On the structure and dynamics of the hydrated sulfite ion in aqueous solution--an ab initio QMCF MD simulation and large angle X-ray scattering study.

Theoretical ab initio quantum mechanical charge field molecular dynamics (QMCF MD) formalism has been applied in conjunction to experimental large angle X-ray scattering to study the structure and dynamics of the hydrated sulfite ion in aqueous solution. The results show that there is a considerable effect of the lone electron-pair on sulfur concerning structure and dynamics in comparison with the sulfate ion with higher oxidation number and symmetry of the hydration shell. The S-O bond distance in the hydrated sulfite ion has been determined to 1.53(1) Å by both methods. The hydrogen bonds between the three water molecules bound to each sulfite oxygen are only slightly stronger than those in bulk water. The sulfite ion can therefore be regarded as a weak structure maker. The water exchange rate is somewhat slower for the sulfite ion than for the sulfate ion, τ(0.5) = 3.2 and 2.6 ps, respectively. An even more striking observation in the angular radial distribution (ARD) functions is that the for sulfite ion the water exchange takes place in close vicinity of the lone electron-pair directed at its sides, while in principle no water exchange did take place of the water molecules hydrogen bound to sulfite oxygens during the simulation time. This is also confirmed when detailed pathway analysis is conducted. The simulation showed that the water molecules hydrogen bound to the sulfite oxygens can move inside the hydration shell to the area outside the lone electron-pair and there be exchanged. On the other hand, for the hydrated sulfate ion in aqueous solution one can clearly see from the ARD that the distribution of exchange events is symmetrical around the entire hydration sphere.

Structure and dynamics of the chromate ion in aqueous solution. An ab initio QMCF-MD simulation.

An ab initio quantum-mechanical charge-field molecular-dynamics (QMCF-MD) simulation of the chromate ion in aqueous solution at ambient temperature was performed to study the structure and dynamics of this ion and its hydration shell. In contrast to conventional quantum-mechanical molecular-mechanics molecular-dynamics (QM/MM-MD) simulations, the QMCF-MD approach offers the possibility of investigating composite systems with the accuracy of a QM/MM method but without the time-consuming construction of solute-solvent potential functions. The data of the simulation give a clear picture of the first hydration shell of the chromate anion, which consists of 14 water molecules. The mean distance between the oxygen atoms of the chromate and the hydrogen atoms of water is 1.82 A. Each chromate oxygen atom is in average coordinated to 2.6 water molecules. The first-shell mean ligand residence time was evalulated as 2.2 ps; the vibrational frequency of the nu(OH) mode was found to be 185 cm(-1). Several structural parameters such as the radial distribution functions, angular distribution functions, and coordination number distributions enable a full characterization of the embedding of the chromate ion in the solvent water. The dynamics of the hydration structure are described by mean residence times of the water molecules in the first hydration shell, distance plots, and velocity autocorrelation functions.

Structure and dynamics of the UO2+ ion in aqueous solution: an ab initio QMCF-MD study.

The quantum mechanical charge field molecular dynamics (QMCF-MD) framework was applied in a simulation of the uranyl(v) ion in aqueous solution. The structure was evaluated on the basis of overall and sectorial radial distribution functions, angular distribution functions, tilt- and Theta-angle distribution functions and coordination number distributions. The cation is strongly coordinated by 4 water ligands at an average distance of 2.51 A, while the oxygen atoms are on average bound to 1.2 water molecules at a distance of 2.9 A. A mean residence time of 2.83 ps was evaluated for the oxygen sites of the uranyl(v) ion. The results are in good agreement with previous experimental and theoretical data on the hydration of similar ions.

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

Ab initio quantum mechanical charge field molecular dynamics (QMCF-MD) simulations have been performed for aqueous solutions of Fe(2+) and Fe(3+) ions at the Hartree-Fock level of theory to describe and compare their structural and dynamical behavior. The structural features of both hydrated ions are characterized by radial distribution functions that give the maximum probability of the ion-O distance for Fe(2+) and Fe(3+) ions at 2.15 and 2.03 A, respectively. The angular distribution functions of both ions prove the octahedral arrangement of six water ligands, whereas the second shells of these ions differ. Both ions show influence on the water molecules beyond the second shells. The structure-forming abilities of both ions are visible from the ligand mean residence times and ion-O stretching frequencies evaluated for both ions. The substantially improved data obtained from these QMCF-MD simulations show better correlation with available experimental results than the conventional quantum mechanics/molecular mechanics molecular dynamics (QM/MM MD) approaches with one hydration shell treated by quantum mechanics.

Hydrolytic conversion of AsO(-3)4 to HAsO(-2)4: a QMCF MD study.

A quantum mechanical charge field molecular dynamics (QMCF MD) study of AsO in water was carried out to gain insight into its conversion from the hydrated anion resulting in OH(-) ions and HAsO, which occurs on the scale of a few hundred femtoseconds. The OH(-) ion undergoes further proton exchange with water molecules, while HAsO is a stable species.

Structural and dynamic aspects of hydration of HAsO4(-2): an ab initio QMCF MD simulation.

An ab initio quantum mechanical charge field simulation has been carried out in order to obtain molecular level insight into the hydration behavior of HAsO4(-2), one of the major biologically active components of As(V) oxoanion in neutral to slightly alkaline aqueous medium. Moreover, a geometrical definition of hydrogen bonding has been used to probe and characterize both solute-solvent and solvent-solvent hydrogen bonding present in the system. The asymmetry of the anion induced by the protonation of one of the oxygens of the arsenate anion causes rather irregular hydration structure. The nonprotonated oxygen atoms preferably form relatively stable hydrogen bonds with two to three water molecules in their vicinity, while the protonated oxygen forms one or two hydrogen bonds, weaker than water-water hydrogen bonds. The two types of As-O distances obtained from the simulation (1.68 and 1.78 A for the protonated and nonprotonated oxygens, respectively) are in good agreement with the experimental data. The two types of As-O stretching frequencies obtained from the simulation (855 and 660 cm(-1) reproduce well the experimental ATR-FTIR results (859 and 680-700 cm(-1)).

Temperature dependence of structure and dynamics of the hydrated Ca2+ ion according to ab initio quantum mechanical charge field and classical molecular dynamics.

Simulations using ab initio quantum mechanical charge field molecular dynamics (QMCF MD) and classical molecular dynamics using two-body and three-body potentials were performed to investigate the hydration of the Ca(2+) ion at different temperatures. Results from the simulations demonstrate significant effects of temperature on solution dynamics and the corresponding composition and structure of hydrated Ca(2+). Substantial increase in ligand exchange events was observed in going from 273.15 K to 368.15 K, resulting in a redistribution of coordination numbers to lower values. The effect of temperature is also visible in a red-shift of the ion-oxygen stretching frequencies, reflecting weakened ligand binding. Even the moderate increase from ambient to body temperature leads to significant changes in the properties of Ca(2+) in aqueous environment.

Ab initio quantum mechanical charge field study of hydrated bicarbonate ion: structural and dynamical properties.

The ab initio quantum mechanical charge field molecular dynamics (QMCF MD) formalism was applied to simulate the bicarbonate ion, HCO(3)(-), in aqueous solution. The difference in coordination numbers obtained by summation over atoms (6.6) and for the solvent-accessible surface (5.4) indicates the sharing of some water molecules between the individual atomic hydration shells. It also proved the importance to consider the hydration of the chemically different atoms individually for the evaluation of structural and dynamical properties of the ion. The orientation of water molecules in the hydration shell was visualized by the theta-tilt surface plot. The mean residence time in the surroundings of the HCO(3)(-) ion classify it generally as a structure-breaking ion, but the analysis of the individual ion-water hydrogen bonds revealed a more complex behavior of the different coordination sites.

Revisiting the hydration of Pb(II): a QMCF MD approach.

A quantum mechanical charge field (QMCF) molecular dynamics (MD) study of Pb(II) in an aqueous medium was carried out in order to gain insight into its solvation behavior, for both structural and dynamic aspects. Applying the advanced methodology and different basis sets, some new aspects concerning the solvation of Pb(II) have been revealed. One of the most interesting outcomes of the current simulation is the variation of first shell coordination number from 7 to 9 in the Pb(H2O)n(2+) complex with Pb(H2O)8(2+) as a major species. Moreover, a far more dynamic and labile hydration shell was found compared to previous QM/MM MD simulation with only the first hydration shell treated by quantum mechanics, which reported a very rigid first hydration shell with a fixed coordination number of 9. The current simulation results are in much better agreement with the properties reported from the recent thermodynamic studies than the previous QM/MM MD study.

Structure and dynamics of the UO(2)(2+) ion in aqueous solution: an ab initio QMCF MD study.

A comprehensive theoretical investigation on the structure and dynamics of the UO(2)(2+) ion in aqueous solution using double-zeta HF level quantum mechanical charge field molecular dynamics is presented. The quantum mechanical region includes two full layers of hydration and is embedded in a large box of explicitly treated water to achieve a realistic environment. A number of different functions, including segmential, radial, and angular distribution functions, are employed together with tilt- and Theta-angle distribution functions to describe the complex structural properties of this ion. These data were compared to recent experimental data obtained from LAXS and EXAFS and results of various theoretical calculations. Some properties were explained with the aid of charge distribution plots for the solute. The solvent dynamics around the ion were investigated using distance plots and mean ligand residence times and the results compared to experimental and theoretical data of related ions.

Beryllium(II): the strongest structure-forming ion in water? A QMCF MD simulation study.

A quantum mechanical charge field (QMCF) molecular dynamics (MD) simulation including the first and second hydration shells in the QM region has been carried out to describe the structural and dynamical properties of Be(2+) in aqueous solution. In this methodology, the full first and second hydration shells are treated by ab initio quantum mechanics supplemented by a fluctuating electrostatic embedding technique. From the simulation, structural properties were extracted and were found to be in good agreement with previously published experimental and theoretical results. The radial distribution function (RDF) showed the maximum probability of the Be-O bond length at 1.62 A. The first tetrahedrally arranged hydration shell is highly inert with respect to ligand-exchange processes. Application of local-density-corrected three-body correlation analysis showed minor structural influence of the ion beyond the second hydration layer, contrary to the findings of a previous QM/MM MD simulation. The dynamics of the hydrate were studied in terms of ligand mean residence times (MRTs) and the power spectrum of the Be(2+)-O stretching frequency. A comparison of the "classical" QM/MM framework with the QMCF method clearly demonstrated the advantages of the latter, as ambiguities arising from the coupling of the subregions occurring in QM/MM MD simulations did not appear when the QMCF ansatz was applied.

Structure and dynamics of the U4+ ion in aqueous solution: an ab initio quantum mechanical charge field molecular dynamics study.

The structure and dynamics of the stable four-times positively charged uranium(IV) cation in aqueous solution have been investigated by ab initio quantum mechanical charge field (QMCF) molecular dynamics (MD) simulation at the Hartree-Fock double-zeta quantum mechanical level. The QMCF-MD approach enables investigations with the accuracy of a quantum mechanics/molecular mechanics approach without the need for the construction of solute-solvent potentials. Angular distribution functions; radial distribution functions; coordination numbers of the first, second, and third shell (9, 19, and 44, respectively); coordination number distribution functions; tilt- and Theta-angle distribution functions; as well as local density corrected triangle distribution functions have been employed for the evaluation of the hydrated ion's structure. Special attention was paid to the determination of the geometry of the first hydration layer, and the results were compared to experimental large-angle X-ray scattering and extended X-ray absorption fine structure data. The solvent dynamics around the ion were also investigated using mean ligand residence times and related data and, resulting from the unavailability of any experimental data, were compared to ions with similar properties.

The hydration structure of Sn(II): an ab initio quantum mechanical charge field molecular dynamics study.

The structural properties of the hydrated Sn(2+) ion have been investigated using ab initio quantum mechanical charge field molecular dynamics (QMCF MD) simulations at double-xi HF quantum mechanical level. The results from the work significantly extend previous study using QM/MM MD simulation and are in good agreement with X-Ray and EXAFS diffraction experiments. The data indicate a set of characteristics for the first hydration shell uncommon among metal ions. Although frequent ligand exchange prevents the formation of a well defined structure, more detailed analyses reveal an asymmetric distribution of ligands around Sn(II). An average of eight water molecules coordinate with the Sn(2+) ion and are distributed at proximal and distal positions that are distinguishable from the second hydration shell and manifest dissimilar degrees of lability.

Structure and dynamics of phosphate ion in aqueous solution: an ab initio QMCF MD study.

A simulation of phosphate in aqueous solution was carried out employing the new QMCF MD approach which offers the possibility to investigate composite systems with the accuracy of a QMMM method but without the time consuming creation of solute-solvent potential functions. The data of the simulations give a clear picture of the hydration shells of the phosphate anion. The first shell consists of 13 water molecules and each oxygen of the phosphate forms in average three hydrogens bonds to different solvent molecules. Several structural parameters such as radial distribution functions and coordination number distributions allow to fully characterize the embedding of the highly charged phosphate ion in the solvent water. The dynamics of the hydration structure of phosphate are described by mean residence times of the solvent molecules in the first hydration shell and the water exchange rate.

Structure and dynamics of solvated Sn(II) in aqueous solution: an ab initio QM/MM MD approach.

Structural and dynamical properties of the hydrated Sn(II) ion have been investigated by ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations at double-zeta HF quantum mechanical level. The results indicate Sn(II)aq to be a rather peculiar, if not unique, case of a hydrated ion: four of its eight first-shell ligands do not take place in the otherwise frequent ligand-exchange processes, forming an approximately tetrahedral cage around the ion. The remaining ligands, however, exchange at a rate that is rather comparable to monovalent than divalent ions. This very surprising behavior of ligand exchange not yet observed in any previous simulation of over 30 hydrated metal ions is consistently confirmed by vibrational spectra, bond lengths, and a detailed analysis of the trajectories of the simulation.