Effect of Basicity on the Hydrolysis of the Bi(III) Aqua Ion in Solution: An Ab Initio Molecular Dynamics Study

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ "This document is the Accepted Manuscript version of a Published Work that appeared in final form in The Journal of Physical Chemistry A, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.jpca.7b12402 ." Effect of Basicity on the Hydrolysis of the Bi(III) Aqua Ion in Solution: an Ab Initio Molecular Dynamics Study Regla Ayala,†,§Jos´e Manuel Mart´ınez,‡Rafael R. Pappalardo,‡Keith Refson,¶,k and Enrique S´anchez Marcos∗,‡ †Universidad de Sevilla, CSIC, ICMSE ‡Universidad de Sevilla, Departamento Qu´ımica F´ısica, 41012 Seville, Spain ¶Royal Holloway University of London, Egham Hill, Egham, Surrey TW20 0EX, United Kingdom. §Departamento Qu´ımica Inorg´anica, 41012 Seville, Spain kISIS Facility, Science and Technology Facility Council, Harwell Science Campus, Didcot, OX11 0QX, UK E-mail: [email protected] 1 Abstract Hydrolysis of the Bi(III) aqua ion under a range of solution conditions has been studied by means of ab initio molecular dynamics simulations. While the Bi(III) aqua ion is stable in pure water, there is an increasing degree of hydrolysis with the number of hydroxide anions in the medium. This is accompanied by a monotonic decrease of the total coordination number to an asymptotic value of ∼6, reached under extreme basicity conditions. Comparison of the simulated Bi(III) hydrolyzed species with the experimental species distribution at different degrees of basicity suggests that at the PBE/DFT level of theory here employed, liquid water shows an overly acidic character. Predictions of theoretical EXAFS and XANES spectra were generated from the AIMD trajectories for different Bi hydrolyzed species, [Bi(HO)m(H2O)n]3–m+, m=0-3 and n=7-2. Comparison with available experimental spectra is presented. Spectral features joined to the degree of hydrolysis and hydration are analyzed. 2 Introduction. Although ion hydration is a central topic of solution chemistry, the description of highlycharged ion-water interactions remains a challenge, particularly for some multivalent cations involved in a series of hydrolysis reactions and polynucleation processes.1–6 This is the case of Bi(III) in water, which plays important roles in chemistry and medicine; for example as an antimicrobial and anticancer agent or in radio-therapy, to name just a few.2,3,5–18 The aqua ion, [Bi(H2O)n]3+, was characterized by N¨aslund et al.17 from acidic solutions of its triflate salt. These authors emphasized the importance of hydrolysis in its solution chemistry. The strong propensity of Bi(III) to suffer hydrolysis in aqueous solution derives from its rather complex chemistry in water. Hydrolyzed species occur in both monomeric, [Bi(OH)m(H2O)n]3−m, and polymeric, [Bip(OH)m(H2O)n]3p–mspecies.1Hydrolysis, polymerization and anion complexation are all involved as a function of the concentration, degree of acidity, counterions and supporting electrolytes. Tooth et al.19 carried out an study on the nature and stability of Bi(III) aqueous complexes under natural hydrothermal conditions, characterizing monomeric species bearing up to four OH−groups. Figure 1 shows the species distribution diagram of the aqueous Bi(III) ion as a function of pH.20 Monomeric species appear only at low Bi(III) concentrations and no evidence of polynuclear species formation in highly dilute solution (∼10−5M) has been found.5,6,19,20 Therefore, the Bi(III) concentration determines which are the main species present: polynuclear species at high or moderate Bi(III) concentrations and mononuclear species at low concentrations. The aqueous oxo species (BiO+) is not usually included in distribution species diagrams, due in part to the difficulty of discriminating between O2−and OH−ligands.1The BiOX species in water can be also formulated as Bi(OH)2X,10 X−being the salt counterion (which is chosen to be non-complexing, or at least very weakly complexing). Nonetheless, the presence of BiOX species can not be disregarded. The formation of BiOCl,11 BiOClO49,13,17 and BiONO3,11,12 species, have all been reported, the last two being very insoluble.11 Therefore, the degree of hydrolysis is lower due to the nitrate or chloride 3 complexing ability with Bi(III) as mononuclear species.12–16 Figure 1: Experimental species distribution diagram of Bi(III) aqueous solutions at room temperature taken from Ref.[ 20]. The species distribution is governed by the equilibrium constants associated with the hydrolysis processes9which allow the formulation of the complex equilibria [Bi(H2O)n]3+ –H+ + H+ [Bi(OH)(H2O)n–1]2+ –H+ + H+ [Bi(OH)2(H2O)n–2]+ [Bi(OH)2(H2O)n–2]+–H+ + H+ [Bi(OH)3(H2O)n–3]–H+ + H+ [Bi(OH)4(H2O)n–4]– involving monomeric species under particular pH conditions, as detailed in Figure 1.1,20 The theoretical study of hydrolysis processes and determination of the dominant species as a function of the pH medium is not easy. In a previous quantum mechanical (QM) study21 on Bi(III) aqueous solutions using a semi-continuum solvent model, the analysis of the equilibria and the different structures involved showed a decrease in the coordination number as hydrolysis processes proceed. Starting from the octa-hydrate, [Bi(H2O)8]3+, a threefold coordination was observed when a degree of hydrolysis of 3 (understood as the number of hydroxyl ligands in the first solvation shell) was reached, [Bi(OH)3]. However, 4 the analysis of these equilibrium structures could only be interpreted assuming that solvent effects were provided by a continuum dielectric without accounting for a particular acid or basic medium. Thus, the differences arose only from the number of water molecules in the cluster without any formal change in the chemical nature of the bulk solvent. QM methodology based on the Self Consistent Reaction Field22 does not allow us to connect the number of OH−ligands in [Bi((OH)m(H2O)n]3−mclusters with the medium conditions. That is, the OH-ligand number was imposed and the evaluation of the optimum number of water molecules in the [Bi(OH)m(H2O)n](3–m)+ ion was subsequently obtained by changing (n). In other words, once the degree of hydrolysis (m) was fixed, the total coordination number (n+m) was determined. The present work expands on the previous static QM view by modeling the solvent at the molecular level, by considering now the evolution of the Bi(III) species as a function of medium conditions. To this aim ab initio molecular dynamics (AIMD) simulations become an appropriate tool. In a previous work23 on the Po(IV) hydration, medium conditions were modeled by specifying the initial configuration, that is, starting from partially or fully hydrolyzed aqua ion species, but retaining the same number of atoms in the simulation box. Every hydroxyl group in the initial Po(IV) coordination sphere was compensated by an extra proton in the bulk ([Po(OH)m(H2O)n](4–m)+ +mH+). The starting clusters considered (m=0, 2 and 3) converged to a similar equilibrated species distribution.23 In the present case, we undertake the analysis of the evolution of Bi(H2O)n 3+ aquaion and some of its hydrolyzed forms as a function of the number of hydroxide anions present in bulk water. The AIMD simulations allow us to follow the chemical evolution of the Bi(III) species and its bulk environment. The goal of determining the changes induced in the dominant species in solution on the addition and removal of proton species in the medium is within current capabilities. The structural analysis of AIMD trajectories provides valuable information on the connection between the degree of basicity of the medium and the number of OH−ligands in the close environment of the Bi(III) ion. This work is therefore a first step in the use of AIMD simulations to evaluate how solvent conditions alter the population of 5 hydrolyzed species of monomeric Bi(III) aquaion in aqueous solutions. It must be pointed out that this theoretical study can be carried out at a qualitative level given that the pH values, as experimentally given in Figure 1, can not be obtained in our case with the methodology employed. Therefore, the speciation distribution diagram of the different Bi(III) monomeric species is only a qualitative guide of species detected under different media, an index of their appearance sequence when the basicity conditions are changed in the medium. Although DFT description of liquid water is not fully satisfactory because there is no functional that can simultaneously describe its structure, dynamics and thermodynamics,24–30 among GGA functionals, PBE is capable of predicting the energetics of water clusters reasonably well.24,29 It is worth noting that in this study we are dealing with highly-charged hydrated ion solutions where the ion-water interactions are significantly larger than the energies associated to the water-water interactions. This fact gives us more confidence in the description of the closest Bi(III) environment using PBE. Even more, an important point for the proper description of hydrolysis processes is the average number of hydrogen bonds. For PBE water, the value is 3.7,30 which is quite close to the experimental estimation (3.5) at ambient conditions.31 The behavior of OH−in water has been the central subject of many theoretical studies and we refer to recent reviews on this topic32–34 which reveal that the DFT level of theory is inadequate for an accurate description of the aqueous hydrolysis structure and thermodynamics of hydroxide anions. In the present study we focus our attention on the Bi(III) aquaion and its hydrolyzed forms which are mediated by hydroxyl ligands, rather than by the hydroxide anions. The relative errors of the DFT energetics will be smaller in the highly polarized ligand environment than in bulk aqueous solution, so it is reasonable to suppose that the effect will be less severe. From an experimental side, X-ray absorption spectroscopies (EXAFS and XANES) are appropriate techniques to help in the structural elucidation of the species in solution.35 However, this complex behavior makes difficult the collection of XAS spectra of the [Bi(H2O)m]3+ 6 aqua ion, an important reference for the follow up of the chemical changes induced by the medium. N¨aslund et al.17 carried out a systematic EXAFS study on Bi(III) solvation in several solvents, including water, and their related solvated crystal. The difficult issue is to discriminate the presence of one or several Bi(III) species in solution contributing to the global EXAFS signal. Tooth et al.19 carried out a systematic XAS study on monomeric [Bi(H2O)n(OH)m]3−min water under hydrothermal conditions which can also be taken as a valuable experimental reference. In this sense, it would be of general interest to simulate XAS spectra from the structural information provided by our MD simulations. Because the chemical species involved are well characterized, the XAS spectrum and the solution structure can be easily correlated, and contributions to the total signal identified.36–40 Methodology. Simulation Protocols. Plane-wave AIMD simulations were performed using the Car-Parrinello scheme41 as implemented in the CPMD code42 The NVT ensemble at a temperature of 350 K using a Nos´e-Hoover chain of thermostats on the ionic degrees of freedom was employed.43–45 For an initial period of 0.6 ps the timestep was set at 0.060 fs while the electronic system relaxed towards the ground state, switching to 0.097 fs thereafter. The fictitious mass of the additional electronic degrees of freedom was 400 a.u. Periodic boundary conditions were applied to a cubic simulation box of side 13.136 ˚ A. Vanderbilt ultrasoft pseudopotentials46 (USP) were used to describe the interactions between the ionic cores and the valence electrons. A plane wave cutoff of 30 Ry was used. The electronic wavefunction was solved using DFT with the PBE exchange-correlation functional.47 Oxygen and hydrogen pseudopotentials were taken from standard libraries, but for Bi we generated a new pseudopotential using a 6s26p2reference configuration and rc= 2.0a0. The use of two projectors per angular momentum channel and the direct generation of fully-local projectors have proven the good 7 behavior of this set of pseudopotentials.48 This was tested against an accurate reference USP by computing forces for a single hydrated Bi(III) in crystalline Bi2O3yielding a standard deviation of 0.016 eV/˚ A. To check the coupling of this new pseudopotential with the rest of QM elements employed in the Bi(III) clusters, we have optimized a set of hydrolyzed species, [Bi(OH)m(H2O)n](3–m)+, and compared with the optimized structures obtained for the same species in our previous DFT(MPW1PW91) study using Gaussian basis sets.21 Table 1 gives the Bi–O(OH) and Bi–O(H2O) distances and the optimized geometries for clusters having up to four OH groups. It is shown a satisfactory agreement that supports the good performance of the Bi pseudopotential proposed. Table 1: Average Bi-O(OH) and Bi-O(H2O) distances taken from Ref. 21 for quantum mechanical optimizations (MPW1PW91/Gaus.) and from PBE and Plane-wave using the ultrasoft pseudopotential developed in this study. Structure Method d(BiOH2O)/˚ A d(BiOH2O)/˚ A 7+0 MPW1PW91/Gaus. 3×2.43, 2×2.47, 2×2.49 — PBE/PW 2×2.41, 2.42, 2.44, 2.46, 2.47, 2.48 — 5+1 MPW1PW91/Gaus. 2×2.46, 2.47, 2.52, 2.54 2.03 PBE/PW 3×2.45, 2.51, 2.53 2.04 3+2 MPW1PW91/Gaus. 2.48, 2.49, 2.91 2.05, 2.06 PBE/PW 2.47, 2.48, 2.82 2.06, 2.07 3+3 MPW1PW91/Gaus. 2.66, 2.68, 2.74 2.11, 2.14, 2.18 PBE/PW 2.64, 2.65, 2.69 2.12, 2.15, 2.19 2+3 MPW1PW91/Gaus. 2.64, 2.69 2.10, 2.13, 2.15 PBE/PW 2.63, 2.65 2.11, 2.14, 2.16 1+4 MPW1PW91/Gaus. 3.52 2.14, 2.15, 2.22, 2.28 PBE/PW 3.53 2.14, 2.15, 2.21, 2.26 The thermalization and production periods were 8.6 and 40 ps long, respectively. Hydrolysis events were identified using the following criteria:49 i)for each O atom, the H atoms being at 1.2 ˚ A or closer are assigned to that oxygen atom; ii)each H atom that 8 Figure 2: Bi-O RDFs (black) along with their running integration numbers (blue) and Bi-H RDFs (red) for simulations 1-4. 15 almost identical Bi-O RDFs. This demonstrates that our simulation times were sufficiently long to exceed the characteristic timescales of hydrolysis events and achieve configurational convergence. The Bi-H RDFs support this analysis. For simplicity, running integration number curves for hydrogen are not included in Figure 2. The first peak is split into two by the hydrolysis events previously discussed when going from simulation 1 to 2C. The peak at shorter distances corresponds to the HOH atoms whereas the second one is due to the HH2Oatoms. Bi-H RDFs of simulations 3A-3C are similar, although a slight shift toward longer distances occurs with the removal of protons. As expected, Bi-H RDF of simulation 4 is alike to simulation 2B. Due to the peculiar shape of RDF series, we have performed an RDF error analysis for the Bi-O pair in the cases of simulations 1 and 2A. For both pair distribution functions, Figure 1 of SI includes the estimation of the standard error of the mean (SEM) for each value of r, that is, in addition to g(r) distribution, g(r)±SEM(r) lines have been included as well. This is a measure of the dispersion of values around the mean g(r) values. For computing of SEM(r), the blocking method of Flyvberg and Petersen58 has been employed. Results show the consistency of our trajectories. Once the RDFs have been analyzed, we can supply a detailed picture of the set of species involved in solution under different degree of hydrolysis. The evolution of hydroxide anion in our simulations leads to a quick transformation of these ions in hydroxyl ligands in the Bi(III) first coordination shell. We have included in SI (Figure 2) a comparison between OOH-OH2ORDF of aqueous solutions containing OH−32 and the RDF of the same pair of the [Bi(OH)(H2O)5]2+ solution. In this case the hydroxyl ligand is immersed in a metal complex which certainly induces a strong asymmetry in the water distribution. For this reason, apart from the total RDF, we have plotted an RDF where oxygen atoms belonging to the first-shell water molecules have been excluded. Likewise, for completeness reason the OOH-OH2O(1st.shell) RDF is also included in Figure 2 of SI. This 16 total RDF is only partially similar to the RDFs of OH−in water.32–34 The peaks are shifted to longer Rbecause the hydrogen bonds formed by the hydroxyl ligand are weaker than those of the OH−. Contrarily, the Bi(III)-monohydrolyzed RDF is more similar to the structure of the OH−surface water clusters obtained by Crespo et al.59 This is due to the fact that one half of the sphere centered in the hydroxyl oxygen atom is controlled by the Bi(III) and not by the OH−anion by itself. First coordination shell analysis. Simulation 1 starts with the aqua ion [Bi(H2O)8]3+ + 56 H2O and ends at the [Bi(H2O)7]3+ + 57 H2O. No hydrolysis events were observed during the 40 ps simulation, just the loss of one water molecule from the first hydration shell at the fifth picosecond. Since that event, the heptahydrate is the representative species along the simulation, suffering short transient events involving first shell water exchanges. Analysis of EXAFS spectra of N¨aslund et al.17 uses a fix coordination number of 8 in an aqueous solution highly concentrated in perchloric acid to prevent aqua ion hydrolysis and polymerization phenomena.1To obtain a good fitting, their structural model included the presence of perchlorate anions in the first coordination shell and contributions of second-shell water molecules. Our simulation conditions correspond formally to about a 1 M Bi(III) aqueous solution without counterions which could be responsible in part of the disagreement in the coordination number of the aqua ion. Rode and coworkers60 using an ab initio quantum mechanical charge field molecular dynamics (QMCF-MD) approach propose a 9-fold coordination with a maximum probability of the Bi-O distance at 2.51 ˚ A. We will reexamine this point in the discussion of the EXAFS results. When one proton was removed from the simulation box (simulation 2A), the system evolved as expected from our previous QM calculations,21 that is, one hydrolysis event is observed together with the loss of one water molecule, yielding the [Bi(OH)(H2O)5]2+ aggregate at the thermalized situation. Subsequent hydrolysis processes were favored when 17 two protons were removed (simulation 2B). Starting from the aqua ion, a degree of hydrolysis of 2 was reached involving mainly two species with different number of water molecules: [Bi(OH)2(H2O)3]+and [Bi(OH)2(H2O)4]+, the average coordination number for this system becoming 5.6. Although a mechanistic study of the OH−diffusion in these solutions is not the central objective of this work, we have qualitatively examined how evolves the initial OH−anions placed on the box boundary in the set of simulations B. Grotthuss diffusion33,34 easily favored the system reorganization and equilibration during the first picoseconds of simulations. Figure 3 displays time evolution of the representative aggregates found in simulations 2C, 3A and 3C during the simulation. The removal of a third proton (simulation 2C, see top of Figure 3) leads to more hydrolyzed clusters, [Bi(OH)3(H2O)n], being the hydration degree between 2 and 4, and the mean coordination number 6. Although hydrolysis and dehydration are coupled, when passing from simulation 2B to 2C a higher degree of hydrolysis does not imply the loss of more water molecules. This suggests an asymptotic limit for the coordination number of Bi(III) ion in aqueous solutions around 6. In our previous quantum mechanical study,21 we found that when two or three OH− groups become part of the first solvation shell of the Bi(III) they avoid a trans arrangement among them. The water molecules in a trans position to the hydroxyl ligands lengthen their Bi-O distances with respect to the remaining water molecules in the cluster. This behaviour is also found in simulations 2B and 2C and illustrated in Table 1. To remove a fourth proton, the final configuration of simulation 2C was chosen as starting point. Its removal in simulation 3A leads to the evolution shown in the middle of Figure 3, where the main representative species are [Bi(OH)4(H2O)]−and [Bi(OH)4(H2O)2]−. These species were not considered in our previous study because we evaluated up to a maximum degree of hydrolysis of 3. However, they appear in the experimental species distribution diagram of Figure 1 at extreme alkaline conditions.19 When two and three protons (sim- 18 Figure 3: Time evolution (bar charts) of the Bi(III) environment in terms of number of water (blue bars) and hydroxyl ligands (red bars) along simulations 2C(top), 3A(middle) and 3C(bottom). Total bar height gives the total coordination number, partial coordination number for OH−and H2O are represented by the respective heights of blue and red bars. 19 ulations 3B and 3C, respectively) were removed from the final configuration of simulation 2C, both situations lead to highly hydrolyzed species, [Bi(OH)5]2−, [Bi(OH)6]−3and some hydrated forms of them. Bottom of Figure 3 shows the evolution of simulation 3C where [Bi(OH)5(H2O]2−is dominant although [Bi(OH)5]2−and [Bi(OH)6]3−are present along the simulation. Although our results show the correct trend, there is an overestimate of the degree of hydrolysis, likely due to an incorrect balance between H bonding and water coordination that gives rise to a too acidic water model. The absence of counterions may also influence the observed behavior.12–16 The removal of 6 protons from simulation 1 to simulation 3C can be regarded as an extreme scenario of alkaline conditions collected in Fig. 3 (bottom). In our previous work21 the possibility of four or more OH−ligands around the Bi(III) was not considered. However the study of other highly charged cations like Po(IV) and Te(IV)21,23,56 indicates that a first coordination shell with four OH−ligands can give rise to two OH−groups opposite to each other. In this case metal-OOH distances are larger than those found in less hydrolyzed clusters. The same conclusion holds up when the structures of aqueous Bi(III) derived from simulations 3A-3C are analyzed (see Table 1). This arrangement explains the shift toward longer distances found in the Bi–O RDF first peak of simulations 3A-3C in Figure 2. To test whether a high degree of hydrolysis could lead to the formation of oxo species, simulation 4 was started from the [BiO]+aggregate. The results indicate that this oxo species is not stable at all. The Bi-O species evolved to the [Bi(OH)]2+ one during the very initial ∼9 fs equilibration stage, which means that the protonation of the oxo bond oxygen atom occurred despite the frozen Bi-O distance. In the subsequent run, the system progressed to a chemical distribution of [Bi(OH)2(H2O)3]+and [Bi(OH)2(H2O)4]+, something very similar to that of simulation 2B. This shows that all memory of the initial condition has been lost and both simulations reach the same chemical composition. 20 EXAFS and XANES spectra of Bi(III) aqueous solutions In Figure 4 we compare some calculated model EXAFS spectra to the experiment. The top panel compares the experimental L3-edge k3-weighted spectrum of an acidic Bi(ClO4)3 aqueous solution (dotted blue line) with that of a [Bi(H2O)9](CF3SO3)3crystal (dotted black line), both obtained by N¨aslund et al.17 The middle panel compares the EXAFS spectrum derived from our AIMD simulation 1 (solid black line) with the experimental one recorded by Naslund et al.17 for the Bi(ClO4)3aqueous solution (dotted blue line). The bottom panel shows a spectrum obtained from the classical MD simulation that has used the [Bi(H2O)8]3+ –H2O intermolecular potential specifically fitted to reproduce the structural data reported by N¨aslund et al.17 (solid red line). The red curve was obtained from a 250 ps classical MD simulation. First of all we will analyze the spectra of Bi(III) aqueous solution plotted in the middle panel of Figure 4. The theoretical spectrum from our AIMD simulation (black solid line) has a smaller amplitude and frequency than the experimental one of the Bi(ClO4)3aqueous solution (dotted blue line). If this was an analysis of single scattering signals, it would indicate that our AIMD simulation has a smaller coordination number and/or a larger DW factor. However in the case of electrolyte solutions this analysis is complicated by the presence of counterions and concentration. Only in very dilute solution can we be sure that counterion effects do not disturb the ion solvation structure. Our simulated spectrum corresponds to the high-dilution limit as our AIMD simulation 1 describes a single Bi(III) only surrounded by water molecules with a dominant backscattering contribution due to the first-shell Bi-OH2Opaths. This is confirmed by the simple shape of the EXAFS function (black line in the middle of Figure 4) that corresponds to an average RBi−OI= 2.47 ±0.2 ˚ A. By contrast the experimental EXAFS spectrum of the 0.66M Bi(ClO4)3aqueous solution recorded by N¨aslund et al.17 was modeled by a fitting which included contributions to the EXAFS signal not only from the first-shell Bi-OH2Opaths but also from the Bi-OClO4and the second-shell Bi-O paths. In that case, the fitted first-shell RBi−OIwas 2.41˚ A. 21 Figure 4: Comparison of experimental k3-weighted EXAFS spectra of a Bi(III) perchlorate aqueous solutions (blue dotted line) with solid hydrate [Bi(H2O)9](CF3SO3)3(black dotted line)17 (top), with simulated spectrum derived from the AIMD simulation of Bi(III) in water (black line) (middle), and with the simulated spectrum obtained from a classical MD simulation using an ad-hoc intermolecular Bi-water potential fitted to reproduce the structural data given by the experimental fitting (red line) (bottom). 22 Looking at the top panel of Figure 4, the higher amplitude of the [Bi(H2O)9](CF3SO3)3, solid sample (black dotted line) with respect to the experimental spectrum of the liquid sample reflects two features discriminating both systems: i) the coordination number for the solid sample is higher than for the liquid one, ii)the DW factor is smaller for the solid sample than for the liquid one. The frequency of their EXAFS signals is quite similar even though RBi-O in the crystal is 0.08 ˚ A longer than in the solution.17 Although RBi-O=2.49 ˚ A in the solid is quite close to our AIMD simulation value, the signal frequency is rather different for both spectra. The bottom panel of Figure 4 shows a second experimental-theoretical comparison of the EXAFS liquid sample (blue dotted line) with the simulated spectrum (red line) obtained from a classical MD simulation which used the ad-hoc classical interaction potential previously described. In this case, the simple signal provided by the simulated EXAFS (red line) exhibits a frequency lower than that of the experimental signal solution (blue dotted line), although structurally for both samples RBi−OI= 2.408 ˚ A. Following N¨aslund et al.’s analysis, the difference between these EXAFS signals is due to two additional contributions coming from the presence of one perchlorate anion in the first-shell and the second hydration shell. Addition of a second hydration shell in the EXAFS modeling from our AIMD simulation makes little difference to the shape of the predicted spectra. Tooth et al.19 provided an independent XANES spectrum of the Bi(III) aqua ion at high T and P (see Figure 5 dotted line). Because the low liquid compressibility and sensitivity of XANES to pressure, it is reasonable to compare the computed XANES from simulation 1, the Bi(III) aqua ion, (red line) with the experimental one. To match the position of the main resonance, the theoretical spectrum has been only shifted -4.5 eV. For the sake of completeness, the MD trajectory derived from the use of the classical Bi-H2O intermolecular potential has been used to simulate a second XANES spectrum. The good agreement of both simulated spectra with the experimental one supports the reliability of the structural description derived from the AIMD and classical MD simulations. 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Ab Initio Quantum Mechanical Charge Field Molecular Dynamics Simulation (QMCF-MD) of Bi3+ in Water. J. Phys. Chem. A 2012,116, 8008–8014. 35 TOC Graphic 36 View publication stats