Acetonitrile real gas phase behavior from quasi-ideal gas to nanodroplets: A microscopical view.

We have applied a recently developed general purpose acetonitrile force field based on first-principles calculations to simulate acetonitrile in the gas phase at different temperatures and densities. These conditions range from nearly ideal to real gas phase behavior and condensation. The molecular dynamics simulation results agree fairly well with the experimental studies available in the literature on the gas samples. The structural analysis of aggregates and their associated interaction energies is examined and related to the early model proposed on molecular association and equilibrium determining the non-ideal behavior. The formation of dimers is mainly responsible for the non-ideal behavior of the gas at very low density, confirming suggested models based on previous experimental studies. However, when the density of the sample rises, the level of aggregation increases and the simple concept of dimerization does not hold anymore. The real behavior adopted by the gas is related to the distribution of molecular structures observed. The macroscopical view of a real gas as a generic interparticle interaction system without a defined form may then be rationalized on the basis of a defined molecular association originated by a distribution of aggregates at the low density regime. The sample with the highest density (∼1.4 × 103 mol m-3) at the lowest temperature exhibits a massive aggregation where most of the acetonitrile (ACN) molecules in the simulation box form a big cluster. Its radial distribution function is similar to that of the liquid ACN. This strongly inhomogeneous distribution in the box can be considered a condensation in the gas phase under specific density-T conditions. This formation opens the door to the potential tuning of its solvent properties as a function of its size in these nanodroplets that in turn are controlled by the density-T conditions.

Shedding light on the metal-phthalocyanine EXAFS spectra through classical and ab initio molecular dynamics.

Extended X-Ray Absorption Fine Structure (EXAFS) theoretical spectra for some 3d transition metal-phthalocyanines-FePc, NiPc, CuPc, and ZnPc-are presented. Their complexity and rigidity make them a good testbed for the development of theoretical strategies that can complement the difficulties present in the experimental spectrum fitting. Classical and ab initio molecular dynamics trajectories are generated and employed as a source of structural information to compute average spectra for each MPc species. The original ZnPc force field employed in the classical molecular dynamics simulations has been modified in order to improve the agreement with the experimental EXAFS spectrum, and the modification strategy-based on MP2 optimized structures-being extended to the rest of MPcs. Both types of trajectories, classical and ab initio, provide very similar results, showing in all cases the main features present in the experimental spectra despite the different simulation timescales employed. Spectroscopical information has been analyzed on the basis of shells and legs contributions, making possible the comparison with the experimental fitting approaches. According to the simulations results, the simple relationships employed in the fitting process to define the dependence of the Debye Waller factors associated with multiple scattering paths with those of single scattering paths are reasonable. However, a lack of multiple backscattering paths contributions is found due to the intrinsic rigidity of the chemical motif (macrocycle). Its consequences in the Debye Waller factors of the fitted contributions are discussed.

A Coupled EXAFS-Molecular Dynamics Study on PuO2+ and NpO2+ Hydration: The Importance of Electron Correlation in Force-Field Building.

The physicochemical properties of the monovalent actinyl cations, PuO2+ and NpO2+, in water have been studied by means of classical molecular dynamic simulations. A specific set of cation-water intermolecular potentials based on ab initio potential energy surfaces has been built on the basis of the hydrated ion concept. The TIP4P water model was adopted. Given the paramagnetic character of these actinyls, the cation-water interaction energies were computed from highly correlated wave functions using the NEVPT2 method. It is shown that the multideterminantal character of the wave function has a relevant effect on the main distances of the hydrated molecular cations. Several structural, dynamical, and energetic properties of the aqueous solutions have been obtained and analyzed. Structural RDF analysis gives An-Oyl distances of 1.82 and 1.84 Å and An-O(water) distances of 2.51 and 2.53 Å for PuO2+ and NpO2+ in water, respectively. Experimental EXAFS spectra from dilute aqueous solutions of PuO2+ and NpO2+ are revisited and analyzed, assuming tetra- and pentahydration of the actinyl cations. Simulated EXAFS spectra have been computed from the snapshots of the MD simulations. Good agreement with the experimental information available is found. The global analysis leads us to conclude that both PuO2+ and NpO2+ cations in water are stable pentahydrated aqua ions.

Hydration of Heavy Alkaline-Earth Cations Studied by Molecular Dynamics Simulations and X-ray Absorption Spectroscopy.

The physicochemical properties of the three heaviest alkaline-earth cations, Sr2+, Ba2+, and Ra2+ in water have been studied by means of classical molecular dynamics (MD) simulations. A specific set of cation-water intermolecular potentials based on ab initio potential energy surfaces has been built on the basis of the hydrated ion concept. The polarizable and flexible model of water MCDHO2 was adopted. The theoretical-experimental comparison of structural, dynamical, energetic, and spectroscopical properties of Sr2+ and Ba2+ aqueous solutions is satisfactory, which supports the methodology developed. This good behavior allows a reasonable reliability for the predicted Ra2+ physicochemical data not experimentally determined yet. Simulated extended X-ray absorption fine-structure (EXAFS) and X-ray absorption near-edge spectroscopy spectra have been computed from the snapshots of the MD simulations and compared with the experimental information available for Sr2+ and Ba2+. For the Ra2+ case, the Ra L3-edge EXAFS spectrum is proposed. Structural and dynamical properties of the aqua ions for the three cations have been obtained and analyzed. Along the [M(H2O)n]m+ series, the M-O distance for the first-hydration shell is 2.57, 2.81, and 2.93 Å for Sr2+, Ba2+, and Ra2+, respectively. The hydration number also increases when one is going down along the group: 8.1, 9.4, and 9.8 for Sr2+, Ba2+, and Ra2+, respectively. Whereas [Sr(H2O)8]2+ is a typical aqua ion with a well-defined structure, the Ba2+ and Ra2+ hydration provides a picture exhibiting an average between the ennea- and the deca-hydration. These results show a similar chemical behavior of Ba2+ and Ra2+ aqueous solutions and support experimental studies on the removal of Ra-226 of aquifers by different techniques, where Ra2+ is replaced by Ba2+. A comparison of the heavy alkaline ions, Rb+ and Cs+, with the heavy alkaline-earth ions is made.

Combining EXAFS and Computer Simulations to Refine the Structural Description of Actinyls in Water.

EXAFS spectroscopy is one of the most used techniques to solve the structure of actinoid solutions. In this work a systematic analysis of the EXAFS spectra of four actinyl cations, [UO2]2+, [NpO2]2+, [NpO2]+ and [PuO2]2+ has been carried out by comparing experimental results with theoretical spectra. These were obtained by averaging individual contributions from snapshots taken from classical Molecular Dynamics simulations which employed a recently developed [AnO2]2+/+ -H2O force field based on the hydrated ion model using a quantum-mechanical (B3LYP) potential energy surface. Analysis of the complex EXAFS signal shows that both An-Oyl and An-OW single scattering paths as well as multiple scattering ones involving [AnO2]+/2+ molecular cation and first-shell water molecules are mixed up all together to produce a very complex signal. Simulated EXAFS from the B3LYP force field are in reasonable agreement for some of the cases studied, although the k= 6-8 Å-1 region is hard to be reproduced theoretically. Except uranyl, all studied actinyls are open-shell electron configurations, therefore it has been investigated how simulated EXAFS spectra are affected by minute changes of An-O bond distances produced by the inclusion of static and dynamic electron correlation in the quantum mechanical calculations. A [NpO2]+-H2O force field based on a NEVPT2 potential energy surface has been developed. The small structural changes incorporated by the electron correlation on the actinyl aqua ion geometry, typically smaller than 0.07 Å, leads to improve the simulated spectrum with respect to that obtained from the B3LYP force field. For the other open-shell actinyls, [NpO2]2+ and [PuO2]2+, a simplified strategy has been adopted to improve the simulated EXAFS spectrum. It is computed taking as reference structure the NEVPT2 optimized geometry and including the DW factors of their corresponding MD simulations employing the B3LYP force field. A better agreement between the experimental and the simulated EXAFS spectra is found, confirming the a priori guess that the inclusion of dynamic and static correlation refine the structural description of the open-shell actinyl aqua ions.

A general study of actinyl hydration by molecular dynamics simulations using ab initio force fields.

A set of new ab initio force fields for aqueous [AnO2]2+/+ (An = Np(vi,v), Pu(vi), Am(vi)) has been developed using the Hydrated Ion (HI) model methodology previously used for [UO2]2+. Except for the non-electrostatic contribution of the HI-bulk water interaction, the interaction potentials are individually parameterized. Translational diffusion coefficients, hydration enthalpies, and vibrational normal mode frequencies were calculated from the MD simulations. Physico-chemical properties satisfactorily agree with experiments validating the robustness of the force field strategy. The solvation dynamics and structure for all hexavalent actinoids are extremely similar and resemble our previous analysis of the uranyl cation. This supports the idea of using the uranyl cation as a reference for the study of other minor actinyls. The comparison between the NpO2 2+ and NpO2 + hydration only provides significant differences in first and second shell distances and second-shell mean residence times. We propose a single general view of the [AnO2]2+/+ hydration structure: aqueous actinyls are amphiphilic anisotropic solutes which are equatorially conventional spherically symmetric cations capped at the poles by clathrate-like water structures.

Hydration Structure of the Elusive Ac(III) Aqua Ion: Interpretation of X-ray Absorption Spectroscopy (XAS) Spectra on the Basis of Molecular Dynamics (MD) Simulations.

Knowledge of actinoid solution chemistry has been enriched with the recent synthesis and characterization of the elusive Ac(III) aqua ion, the first one of the series, for which extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectra has been recorded. Structural analysis combined with Born-Oppenheimer molecular dynamics simulations lead to suggest a 2.63-2.69 Å range for the Ac-O distance, and a coordination number between 9 and 11. A hydration number as high as 11 would imply the appearance of a sharp coordination number contraction at the beginning of the series. In this work, we present a specific Ac(III)-H2O first-principles-based intermolecular potential, which has been developed following the exchangeable Hydrated Ion model. This potential has been used in classical molecular dynamics (MD) simulations of Ac(III) in water. Results show a well-defined Ac(III) ennea-hydrated aqua ion with a mean Ac-O distance of 2.66 ± 0.02 Å, surrounded by a compact second hydration shell formed by ∼20 H2O centered at 4.9 ± 0.1 Å. The results obtained for the first element of the actinoid series confirm the regular contraction of their aqua ions along the series. Simulated EXAFS and XANES spectra have been computed from the structural information provided by the MD simulation. The agreement with the experimental spectra is satisfactory, validating the results from the computer simulation. An observed hump in the experimental XANES spectrum is interpreted and ascribed to the second hydration shell, being an evidence of the consistency of the Ac(III) hydration shells.

Extracting the Americyl Hydration from an Americium Cationic Mixture in Solution: A Combined X-ray Absorption Spectroscopy and Molecular Dynamics Study.

Am(VI) solution chemistry differs from that of lighter actinoids, as U, Pu, and Np, where the actinyl [AnO2]2+ is the most stable form and plays an important role in nuclear fuel technology. The behavior of americium in solution shows the trend to stabilize lower oxidation states, mainly Am(III). Riddle and co-workers recently reported the EXAFS and first XANES spectra of an americium-containing aqueous solution where the americyl species is detected in a mixture. We have developed Am3+-H2O and [AmO2]2+-H2O intermolecular potentials based on quantum-mechanical calculations to carry out classical MD simulations of these two cations in water. Structural information extracted from the statistical trajectories has been used to simulate EXAFS and XANES spectra of both solutions. For the Am3+ case the theoretical-experimental agreement for both EXAFS and XANES spectra is satisfactory. This is not the case for the [AmO2]2+ aqueous solutions. However, when an aqueous solution mixture of both cationic forms in a 55/45 [AmO2]2+/Am3+ ratio is considered, the theoretical-experimental agreement is recovered. EXAFS and XANES spectra which would correspond to a pure [AmO2]2+ aqueous solution are proposed. In the XANES case, the main features characterizing the simulated spectrum are consistent with those previously found in the experimental XANES spectra of stable [UO2]2+ and [PuO2]2+ in water.

The hydration structure of the heavy-alkalines Rb+ and Cs+ through molecular dynamics and X-ray absorption spectroscopy: surface clusters and eccentricity.

Physicochemical properties of the two heaviest stable alkaline cations, Rb+ and Cs+, in water have been examined from classical molecular dynamics (MD) simulations. Alkaline cation-water intermolecular potentials have been built from ab initio interaction energies of [M(H2O)n]+ clusters. Unlike in the case of other monatomic metal cations, the sampling needed the inclusion of surface clusters to properly describe the interactions. The first coordination shell is found at an average M-O distance of 2.87 Å and 3.12 Å for Rb+ and Cs+, respectively, with coordination numbers of 8 and 10. Structural, dynamical and energetic properties are discussed on the basis of the delicate compromise among the ion-water and water-water interactions which contribute almost on the same foot to the definition of the solvent structure around the ions. A significant asymmetry is detected in the Rb+ and Cs+ first hydration shell. Reorientational times of first-shell water molecules for Cs+ support a clear structure-breaking nature for this cation, whereas the Rb+ values do not differ from pure water behavior. Experimental EXAFS and XANES spectra have been compared to simulated ones, obtained by means of application of the FEFF code to a set of statistically significant structures taken from the MD simulations. Due to the presence of multi-excitations in the absorption spectra, theoretical-experimental agreement for the EXAFS spectra is reached when the multi-excitations are removed from the experimental spectra.

A hydrated ion model of [UO2]2+ in water: Structure, dynamics, and spectroscopy from classical molecular dynamics.

A new ab initio interaction potential based on the hydrated ion concept has been developed to obtain the structure, energetics, and dynamics of the hydration of uranyl in aqueous solution. It is the first force field that explicitly parameterizes the interaction of the uranyl hydrate with bulk water molecules to accurately define the second-shell behavior. The [UO2(H2O)5]2+ presents a first hydration shell U-O average distance of 2.46 Å and a second hydration shell peak at 4.61 Å corresponding to 22 molecules using a coordination number definition based on a multisite solute cavity. The second shell solvent molecules have longer mean residence times than those corresponding to the divalent monatomic cations. The axial regions are relatively de-populated, lacking direct hydrogen bonding to apical oxygens. Angle-solved radial distribution functions as well as the spatial distribution functions show a strong anisotropy in the ion hydration. The [UO2(H2O)5]2+ solvent structure may be regarded as a combination of a conventional second hydration shell in the equatorial and bridge regions, and a clathrate-like low density region in the axial region. Translational diffusion coefficient, hydration enthalpy, power spectra of the main vibrational modes, and the EXAFS spectrum simulated from molecular dynamics trajectories agree fairly well with the experiment.

Identifying Coordination Geometries of Metal Aquaions in Water: Application to the Case of Lanthanoid and Actinoid Hydrates.

The angular distribution function (ADF) associated with the oxygen-metal ion-oxygen angle (OMO) of several trivalent lanthanoid and actinoid aquaions has been used to identify the most probable coordination geometry of these aquaions in aqueous solutions. The ADFs extracted from the molecular dynamics trajectories have been compared with continuous distribution functions corresponding to the geometry of a reference polyhedron pattern. The procedure incorporates specific quantum-mechanical information on the aquaion under study. The new method is applied to the analysis of four M(H2O)n3+ aquaions in water, M = Lu and Cf for n = 8, and M = La and Ac for n = 9. For those that are 8-coordinated, the square antiprism (SA) coordination geometry is preferred. For the 9-fold coordination, the simulation ADFs are more similar to the continuous ADF derived from a Gyro-elongated-SA rather than to the usually proposed trigonal tricapped prism. Advantages of these continuous distributions with respect to the usually employed discrete distributions are emphasized as well as further applications are suggested.

Hydration of Two Cisplatin Aqua-Derivatives Studied by Quantum Mechanics and Molecular Dynamics Simulations.

The hydration of the cisplatin aqua-derivatives, cis-[PtCl(H2O)(NH3)2](+) (w-cisplatin) and cis-[Pt(H2O)2(NH3)2](2+) (w2-cisplatin), has been studied by means of classical molecular dynamics simulations. The new platinum complex-water interaction potential, w-cisplatin-W, has been built on the basis of the already obtained cisplatin-water interaction potential (cisplatin-W) [J. Chem. Theory Comput. 2013 9, 4562]. That potential has been then transferred to the w2-cisplatin-W potential. The w-cisplatin and w2-cisplatin atomic charges were specifically derived from their solute's wave functions. Bulk solvent effects on the complex-water interactions have been included by means of a continuum model. Classical MD simulations with 1 platinum complex and 1000 SPC/E water molecules have been carried out. Angle-solved radial distribution functions and spatial distribution functions have been used to provide detailed pictures of the local hydration structure around the ligands (water, chloride, and ammine) and the axial region. A novel definition of a multisite cavity has been employed to compute the hydration number of complexes in order to provide a consistent definition of their first-hydration shell. Interestingly, the hydration number decreases with the increase of the complex net charge from 27 for cisplatin to 23 and 18 for w-cisplatin and w2-cisplatin, respectively. In parallel to this hydration number behavior, the compactness of the hydration shell increases when going from the neutral complex, i.e. cisplatin, to the doubly charged complex, w2-cisplatin. Quantum mechanics estimation of the hydration energies for the platinum complexes allows the computation of the reaction energy for the first- and second-hydrolysis of cisplatin in water. The agreement with experimental data is satisfactory.

Collecting high-order interactions in an effective pairwise intermolecular potential using the hydrated ion concept: the hydration of Cf³âº.

This work proposes a new methodology to build interaction potentials between a highly charged metal cation and water molecules. These potentials, which can be used in classical computer simulations, have been fitted to reproduce quantum mechanical interaction energies (MP2 and BP86) for a wide range of [M(H2O)n](m+)(H2O)ℓ clusters (n going from 6 to 10 and ℓ from 0 to 18). A flexible and polarizable water shell model (Mobile Charge Density of Harmonic Oscillator) has been coupled to the cation-water potential. The simultaneous consideration of poly-hydrated clusters and the polarizability of the interacting particles allows the inclusion of the most important many-body effects in the new polarizable potential. Applications have been centered on the californium, Cf(III) the heaviest actinoid experimentally studied in solution. Two different strategies to select a set of about 2000 structures which are used for the potential building were checked. Monte Carlo simulations of Cf(III)+500 H2O for three of the intermolecular potentials predict an aquaion structure with coordination number close to 8 and average R(Cf-O) in the range 2.43-2.48 Å, whereas the fourth one is closer to 9 with R(Cf-O) = 2.54 Å. Simulated EXAFS spectra derived from the structural Monte Carlo distribution compares fairly well with the available experimental spectrum for the simulations bearing 8 water molecules. An angular distribution similar to that of a square antiprism is found for the octa-coordination.

Hydration of Cisplatin Studied by an Effective Ab Initio Pair Potential Including Solute-Solvent Polarization.

The hydration of cis-[PtCl2(NH3)2] (cisplatin) has been studied by means of classical molecular dynamics simulations using a new interaction potential obtained by fitting about 4000 ab initio interaction energies calculated at the MP2 level. The functional form included several r(-n) terms (n = 4, 6, 8, 12) to achieve an accurate description of the interactions in the different regions around the cisplatin. Bulk solvent effects on the cisplatin-water molecule interactions have been included by means of a continuum model. Radial Distribution Function (RDF) analysis does not provide a clear enough description of the hydration pattern due to the intricate solvent arrangement around the solute. Angle-solved RDFs and spatial distribution functions have been used to provide more detailed pictures of the local hydration structure around the two ligands, chloride and ammine groups, and the axial region. Based on this information, it is shown a more convenient way to compute the running coordination number for the first hydration shell by simultaneously considering angle-solved RDFs centered on the ligand representative atoms of the complex: ammino N, Cl, and Pt atoms. This way, the hydration number is obtained by integrating over an interlocking-sphere volume built by the spheres centered on the cation and the main atoms of each ligand. Compared to previous works dealing with cisplatin hydration, the global hydration number for the first coordination shell is now higher and involves about 27 water molecules. The importance of the structural sampling, the computational level, as well as the functional form adopted for the interaction potential are thoroughly discussed with respect to the previous proposed intermolecular potential.

Quantum-mechanical study on the aquaions and hydrolyzed species of Po(IV), Te(IV), and Bi(III) in water.

A systematic study of [M(H(2)O)(n)(OH)(m)](q+) complexes of Te(IV) and Bi(III) in solution has been undertaken by means of quantum mechanical calculations. The results have been compared with previous information obtained for the same type of Po(IV) complexes ( J. Phys. Chem. B 2009 , 113 , 487 ) to get insight into the similarities and differences among them from a theoretical view. The evolution of the coordination number (n + m) with the degree of hydrolysis (m) for the stable species shows a systematic decrease regardless the ion. A general behavior on the M-O distances when passing from the gas phase to solution, represented by the polarizable continuum model (PCM), is also observed: R(M-O) values corresponding to water molecules decrease, while those of the hydroxyl groups slightly increase. The hydration numbers of aquaions are between 8 and 9 for the three cations, whereas hydrolyzed species behave differently for Te(IV) and Po(IV) than for Bi(III), which shows a stronger trend to dehydrate with the hydrolysis. On the basis of the semicontinuum solvation model, the hydration Gibbs energies are -800 (exptl -834 kcal/mol), -1580 and -1490 kcal/mol for Bi(III), Te(IV), and Po(IV), respectively. Wave function analysis of M-O and O-H bonds along the complexes has been carried out by means of quantum theory of atoms in molecule (QTAIM). Values of electron density and its Laplacian at bond critical points show different behaviors among the cations in aquaions. An interesting conclusion of the QTAIM analysis is that the prospection of the water O-H bond is more sensitive than the M-O bond to the ion interaction. A global comparison of cation properties in solution supplies a picture where the Po(IV) behavior is between those of Te(IV) and Bi(III), but closer to the first one.

An ab initio molecular dynamics study on the hydrolysis of the Po(IV) aquaion in water.

Po(IV) in water has been studied by means of Car-Parrinello molecular dynamics (CPMD) simulations. A new Trouiller-Martins pseudopotential for Po(IV) has been developed. This pseudopotential was tested by comparing the structure and energetics of small [Po(H(2)O)(n)(OH)(m)](4-m) clusters optimized quantum-mechanically. CP-MD simulations of 1 Po + 60 H(2)O were carried out starting from three different degrees of hydrolysis of the aquaion (m = 0, 2, and 3), in order to check the stability of the hydrolyzed forms under the simulation conditions. The three simulations converge to a description of the solution where the same hydrolyzed species are present. Dynamics of the octahydrate aquaion in water indicates that dehydration couples to hydrolysis processes, and the total coordination number decreases with the hydrolysis degree. The time evolution of the initial [Po(H(2)O)(8)](4+) aquaion in aqueous solution indicates that hydrolysis precedes to dehydration in the process from aquaion to hydroxoaquaion. Structural and dynamical properties of the ligands in the first coordination shell are analyzed. The power spectra and its contribution from fragments of the first coordination shell are also examined.

Characterizing Pt-derived anticancer drugs from first principles: the case of oxaliplatin in aqueous solution.

The molecular compound ethyldiamine-oxalatoplatinum(II), EDO-Pt, is used as a model to study the oxaliplatin anticancer drug in aqueous solution by means of ab initio computer simulation. Gas-phase structure optimizations have been performed for both oxaliplatin and its EDO-Pt mimic along with Car-Parrinello molecular dynamics simulations of EDO-Pt in gas phase and in aqueous solution. The coordination of Pt(II) is square-planar on average, with Pt-N and Pt-O(I) distances of 2.04 A in solution. The diamine ligand has a bent structure, while the oxalate ligand is planar on average. The complex features a very rigid structure during the simulation and the charge distribution describes a dipole with its negative pole on the oxalate ligand and the positive pole on the Pt-diamine side. The solvation pattern of EDO-Pt is most well-defined around the amine and oxalate groups and is quantified by means of radial and spatial distribution functions of water molecules around the complex. Decomposition of radial distribution functions into their contributions from different regions (axial and equatorial) reveals an "anionic hydration" pattern of the metal cation by the solvent, which is analogous in nature to the bare Pt(II) aqua ion. A qualitative prediction on the kinetics of ligand exchange in oxaliplatin is derived based on its axial hydration pattern.

General quantum-mechanical study on the hydrolysis equilibria for a tetravalent aquaion: the extreme case of the Po(IV) in water.

A systematic study of the different hydrolyzed species derived from the hydrated Po(IV) in water, [Po(H(2)O)(n)(OH)(m)]((4-m)) for 1 m 4, and 4 m + n 9, has been carried out by means of quantum mechanical computations. The effects of outer solvation shells have been included using a polarizable continuum dielectric model. For a fixed number of hydroxyl groups, the preferred hydration number for the Po(IV) can be determined in terms of Gibbs energy. It is shown that the hydration number (n) systematically decreases with the increase in the number of hydroxyl groups (m) in such a way the total coordination number (n + m) becomes smaller, being 9 in the aquocomplex and 4 in the neutral hydroxo-complex. Free energies for the hydrolysis processes involving Po(IV) complexes and a different number of hydroxyl groups have been computed, revealing the strong tendency toward hydrolysis exhibited by these complexes. The predominant species of Po(IV) in aqueous solutions are ruled by a dynamical equilibrium involving aggregates containing in the first coordination shell OH(-) groups and water molecules. Although there is not experimental information to check the theoretical predictions, theoretical computations in solution seem to suggest that the most likely clusters are [Po(H(2)O)(5)(OH)(2)](2+) and [Po(H(2)O)(4)(OH)(2)](2+). The geometry of the different clusters is ruled by the trend of hydroxyl groups to be mutually orthogonal and to promote a strong perturbation of the water molecule in trans-position by lengthening the Po-H(2)O distances and tilting the corresponding bond angle. A general thermodynamic cycle is defined to compute the Gibbs free energy associated to the formation of the different hydrolyzed forms in solution. From it, the estimates of pK(a) values associated to the different protolytic equilibria are provided and discussed. Comparison of the relative values of pK(a) along a hydrolysis series with the experimental values for other tetravalent cations supports its consistency.

Po(IV) hydration: a quantum chemical study.

This work presents a theoretical study on the hydration of Po(IV) in solution. Three points have been addressed: (i) the level of calculation needed to properly describe the system under study, (ii) the hydration number of Po(IV), and (iii) the nature of the polonium-water bonding. The condensed medium effects have been included by means of a continuum solvation model, thus different [Po(H(2)O)(n)](4+) hydrates were embedded in a cavity surrounded by a polarizable dielectric medium. Among the quantum-mechanical calculation levels here considered, the MPW1PW91 functional was shown to be the most suitable, allowing a proper description of the Po-H(2)O interactions at affordable cost. The hydration number of Po(IV) was found to be between 8 and 9. This value is ruled by a dynamic equilibrium involving the octa- and ennea-hydrates, although the 7-fold coordination cannot be completely excluded. The hydration free energy of Po(IV) is estimated to be around -1480 kcal/mol. The Po-H(2)O bonding is dominated by strong electrostatic contributions although a small covalent contribution is responsible for the peculiar arrangement adopted by the smaller hydrates (n < or = 5). A natural bond order (NBO) analysis of the hydrate wave functions shows that the covalent bond involves the empty 6p orbitals of the polonium ion and one lone pair on the oxygen atom of the water molecule. A parallel investigation to the hydrate study, where the polonium ion was replaced by a tetravalent point charge plus a repulsion potential, was carried out. These results allowed a detailed examination of the electrostatic and nonelectrostatic contributions to the polonium hydrate formation.