvation─the Poisson-Boltzmann (PB) and generalized Born (GB) models─face fundamental accuracy limitations. Here, we propose a computationally efficient implicit solvation framework, the Implicit Water Multipole GB (IWM-GB) model, that systematically incorporates the effects of multipole moments of water molecules in the first hydration shell of a solute, beyond the dipole water polarization already present at the PB/GB level. The framework explicitly accounts for coupling between polar and nonpolar contributions to the total solvation energy, which is missing from many implicit solvation models. An implementation of the framework, utilizing the GAFF force field and AM1-BCC atomic partial charges model, is parametrized and tested against the experimental hydration free energies of small molecules from the FreeSolv database. The resulting accuracy on the test set (RMSE ∼ 0.9 kcal/mol) is 12% better than that of the explicit solvation (TIP3P) treatment, which is orders of magnitude slower. We also find that the coupling between polar and nonpolar parts of the solvation free energy is essential to ensuring that several features of the IWM-GB model are physically meaningful, including the sign of the nonpolar contributions.

New QSPR molecular descriptors based on low-cost quantum chemistry computations using DFT/COSMO approach

This work presents a simple computational methodology to determine new theoretical molecular descriptor scales convenient for use in QSPR correlations and the analysis of solvation-related properties. On the basis of low-cost quantum chemical computations using DFT/COSMO approach, in particular that implemented in the ADF/COSMO-RS module of the Amsterdam Modeling Studio program package, optimized geometry and local screening charge density of a considered molecule are obtained and related to the proposed descriptors through an a priori simple reasoning. Values of four molecular descriptors, namely volume VCOSMO∗, hydrogen bond/Lewis acidity αCOSMO and basicity βCOSMO, and charge asymmetry of the nonpolar region of the molecule δCOSMO have been calculated and are presented for sets of 128 non-ionic organic molecules and 47 ions composing ionic liquids. Relation of the proposed scales to some others that are widely known or presented recently in the literature has been examined. Although the present methodology is completely independent of any experimental data, the theoretical descriptor scales proposed here have been found to correlate linearly quite well with the respective empirical scales (mostly R2 > 0.8, and for some R2 > 0.9). For non-ionic organics, just the direct proportionality provided adequate fits of the well-established prominent scales such as those of Abraham, Klamt, Kamlet-Taft, Catalan, Gutmann, and Laurence and showed only a few statistically justifiable outliers. For ions composing ILs, the linear fits of seven acidity or basicity scales presented previously with those proposed in this work were of similar quality. Examining the observed outliers, several descriptor values reported in the literature were identified as being in error. Good performance of the new descriptor scales has been demonstrated by employing them for LSER fitting of various solvation-related thermodynamic and kinetic properties such as standard vaporization enthalpy, standard hydration enthalpy, air–water partition coefficient, air-IL partition coefficient, and solvent effects on the activation Gibbs energy or rate constant of SN1 and SNAr reactions. The quality of these correlations appears to be comparable to that of the correlations currently available for these properties, regardless of the role, solute or solvent, the involved molecules played.

The Determination of Free Energy of Hydration of Water Ions from First Principles.

We model the autoionization of water by determining the free energy of hydration of the major intermediate species of water ions. We represent the smallest ions─the hydroxide ion OH-, the hydronium ion H3O+, and the Zundel ion H5O2+─by bonded models and the more extended ionic structures by strong nonbonded interactions (e.g., the Eigen H9O4+ = H3O+ + 3(H2O) and the Stoyanov H13O6+ = H5O2+ + 4(H2O)). Our models are faithful to the precise QM energies and their components to within 1% or less. Using the calculated free energies and atomization energies, we compute the pKa of pure water from first principles as a consistency check and arrive at a value within 1.3 log units of the experimental one. From these calculations, we conclude that the hydronium ion, and its hydrated state, the Eigen cation, are the dominant species in the water autoionization process.