A quantum mechanical charge field molecular dynamics study of Fe(2+) and Fe(3+) ions in aqueous solutions

Structure and size of complete hydration shells of metal ions and inorganic anions in aqueous solution

The structures of nine hydrated metal ions in aqueous solution have been redetermined by large angle X-ray scattering to obtain experimental data of better quality than those reported 40-50 years ago. Accurate M-OI and M-(OI-H)⋯OII distances and M-OI(H)⋯OII bond angles are reported for the hydrated magnesium(II), aluminium(III), manganese(II), iron(II), iron(III), cobalt(II), nickel(II), copper(II) and zinc(II) ions; the subscripts I and II denote oxygen atoms in the first and second hydration sphere, respectively. Reported structures of hydrated metal ions in aqueous solution are summarized and evaluated with emphasis on a possible relationship between M-OI-OII bond angles and bonding character. Metal ions with high charge density have M-OI-OII bond angles close to 120°, indicative of a mainly electrostatic interaction with the oxygen atom in the water molecule in the first hydration shell. Metal ions forming bonds with a significant covalent contribution, as e.g. mercury(II) and tin(II), have M-OI-OII bond angles close to 109.5°. This implies that they bind to one of the free electron pairs in the water molecule. Comparison of M-O bond distances of hydrated metal ions in the solid state with one hydration shell, and in aqueous solution with in most cases at least two hydration shells, shows no significant differences. On the other hand, the X-O bond distance in hydrated oxoanions increases by ca. 0.02 Å in aqueous solution in comparison with the corresponding X-O distance in the solid state. A linear correlation is observed between volume, calculated from the van der Waals radius of the hydrated ion, and the ionic diffusion coefficient in aqueous solution. This correlation strongly indicates that monovalent metal ions, except lithium and silver(I), and singly-charged monovalent oxoanions have a single hydration shell. Divalent metal ions, bismuth(III) and the lanthanoid(III) and actinoid(III) ions have two hydration shells. Trivalent transition and tetravalent metal ions have two full hydration shells and portion of a third one. Doubly charged oxoanions have one well-defined hydration shell and an ill-defined second one.

Characterization of iron(III) in aqueous and alkaline environments with ab initio and ReaxFF potentials

The iron(III) complexes [Fe(H2O)n(OH)m]3-m (n + m = 5, 6, m ≤ 3) and corresponding proton transfer reactions are studied with total energy calculations, the nudged elastic band (NEB) method, and molecular dynamics (MD) simulations using ab initio and a modification of reactive force field potentials, the ReaxFF-AQ potentials, based on the implementation according to Böhm et al. [J. Phys. Chem. C 120, 10849-10856 (2016)]. Applying ab initio potentials, the energies for the reactions [Fe(H2O)n(OH)m]3-m + H2O → [Fe(H2O)n-1(OH)m+1]2-m + H3O+ in a gaseous environment are in good agreement with comparable theoretical results. In an aqueous (aq) or alkaline environment, with the aid of NEB computations, respective minimum energy paths with energy barriers of up to 14.6 kcal/mol and a collective transfer of protons are modeled. Within MD simulations at room temperature, a permanent transfer of protons around the iron(III) ion is observed. The information gained concerning the geometrical and energetic properties of water and the [Fe(H2O)n(OH)m]3-m complexes from the ab initio computations has been used as reference data to optimize parameters for the O-H-Fe interaction within the ReaxFF-AQ approach. For the optimized ReaxFF-AQ parameter set, the statistical properties of the basic water model, such as the radial distribution functions and the proton hopping functions, are evaluated. For the [Fe(H2O)n(OH)m]3-m complexes, it was found that while geometrical and energetic properties are in good agreement with the ab initio data for gaseous environment, the statistical properties as obtained from the MD simulations are only partly in accordance with the ab initio results for the iron(III) complexes in aqueous or alkaline environments.

Solvation energies of the ferrous ion in water and in ammonia at various temperatures

CONTEXT

The solvation of metal ions is crucial to understanding relevant properties in physics, chemistry, or biology. Therefore, we present solvation enthalpies and solvation free energies of the ferrous ion in water and ammonia. Our results agree well with the experimental reports for the hydration free energy and hydration enthalpy. We obtained [Formula: see text] kJ mol[Formula: see text] for the hydration free energy and [Formula: see text] kJ mol[Formula: see text] for the hydration enthalpy of ferrous ion in water at room temperature. At ambient temperature, we obtained [Formula: see text] kJ mol[Formula: see text] as the [Formula: see text] ammoniation free energy and [Formula: see text] kJ mol[Formula: see text] for the ammoniation enthalpy. In addition, the free energy of solvation is deeply affected when the temperature increases. This pattern can be attributed to the rise of entropy when the temperature rises. Besides, the temperature does not affect the ammoniation enthalpies and the hydration enthalpy of the [Formula: see text] ion.

METHOD

All the geometry optimizations are performed at the MP2 methods associated with the 6-31++g(d,p) basis set of Pople. solvated phase structures of [Formula: see text] ion in water or in ammonia are performed using the PCM model. The [Formula: see text] program suite was used to perform all the calculations. The program TEMPO was also used to evaluate the temperature sensitivity of the different obtained geometries.

Speeding-up Hybrid Functional-Based Ab Initio Molecular Dynamics Using Multiple Time-stepping and Resonance-Free Thermostat

Ab initio molecular dynamics (AIMD) based on density functional theory (DFT) has become a workhorse for studying the structure, dynamics, and reactions in condensed matter systems. Currently, AIMD simulations are primarily carried out at the level of generalized gradient approximation (GGA), which is at the second rung of DFT functionals in terms of accuracy. Hybrid DFT functionals, which form the fourth rung in the accuracy ladder, are not commonly used in AIMD simulations as the computational cost involved is 100 times or higher. To facilitate AIMD simulations with hybrid functionals, we propose here an approach using multiple time stepping with adaptively compressed exchange operator and resonance-free thermostat, that could speed up the calculations by ∼30 times or more for systems with a few hundred of atoms. We demonstrate that by achieving this significant speed up and making the compute time of hybrid functional-based AIMD simulations at par with that of GGA functionals, we are able to study several complex condensed matter systems and model chemical reactions in solution with hybrid functionals that were earlier unthinkable to be performed.

Hybrid Functional and Plane Waves based Ab Initio Molecular Dynamics Study of the Aqueous Fe2+ /Fe3+ Redox Reaction

Kohn-Sham density functional theory and plane wave basis set based ab initio molecular dynamics (AIMD) simulation is a powerful tool for studying complex reactions in solutions, such as electron transfer (ET) reactions involving Fe²⁺ /Fe³⁺ ions in water. In most cases, such simulations are performed using density functionals at the level of Generalized Gradient Approximation (GGA). The challenge in modelling ET reactions is the poor quality of GGA functionals in predicting properties of such open-shell systems due to the inevitable self-interaction error (SIE). While hybrid functionals can minimize SIE, standard plane-wave based AIMD at that level of theory is typically 150 times slower than GGA for systems containing ∼100 atoms. Among several approaches reported to speed-up AIMD simulations with hybrid functionals, the noise-stabilized MD (NSMD) procedure, together with the use of localized orbitals to compute the required exchange integrals, is an attractive option. In this work, we demonstrate the application of the NSMD approach for studying the Fe²⁺ /Fe³⁺ redox reaction in water. It is shown here that long AIMD trajectories at the level of hybrid density functionals can be obtained using this approach. Redox properties of the aqueous Fe²⁺ /Fe³⁺ system computed from these simulations are compared with the available experimental data for validation.

Liquid-Gas Interface of Iron Aqueous Solutions and Fenton Reagents

Fenton chemistry, involving the reaction between Fe²⁺ and hydrogen peroxide, is well-known due to its applications in the mineralization of extremely stable molecules. Different mechanisms, influenced by the reaction conditions and the solvation sphere of iron ions, influence the fate of such reactions. Despite the huge amount of effort spent investigating such processes, a complete understanding is still lacking. This work combines photoelectron spectroscopy and theoretical calculations to investigate the solvation and reactivity of Fe²⁺ and Fe³⁺ ions in aqueous solutions. The reaction with hydrogen peroxide, both in homogeneous Fenton reagents and at the liquid-vapor interface, illustrates that both ions are homogeneously distributed in solutions and exhibit an asymmetric octahedral coordination to water in the case of Fe²⁺. No indications of differences in the reaction mechanism between the liquid-vapor interface and the bulk of the solutions have been found, suggesting that Fe³⁺ and hydroxyl radicals are the only intermediates.

Structures and relative stability of hydrated ferrous ion clusters and temperature effects

Structures of solvated ferrous ion clusters have been investigated in the singlet and quintet spin states of the ferrous ion. Relative stabilities of isomers are also discussed at different temperatures.

Evidence for an egg-box-like structure in iron(ii)–polygalacturonate hydrogels: a combined EXAFS and molecular dynamics simulation study

The coordination of Fe(ii) with polygalacturonic acid (polyGalA) in Fe(ii)–polyGalA hydrogels exhibits an octahedral geometry that follows the “egg-box model”.

Adsorption of Pregelatinized Starch for Selective Flocculation and Flotation of Fine Siderite

Pregelatinized starch (PS) was used for the selective flocculation and flotation of fine siderite in a carbonate-containing iron ore. With PS, the flotation of fine siderite was improved. The repulsive forces between fine siderite particles and the attractive forces between siderite and hematite or quartz were decreased after treatment with PS, indicating that the aggregation of siderite was enhanced and the aggregations of mixed minerals were weakened. An analysis of the changes in X-ray photoelectron spectra showed that coordination bonds were formed when PS was adsorbed on siderite and hematite. However, PS could not adsorb on quartz. Moreover, the molecular simulation showed that the main mechanism for PS adsorption on siderite was confirmed as a "tail model" with end -OH coordinated with Fe²⁺. The bridge connection of PS enhanced the flocculation of fine siderite. The flotation of fine siderite was also enhanced. For hematite treated with PS, the combination of coordination bond and hydrogen bond resulted in the "loop model" and "train model" as the main adsorption mechanisms of PS. The molecules covered the hematite surface and prevented the adsorption of the collector. The flotation of hematite was depressed. As a result, the selective flocculation and flotation of fine siderite were realized.

Colorimetric determination of ferrous ion via morphology transition of gold nanorods

A colorimetric method is described for the determination of ferrous ion (Fe²⁺) with high sensitivity and selectivity. The method is based on catalytic etching of gold nanorod (NR). In an acid condition, Fe²⁺ reacts with H₂O₂ to produce superoxide radical (O₂^•-) that etches gold NRs from the low energy surface along the longitudinal direction preferentially. As a result, the changes in the absorption spectrum and color of gold NR can be measured and also can be detected visually. Under the optimal conditions, the assay has very low detection limit (13.5 nM) and a linear response in a concentration range of 75 to 1 μM. The method was applied to the determination of Fe²⁺ in spiked samples of fetal bovine serum and also transferred to a kind of test stripe for use in fast practical applications. A unique colorimetric sensing method is demonstrated for the colorimetric detection of Fe²⁺, again based on the oxidation of gold nanorods which leads to the blue-shift of the absorption. Graphical abstract A unique colorimetric sensing method was shown for the colorimetric detection of Fe²⁺. Fe²⁺reacts with H₂O₂ to generate superoxide radical that oxidize gold nanorods. This leads to a color change from blue-green to pink.

Hydration of iron-porphyrins: ab initio quantum mechanical charge field molecular dynamics simulation study

The ab initio quantum mechanical charge field molecular dynamics (QMCF-MD) simulation approach was successfully applied to Fe²⁺-P and Fe³⁺-P in water to evaluate their structural, dynamical and energetic properties. Based on the structural data, it was found that Fe²⁺-P accommodates one water molecule in the first coordination sphere of the Fe²⁺ ion including the four nitrogen atoms of the porphyrin system coordinating with central metal species. On the other hand, two water molecules were coordinated to Fe³⁺-P, thus forming a hexa-coordinated species. Comparison of dynamical properties such as the vibrational power spectrum and ligand mean residence times to other metal-free porphyrin systems demonstrate the ions' influence on the hydration structure, enabling a characterisation of the strong interaction of the ions which greatly reduces the hydrogen bonding potential of the complex. The association of water molecules with the metal ions in both solutes was quantified by computing the free energy of binding obtained via the potential of mean force. This further confirmed the strong association of water to the metal ions which was conversely weak as inferred from the energetic data for the Fe²⁺-P system.

Zinc- and copper-porphyrins in aqueous solution - two similar complexes with strongly contrasting hydration

We present detailed analysis of the hydration behavior of zinc and copper bound porphyrins treated via ab initio quantum mechanical charge field molecular dynamics which agrees well with available experimental data. The computed metal-water coordination in the case of zinc bound porphyrin demonstrates a strong association of water with zinc compared to the copper-water interaction which correlates well with the calculated free energy of binding. The H-bond dynamics in these hydrated systems yield weaker H-bond interactions as compared to that observed in the case of metal-free porphyrin; nevertheless, the effect of metal association with porphyrin resulted in shifts in the vibrational frequencies. These characteristic data suggest a contrasting behavior between these metalloporphyrins in solution which could also serve to correlate with the properties of biological systems.

Room-temperature phosphorescence by Mn-doped ZnS quantum dots hybrid with Fenton system for the selective detection of Fe2+

Fe²⁺ was selectively detected based on the phosphorescence quenching of MPA–Mn : ZnS QDs caused by hydroxyl radicals from the Fenton reaction.

Structure and dynamics of chromatographically relevant Fe(III)-chelates

Immobilized metal ion affinity chromatography (IMAC) is an important chromatographic technique for biomolecules. In order to get a detailed understanding of the hydration of immobilized Fe(III), complexes of Fe(III) with methyl substituted iminodiacetate ([Fe(MSIDA)(H2O)3](+)) as well as with methyl substituted nitrilotriacetate ([Fe(MSNTA)(H2O)2]) were simulated in aqueous solutions with the quantum mechanical charge field molecular dynamics (QMCF MD) approach. The simulations were carried out at the Hartree-Fock (HF) level of theory, since cluster calculations at the HF, MP2, and B3LYP levels of theory showed that this method results in a good compromise between computational effort and accuracy. None of the coordinating water molecules were exchanged during the simulation period of 15 ps. The Fe-OH2O bond distances as well as the Fe-OH2O stretching motions differed among the coordinating water molecules, indicating different bond strengths. For the water molecules in the second hydration layer, mean residence times of 2.7 and 1.9 ps were obtained for [Fe(MSIDA)(H2O)3](+) and [Fe(MSNTA)(H2O)2], respectively. Furthermore, infrared measurements were carried out to characterize the most prominent bond features of aqueous Fe(III)-NTA and to discuss these results in conjunction with the computationally derived frequencies.

Parameterization of highly charged metal ions using the 12-6-4 LJ-type nonbonded model in explicit water

Highly charged metal ions act as catalytic centers and structural elements in a broad range of chemical complexes. The nonbonded model for metal ions is extensively used in molecular simulations due to its simple form, computational speed, and transferability. We have proposed and parametrized a 12-6-4 LJ (Lennard-Jones)-type nonbonded model for divalent metal ions in previous work, which showed a marked improvement over the 12-6 LJ nonbonded model. In the present study, by treating the experimental hydration free energies and ion–oxygen distances of the first solvation shell as targets for our parametrization, we evaluated 12-6 LJ parameters for 18 M(III) and 6 M(IV) metal ions for three widely used water models (TIP3P, SPC/E, and TIP4P_EW). As expected, the interaction energy underestimation of the 12-6 LJ nonbonded model increases dramatically for the highly charged metal ions. We then parametrized the 12-6-4 LJ-type nonbonded model for these metal ions with the three water models. The final parameters reproduced the target values with good accuracy, which is consistent with our previous experience using this potential. Finally, tests were performed on a protein system, and the obtained results validate the transferability of these nonbonded model parameters.

Erbium(III) in aqueous solution: an ab initio molecular dynamics study

Structural and dynamical properties of the erbium(III) ion in water have been obtained by means of ab initio quantum mechanical charge field molecular dynamics (QMCF-MD) simulations for the ground state and an excited state. The quality of the simulations has been monitored by recording UV/vis and Raman spectra of dilute solutions of ErCl3 and Er(NO3)3 in water and by comparison with EXAFS data from literature. Slight deviations between these data can be mainly attributed to relativistic effects, which are not sufficiently considered by the methodological framework. In both simulations, a mixture of coordination numbers eight and nine and a ligand exchange on the picosecond range are observed. The strength of the Er-ligand bond is considerably lower than that of trivalent transition metal ions but higher than that for La(III) and Ce(III) in aqueous solution. The main difference between ground state and excited state is the ligand exchange rate of the first shell. The second hydration shell is stable in both cases but with significantly different properties.

Computational study of the cerium(III) ion in aqueous environment

This work comprises the first quantum chemical simulation study of the Ce3+ ion in aqueous environment. The structural and dynamical properties have been investigated by means of the quantum mechanical charge field (QMCF) molecular dynamics (MD) approach and the results, where applicable, have been compared to experimental data. Besides conventional analytical tools, angular radial distribution functions have been employed to gain deeper insight into the structure of the hydrate. The ion-oxygen stretching motion's wavenumber, further characterising the Ce-O bond, is in excellent agreement with experimental results, same as the structural values obtained from the simulation.

A QMCF-MD investigation of the structure and dynamics of Ce4+ in aqueous solution

A quantum-mechanical charge-field molecular dynamics simulation has been performed for a tetravalent Ce ion in aqueous solution. In this framework, the complete first and second hydration spheres are treated by ab initio quantum mechanics supplemented by an electrostatic embedding technique, making the construction of non-Coulombic solute-solvent potentials unnecessary. During the 10 ps of simulation time, the structural aspects of the solution were analyzed by various methods. Experimental results such as the mean Ce-O bond distance and the predicted first-shell coordination number were compared to the results obtained from the simulation resolving some ambiguities in the literature. The dynamics of the system were characterized by mean ligand residence times and frequency/force constant calculations. Furthermore, Ce-O and Ce-H angular radial distribution plots were employed, yielding deeper insight into the structural and dynamical aspects of the system.

Hydration of trivalent lanthanum revisited - An ab initio QMCF-MD approach

The previously investigated La³⁺-hydrate has been re-evaluated by means of the quantum mechanical charge field (QMCF) molecular dynamics (MD) approach. Improved description of the hydration characteristics has been realised by including the full second hydration shell into the quantum mechanically treated region and by introducing the influence of the surrounding bulk via an electrostatic embedding technique. Analytical tools such as the ligand angular radial distribution analysis have been employed to gain deeper insight into the structural features of the hydrate. La³⁺ simultaneously forms nona- and decahydrates with capped trigonal and quadratic prismatic structure, besides small amounts of an octahydrate.

Simulation of Ir(III) in Aqueous Solution: The Most Inert Ion Hydrate

The ab initio quantum mechanical charge field (QMCF) molecular dynamics (MD) approach at Hartree-Fock level was used to simulate the tripositive iridium ion in aqueous solution, evaluating structure and dynamics of its hydrate complex. The Ir-OH2 force constant was of particular interest because of the observed high inertness of Ir(iii) in aqueous solution. Iridium forms three hydration shells. Six water molecules coordinate the ion in the first hydration shell in a well defined octahedral geometry, and no exchanges took place during the simulation time of 15 ps. The second hydration shell is very flexible, however, with a mean residence time of a water molecule of 3.6 ps. The third shell can be identified only by a slight ordering effect. This investigation classified the Ir-OH2 force constant as the strongest ion-OH2 bond known to date.

An investigation of biooxidation ability of Acidithiobacillus ferrooxidans using NMR relaxation measurement

NMR relaxation measurements can provide a simple means for understanding biological activity of cells in solution with known composition. It has the advantage that it is an in situ, non-intrusive technique, and the acquisition is fast. The iron oxidation ability of Acidithiobacillus ferrooxidans was investigated using NMR relaxation measurements. The transversal relaxation is characterized by a time constant, T₂, which is sensitive to the chemical environment. Fe³⁺ ion has more significant T₂ shortening than Fe²⁺ ion. In the presence of A. ferrooxidans in solutions containing Fe²⁺ ion, T₂ shortening was found with increasing time as the bacteria oxidize Fe²⁺ to Fe³⁺ ions. In the optimal growth medium, the bacteria concentration increased 80 times and high iron oxidation rate was found. In 10 mM K₂SO₄ medium, however, bacteria concentration remained almost unchanged and the iron oxidation rate was significantly lower.

Hydration of highly charged ions

Based on a series of ab initio quantum mechanical charge field molecular dynamics (QMCF MD) simulations, the broad spectrum of structural and dynamical properties of hydrates of trivalent and tetravalent ions is presented, ranging from extreme inertness to immediate hydrolysis. Main group and transition metal ions representative for different parts of the periodic system are treated, as are 2 threefold negatively charged anions. The results show that simple predictions of the properties of the hydrates appear impossible and that an accurate quantum mechanical simulation in cooperation with sophisticated experimental investigations seems the only way to obtain conclusive results.