Ammonia Solvation of Alkaline Earth Dications: Mg(II), Ca(II), Sr(II), and Ba(II); Hybrid Density Functional Theory Born-Oppenheimer Molecular Dynamics Studies.

Structural and dynamical properties of Sr2+ and Ba2+ dications in ammonia microsolvation environments were studied through hybrid density functional theory Born-Oppenhemier molecular dynamics of [Sr(NH3)n]2+ and [Ba(NH3)n]2+ clusters with n = 2, 3, 4, 5, 6, 8, 10, and 27. The largest cluster models were used to explore bulk phase solvation of Sr2+ and Ba2+ in liquid ammonia for which experimental data are available. Results are discussed in the light of previous results obtained for the [Mg(NH3)n]2+ and [Ca(NH3)n]2+ systems with the same methodology. Vibrational and EXAFS spectra are reported for the first time for [Sr(NH3)n]2+ and [Ba(NH3)n]2+ systems. It was found that alkaline earth dications have coordination numbers (CN) in ammonia as follows: Mg2+ (6) < Ca2+ (8) < Sr2+ (8.3) < Ba2+ (9.4). The coordination structures found turn out to be rather flexible with CN greater than six and these structures depart from the simple geometry of hexamine in the solid phase.

Mg(II) and Ca(II) Microsolvation by Ammonia: Born-Oppenheimer Molecular Dynamics Studies.

We report the structural and energetic features of the Mg2+ and Ca2+ cations in ammonia microsolvation environments. Born-Oppenhemier molecular dynamics studies are carried out for [Mg(NH3)n]2+ and [Ca(NH3)n]2+ clusters with n = 2, 3, 4, 6, 8, 20, and 27 at 300 K based on hybrid density functional theory calculations. We determine binding energies per ammonia molecule and the metal cation solvation patterns as a function of the number of molecules. The general trend for Mg2+ is that the Mg-N distances increase as a function of n until the first solvation shell is populated by six ammonia molecules, and then the distances slightly decrease while CN = 6 does not change. For Ca2+, the first solvation shell at room temperature is populated by eight ammonia molecules for clusters with more than one solvation shell, leading to a different structure from that of [Ca(NH3)6]2+ hexamine. The evaporation of NH3 molecules was found at 300 K only for Mg2+ clusters with n ≥ 10; this was not the case for Ca2+ clusters. Vibrational spectra are obtained for all of the clusters, and the evolution of the main features is discussed. EXAFS spectra are also presented for the [Mg(NH3)27(NH3)27]2+ and [Ca(NH3)27]2+ clusters, which yield valuable data to be compared with experimental data in the liquid phase, as previously done for the aqueous solvation of these dications.

Study of the Elusive Hydration of Pb2+ from the Gas Phase to the Liquid Aqueous Solution: Modeling the Hemidirected Solvation with a Polarizable MCDHO Force-Field.

The Pb2+ presents unique hydration features that make the experimental characterization and its theoretical modeling challenging: classical molecular dynamics (MD) with standard force-fields fails to produce the experimentally determined diffusion coefficient and the EXAFS spectrum. Here we study the hydration of Pb2+ in aqueous solution employing a polarizable model compatible with the MCDHO water model. The MCDHO FF for the Pb2+-water interaction was fitted to reproduce the configurations and interaction energies of various [Pb(H2O)n]2+ clusters obtained with ab initio calculations, with n = 4, 6, and 8. Its use in classical MD simulations yielded qualitative agreement with Born-Oppenheimer molecular dynamics of gas-phase hydrated clusters and MD simulations of the aqueous solution resulted in good agreement with the experimental DPb2+ and EXAFS spectrum. Analysis of the MD trajectories revealed a labile and very dynamic hemidirected first hydration shell in the aqueous solution with a non-well-defined coordination number CN; nonetheless, it was found that the more probable hydration structures have either 3 or 4 water molecules directly bound to the Pb2+ with another 3 or 2 at slightly larger distances. The simulations of the gas-phase [Pb(H2O)29]2+ cluster were found to capture the main structural features of the diluted aqueous solution.

On the aqueous solvation of AsO(OH)3vs. As(OH)3. Born-Oppenheimer molecular dynamics density functional theory cluster studies.

While arsenous acid, As(OH)3, has been the subject of a plethora of studies due to its worldwide ubiquity and its toxicity, pentavalent As in the form of arsenic acid, AsO(OH)3, has recently been found in rivers in central Mexico as the most abundant naturally occurring arsenic species. To better understand the solvation patterns of both toxic acids at the molecular level, we report the results of Born-Oppenheimer molecular dynamics simulations on the aqueous solvation of the AsO(OH)3 and As(OH)3 molecules at room temperature using the cluster microsolvation approach including 30 water molecules at the B3LYP/6-31G** level of theory. We found that the average per-molecule water binding energy is ca. 1 kcal mol-1 larger for the As(v) species as compared to the As(iii) one. To account for the asymmetry of both molecules, the hydration patterns were studied separately for a "lower" hemisphere, defined by the initially protonated oxygens, and for the opposite "upper" hemisphere. Similar lower hydration patterns were found for both As(iii) and As(v), with the same coordination number CN = 7. The upper pattern for As(iii) was found to be of a hydrophobic type, whereas that for As(v) showed the fourth oxygen to be hydrogen-bonded to the water network, yielding CN = 3.7; moreover, a proton "hopped" from the lower to the upper side, through the Grotthuss mechanism. Theoretical EXAFS spectra were obtained that showed good agreement with experimental data for As(iii) and As(v) in liquid water, albeit with somewhat longer As-O distances due to the level of theory employed. Proton transfer processes were also addressed; we found that the singly deprotonated H2AsO3- species largely dominated (99% of the simulation) for the As(iii) case, and that the deprotonated H2AsO4- and HAsO42- species were almost equally present (45% and 55%, respectively) for the As(v) case, which is in line with the experimental data pKa1 = 2.24 and pKa2 = 6.96. Through vibrational analysis the features of the Eigen and Zundel ions were found in the spectra of the microsolvated As(iii) and As(v) species, in good agreement with experimental data in aqueous solutions.

Aqueous solvation of Mg(ii) and Ca(ii): A Born-Oppenheimer molecular dynamics study of microhydrated gas phase clusters.

The hydration features of [Mg(H2O)n]2+ and [Ca(H2O)n]2+ clusters with n = 3-6, 8, 18, and 27 were studied by means of Born-Oppenheimer molecular dynamics simulations at the B3LYP/6-31+G** level of theory. For both ions, it is energetically more favorable to have all water molecules in the first hydration shell when n ≤ 6, but stable lower coordination average structures with one water molecule not directly interacting with the ion were found for Mg2+ at room temperature, showing signatures of proton transfer events for the smaller cation but not for the larger one. A more rigid octahedral-type structure for Mg2+ than for Ca2+ was observed in all simulations, with no exchange of water molecules to the second hydration shell. Significant thermal effects on the average structure of clusters were found: while static optimizations lead to compact, spherically symmetric hydration geometries, the effects introduced by finite-temperature dynamics yield more prolate configurations. The calculated vibrational spectra are in agreement with infrared spectroscopy results. Previous studies proposed an increase in the coordination number (CN) from six to eight water molecules for [Ca(H2O)n]2+ clusters when n ≥ 12; however, in agreement with recent measurements of binding energies, no transition to a larger CN was found when n > 8. Moreover, the excellent agreement found between the calculated extended X-ray absorption fine structure spectroscopy spectra for the larger cluster and the experimental data of the aqueous solution supports a CN of six for Ca2+.

Born-Oppenheimer molecular dynamics studies of Pb(ii) micro hydrated gas phase clusters.

In this work, a theoretical investigation was made to assess the coordination properties of Pb(ii) in [Pb(H2O)n]2+ clusters, with n = 4, 6, 8, 12, and 29, as well as to study proton transfer events, by means of Born-Oppenheimer molecular dynamics simulations at the B3LYP/aug-cc-pVDZ-pp/6-311G level of theory, that were calibrated in comparison with B3LYP/aug-cc-pVDZ-PP/aug-cc-pVDZ calculations. Hemidirected configurations were found in all cases; the radial distribution functions (RDFs) produced well defined first hydration shells (FHSs) for n = 4,6,8, and 12, that resulted in a coordination number CN = 4, whereas a clear-cut FHS was not found for n = 29 because the RDF did not have a vacant region after the first maximum; however, three water molecules remained directly interacting with the Pb ion for the whole simulation, while six others stayed at average distances shorter than 4 Å but dynamically getting closer and farther, thus producing a CN ranging from 6 to 9, depending on the criterion used to define the first hydration shell. In agreement with experimental data and previous calculations, proton transfer events were observed for n≤8 but not for n≥12. For an event to occur, a water molecule in the second hydration shell had to make a single hydrogen bond with a water molecule in the first hydration shell.

A theoretical study of the dissociation of the sI methane hydrate induced by an external electric field.

Molecular dynamics simulations in the equilibrium isobaric-isothermal (NPT) ensemble were used to examine the strength of an external electric field required to dissociate the methane hydrate sI structure. The water molecules were modeled using the four-site TIP4P/Ice analytical potential and methane was described as a simple Lennard-Jones interaction site. A series of simulations were performed at T = 260 K with P = 80 bars and at T = 285 K with P = 400 bars with an applied electric field ranging from 1.0 V nm(-1) to 5.0 V nm(-1). For both (T,P) conditions, applying a field greater than 1.5 V nm(-1) resulted in the orientation of the water molecules such that an ice Ih-type structure was formed, from which the methane was segregated. When the simulations were continued without the external field, the ice-like structures became disordered, resulting in two separate phases: gas methane and liquid water.

The role of hydration in the hydrolysis of pyrophosphate. A Monte Carlo simulation with polarizable-type interaction potentials.

The exchange of energy in biochemical reactions involves, in a majority of cases, the hydrolysis of phosphoanhydrides (P-O-P). This discovery has lead to a long discussion about the origin of the high energy of such bonds, and to a proposal that hydration plays a major role in the energetics of the hydrolysis. This idea was supported by recent ab initio quantum mechanical calculations (Saint-Martin et al. (1991) Biochim. Biophys. Acta 1080, 205-214) that predicted the hydrolysis of pyrophosphate is exothermic in the gas phase. This exothermicity can account for only a half of the total energy release that one measures in aqueous solutions. Here we address the problem of hydration of the reactants and products of the pyrophosphate hydrolysis by means of Monte Carlo simulations, employing polarizable potentials whose parameters are fitted to energy surfaces computed at the SCF/6-31G** level of the theory. The present results show that the hydration enthalpies of the reactants and products contribute significantly to the total energy output of the pyrophosphate hydrolysis. The study predicts that both, the orthophosphate and the pyrophosphate, have hydration spheres with the water molecules acting as proton acceptors in the P-OH ... O(water) hydrogen bonds. These water molecules weakly repel the water molecules in the further hydration spheres. The perturbation of the structure of the solvent caused by the presence of the solute molecules is short ranged: after ca. 5 A from the P atoms, the energy and the structure of water correspond to bulk water. Due mainly to nonadditive effects, the molecular structure of the hydrated pyrophosphate is quite different from two fused structures of the hydrated orthophosphates. The hydration sphere of pyrophosphate is very loose and has a limited effect on the water network, whereas for orthophosphate it has a well developed shell structure. Hence, upon hydration there will be both a gain in hydration enthalpy and a gain in entropy because of distortion of the water molecular network.

Ab initio calculations of the pyrophosphate hydrolysis reaction.

Ab initio quantum mechanical calculations were used to study the hydrolysis reaction H4P2O7 + H2O in equilibrium with 2H3PO4, as well as some molecular properties of the reactants and products. SCF calculations with several basis sets ranging from minimal to extended with polarization functions were used to look at the basis dependency of the reaction enthalpies and optimized geometries. Although the minimal basis sets yield erratic predictions of the enthalpy, when a more extended basis (3-21G*) was used for the geometry optimization, and the total energies of the reactants and products were computed with this and larger basis sets, we obtained more consistent predictions of the structural properties of the P-O-P bridge and of the heat of the hydrolysis reaction (delta E = -7.39 kcal/mol at the SCF/6-31G** level). A comparison is made with previous estimates performed with smaller basis sets and without taking into account the electron correlation effects, which are calculated in the present work. The inclusion of the zero point energy calculated using the harmonic approximation, and of the electronic correlation energy determined at the MBPT(2) level, raised the computed heat of the reaction to -3.83 kcal/mol, and when an estimate for the thermal energy was added, the value obtained was of -3.38 kcal/mol. In conclusion, we found that the hydrolysis of pyrophosphate should be exothermic in the gas phase. The implications of this result in relation to some recent theories about enzyme catalysis are discussed.