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First Principle Molecular Dynamics Investigation of Waterborne As-V Species Sangkha Borah, and Padma Kumar Padmanabhan J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b12482 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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The Journal of Physical Chemistry

First Principle Molecular Dynamics Investigation of Waterborne As−V Species Sangkha Borah and P. Padma Kumar∗ Department of Physics, Indian Institute of Technology Guwahati, Guwahati, Assam, India-781039 E-mail: [email protected] Phone: +91-361-258-2718

Abstract

mental Protection Agency (USEPA) have classified arsenic as a carcinogen. Consumption of arsenic above 10 g/l is considered unhealthy for humans. Millions of people across the globe, especially from south Asian and Latin American countries, are at the risk of elevated levels (> 50 g/l) of arsenic consumption through drinking water. 5,8–12 The environmental contamination is largely due to anthropogenic activities, such as mining and ore smelting, that results in the dissolution of arsenic containing minerals in ground water. 5 Arsenic is found in several oxidation states (−III, 0, + III, + V) in the environment. The toxicity of arsenic poisoning can vary enormously with its speciation. 13 Arsine (−III) is an extremely toxic gas; if exposed, attacks haemoglobin in the red blood cells (RBC), causing them to be eventually destroyed by the body. 5 Trivalent arsenite (As−III) has high affinity for thiol groups (R−SH) and forms kinetically stable bonds with sulfur (S), inducing enzyme inactivation. On the other hand, pentavalent arsenate (As−V) has a very poor affinity toward thiol groups and hence results in a rapid excretion from the body. 5 Organic compounds and methylated compounds such as Mono-methylarsenic acid (MMAA) and Di-methylarsenic (DMAA) are considered less harmful than the inorganic ones. 5,13 The concern over arsenic contamination is not dependent purely on its concentration in the local environmental but also on its specia-

The toxicity, mobility and geochemical behaviors of arsenic are known to vary enormously with its speciation and oxidation states. Present work details results based on ab initio molecular dynamics analysis on various waterborne As−V species, namely H3 AsO4 , H2 AsO4 – , HAsO42 – and AsO43 – . The nature of hydrogen-bonding of these species with water and its influence on the solvent structure and relaxation behavior are discussed. Useful microscopic insights on the structural and spectroscopic signatures of the species in aqueous media are reported. Comparison of normal mode frequencies of the species in gas phases to the vibrational density of states in solution provides insights on the influences of solvation and H-bonding. The results are compared with the previous experimental and simulation studies, where available.

Introduction Arsenic (As) deposits on Earth’s crust are largely in the form of minerals, mostly as sulphides (such as, As2 S3 , As4 S4 ), and often with metals such as Fe, Ni, Co etc. 1–4 It has been widely used in various industrial applications such as, in wood preservation, lead storage batteries, ammunitions, semiconductor devices, pesticides, medicines etc. 5–7 The World Health Organization (WHO) and the U.S. Environ-

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tion, and their individual physical and chemical properties such as dissolution rate in water, mobility, bioavailability etc. 5 In aqueous environment, arsenic is mainly present in As−III and As−V valence states, but are sensitive to factors such as, the pH of the solution, redox potential (Eh), presence of other competing ions or species, temperature and pressure etc. 5,7,9 The toxicity and mobility of As−III are in general higher than those of As−V. 14 Reportedly, As−V is more strongly adsorbed to the surface of several common minerals, such as ferrihydrite, alumina, calcite etc., compared to As−III. 15,16 A recent study reveals that the arsenic adsorption in calcite (CaCO3 ) markedly depends on the arsenite/arsenate ratio in solution. 17 Thus the redox conversion between As−III and As−V is of great importance, and offer a useful strategy for arsenic sequestration. 7 Hence, the understanding of the physicochemical properties of various arsenic species under different environmental conditions are of great importance to humanity. Over the last three decades density functional theory (DFT) 18,19 based ab initio molecular dynamics (AIMD) 20,21 has emerged as a very convenient tool to extract the mechanism of atomic/molecular phenomena at the microscopic length and time scales, which are otherwise too complicated or requires expensive experimental set-ups. 20,22,23 Recently, Hassanali et. al 24 have discussed the utility and limitations of the technique, reviewing previous ab initio simulations in the studies of water dissolving various organic and inorganic species. The present work details AIMD 18–21 simulation studies of arsenic acid (H3 AsO4 ) and its deprotonated derivatives viz. H2 AsO4 – , HAsO42 – and AsO43 – in aqueous environment. Fresh microscopic insights on the hydration structure, nature of hydrogen-bonding (H-bonding) and vibrational properties of these species have been presented comprehensively.

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implemented in the version 3.15.3 of the CPMD package. 25 Four sets of simulations have been carried out at 315 K temperature on various As−V species, namely (i) H3 AsO4 , (ii) H2 AsO4 – , (iii) HAsO42 – and (iv) AsO43 – , solvated by 60 H2 O molecules in a simulation box of length 12.42 ˚ A, corresponding to the density of about 1.05 g/cc. All simulations are performed using the gradient-corrected, norm-conserving, Martin-Troullier (MT) type pseudo-potentials employing Becke, 26 LeeYang-Parr 27 (B-LYP) exchange and correlation functionals. 20 The Kohn-Sham orbitals are expanded in plane-wave basis up to a cut-off of 85 Ry. Simulations are carried out in NVT ensemble with the electronic as well as ionic degrees of freedom controlled using Nose-Hoover thermostats. 28–30 The fictitious kinetic energy of electrons are maintained at 0.03 au, employing an electron mass of 600 au, for the necessary adiabatic separation of electronic and ionic degrees of freedom. 20,31–34 A time step of 0.1 fs is used for integration of equations of motion. The trajectories printed at every 5 MD steps, during the production phases, are subjected to detailed analysis discussed below. The H3 AsO4 +60H2 O system is prepared from a well equilibrated configuration of H2 SeO4 +H2 O employed in a previous study, details of which are discussed elsewhere. 33 This initial structure is further optimized for geometry, and equilibrated for 50 ps using AIMD scheme. A further 80 ps run is analyzed for detailed structural and dynamical properties. The other systems, involving H2 AsO4 – , HAsO42 – and AsO43 – species solvated by 60 H2 O molecules, are prepared from the final structure of H3 AsO4 system simulated earlier. Each of these systems are further geometry optimized and equilibrated for 30−50 ps, before 50−80 ps of production runs are carried out. Additionally, the geometry optimized gasphase molecular structures are obtained for all of the As−V species, H3 AsO4 , H2 AsO4 – , HAsO42 – and AsO43 – , under the same level of theory, for making useful comparisons. Normalmode analysis of these species is carried out about their geometry optimized gas-phase structures, using Martyna-Tuckerman’s Poisson

Methods The present study employs ab initio CarParrinello molecular dynamics (CPMD) 21 as


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solver scheme, 35,36 as implemented in CPMD software. Further, dipole moments of H3 AsO4 , H2 AsO4 – , HAsO42 – and AsO43 – molecules in gas phase have been computed from the optimized structures, at the same theoretical level employing CPMD package. 25 The H-bonds between different solute-solvent species are defined, and analyzed in detail, based on the simultaneous fulfillment of the following criteria: (i) the donor−acceptor O · · · O distance is less than 3.5 ˚ A, (ii) the hydrogenacceptor H · · · O distance is less than 2.45 ˚ A, and (iii) the hydrogen-donor-acceptor angle is less than 30◦ . This definition is widely employed for waters and similar systems in literature. 32–34,37–42 Lifetimes of different types of Hbonds in the systems are computed using the continuous H-bond correlation function given by, hh(0).H(t)i (1) SHB (t) = hhi

rectly related to the experimentally calculated rotational anisotropy measurements of water molecules. 43,45 For convenience of description, we shall use the label OAs for the oxygens of the As species not bonded to hydrogen, and OAs,H for those bonded to hydrogen. The hydrogens and oxygens of H2 O will be labeled as H and O respectively.

Results and Discussions Molecular Structure

where, h(t) = 1, if a tagged pair of donoracceptor species is H-bonded at time t and h(t) = 0 otherwise. H(t) = 1, if the two species remain H-bonded (based on all three criteria above) continually over a period t, else zero. Another time correlation function, CHB (t), known as intermittent H-bond correlation function, defined as, CHB (t) =

hh(0).h(t)i hhi

Figure 1: Gas phase geometry optimized structure of H3 AsO4 . Fig.1 shows the DFT optimized structure of H3 AsO4 . The central As atom is bonded to four oxygen atoms, among which three are bonded to hydrogens (OAs,H −H), thus forming one As−OAs double bond and three As−OAs,H single bonds, as shown in the figure. The As−OAs bond length thus obtained is 1.57 ˚ A, while As−OAs,H single bonds measure ∼ 1.71 ˚ A. The OAs −As−OAs,H angles measures about 115◦ , while the OAs,H −As−OAs,H angles are around 100◦ . The three As−OAs,H −H angles are around 110◦ . Table1 presents the As−OAs bond lengths for H3 AsO4 , H2 AsO4 – , HAsO42 – and AsO43 – obtained from CPMD studies in aqueous environment at 315 K, along with the ones obtained from gas phase calculations. Evidently, the As−OAs,H bond-lengths are longer than As−OAs across the species, and in all three phases. A systematic increase in the ArsenicOxygen bond-lengths with the effective charge

(2)

which is not sensitive to the intermittent breaking of H-bonds, gives a measure of structural relaxation through H-bonds. In the above definitions, the angular brackets, h· · · i, denote the statistical average over all pairs and time origins. 32,33,37–43 The dynamics of orientational relaxation of H2 O molecules have been studied by using the second order Legendre polynomial (P2 ) of vector (~v ) correlation function, given by, C2 (t) = hP2 (ˆ v (0).ˆ v (t))i

(3)

where, vˆ = ~v /|~v | is the unit vector along ~v (along the OH bond of H2 O) and P2 (x) = (3x2 − 1)/2 is the 2nd -order Legendre polynomial. 37,43–46 In this form, these results are di-


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Figure 2: (a-d) Snapshots of As−V species viz., (a) H3 AsO4 (b) H2 AsO4 – (c) HAsO42 – and (d) AsO43 – in aqueous environment, obtained from CPMD simulations at 315 K. The solvent water molecules are rendered by the “stick”-model in cyan, and the solute species by “ball-and-stick”model. (e)-(h)The hydration structures of the species is visualized through their spatial density distributions (SDD) plots, obtained averaged over the CPMD trajectory, where oxygens of water is rendered in red and hydrogens in gray iso-surfaces, around the solute. The iso-surfaces are plotted for a uniform isodensity value of 0.04 ˚ A−3 . of the species is also evident. These results from simulation are in overall consistency with the previous experimental studies carried out in aqueous and crystalline phases (see the article by Mahler et. al. 47 and the supplementary of that paper).

icantly lower than the experimental pKa1 value of 2.25. This results the equilibrium shifted heavily towards the H3 AsO4 species. Thus, it is not possible to investigate with sufficient accuracy the deprotonation events for As−V species within the length- (or, system size) and time-scales of the present study. Aqueous H2 AsO4 – and HAsO42 – did not exhibit any protonation-deprotonation events during the simulation runs owing to the high dissociation constants, pKa2 −7.05 and pKa3 −11.58. 47 However, aqueous AsO43 – exhibited, within the 80 ps of production runs, a few instances of proton 2– – −− transfer, AsO43 – + H2 O ) −* − HAsO4 + OH , lasting for a few fs, which is consistent with its pKa3 of 11.58. 47 Fig. 2(a)-(d) show typical snapshots of the simulated systems with waters represented with the “stick”-model in cyan, and the solute species in “ball-and-stick” representations.

Hydration Structure We have observed a few protonation and −− deprotonation events, H3 AsO4 + H2 O ) −* − – + H2 AsO4 + H3 O during the course of the 80 ps production run, with qualitatively consistency with its reported pKa1 value of 2.25. 47 The deprotonated state lasted only for a few ps, as the H2 AsO4 – is not found to be stable in the highly acidic environment due to the presence of the H3 O+ produced. It shall be noted that the presence of one H3 O+ in 59 other H2 O molecules amounts to a pH of about 0.06, signif-


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Table 1: Comparison of As−OAs and As−OAs,H bond-lengths for the As−V species, in aqueous and gas phases to available experimental studies. 47 Species H3 AsO4 H2 AsO4 – HAsO42 – AsO43 –

Type As−OAs-H As−OAs As−OAs-H As−OAs As−OAs-H As−OAs As−OAs

Solution (˚ A) 1.69 1.61 1.71 1.63 1.74 1.64 1.67

AIMD studies carried out on these systems reveal the important dissimilarities in their solvation structures, shown respectively in Fig.2(e)(h) through the three dimensional Spatial Density Distribution (SDD) plots. These are generated by transforming the coordinates of the solvent molecules, frame by frame, to a selected body fixed axis of the solute species and averaged over the entire stored trajectory. Only those water molecules within the first hydration shell are shown, omitting those beyond the distance, as in criterion (i) and (ii) of the Hbond definition discussed in the methods section. The iso-density surfaces, in Fig.2, are plotted for a value of 0.04 ˚ A−3 , with those for the hydrogens of water molecules in gray and oxygens in red. The SDD plots clearly suggest a systematic increase in the organization of the solvent molecules across the species, starting from H3 AsO4 to AsO43 – .

Gas(˚ A) 1.71 1.57 1.77 1.61 1.87 1.64 1.67

Expt. (˚ A) 1.69 1.64 1.71 1.66 1.74 1.67 1.70

ing the intra-molecular and inter-molecular hydrogens. Thus, the 1st peaks around 1 ˚ A may be identified with the intra-molecular hydrogens of the As−V species, and the 2nd peaks constitute the H-bonded hydrogens of the surrounding water molecules. Again, as in the OAs −O RDFs, the peak heights, or more appropriately the area under the peak, show a systematic increase with the charge of the As−V species. The higher 1st peak of solute-solvent RDFs (Fig.3(a) and (b)) followed by a deeper minimum for AsO43 – is suggestive of its compact, well defined hydration structure. The solute-solvent RDFs (Fig.3(a) and (b)) of all the As−V species reflect distinct intensities even beyond the second hydration shells (spread over 5 − 5.5 ˚ A in OAs −O and 4 − 4.5 ˚ A in OAs −H RDFs), contrasting to those of pure water. This is traced to the water molecues H-bonded to other sites of the same As−V species. These observations are in good qualitative agreement with the SDD plots shown in Fig.2(e)-(h), and the hydrogen bond statistics presented later in this section. The O−O RDFs of the water molecules in the presence of different As−V species, shown in Fig.3(c) presents interesting aspects on the influence of the solute on the solvent structure. The solvent RDFs for H3 AsO4 and HAsO42 – are found to be qualitatively very similar with that of pure H2 O. However, O−O RDFs for H2 AsO4 – exhibits higher maxima and lower minima, predicting more ordered water structure. The corresponding RDF for the AsO43 – case is found to have the shorter peak and a shallow minima following it. Thus, the a compact the hydration structure of the species results a somewhat diminished solvent structure.

Hydrogen-bonding Fig.3(a) shows the radial distribution functions (RDFs) of O of water molecules with respect to oxygen of the As−V species (without distinguishing between OAs and OAs,H ), along with the O−O RDF for pure waters for comparison. The intensities of the 1st peaks in the OAs −O RDFs show a systematic increase with the formal charge of the solute species. The minima following the 1st peaks also show systematic decrease suggesting that with the increase in the formal charge of the solute species hydration shells become more compact. Fig.3(b) presents the corresponding OAs −H RDFs which are calculated without distinguish-


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12

-

Number of H-bonds/molecule

H2AsO4

2-

2

3-

Pure H2O

gO

As

-O

(r)

HAsO4 AsO4

11.25

Donated by OAs,H

H3AsO4

(a)

3

1

Accepted by OAs,H

10

Accepted by OAs H2O---H2O

8.24

8

6 5.14

4

3.41

3.40

3.37

3.62 3.27

2.93

2

2.12 1.72

1.98 1.60 0.98 1.07

0

2

(r)

3.5

4

4.5

5

5.5

6

0

2

gO

1

0

0

1

2

3

4

5

1

2

-

2-

HAsO4

AsO4

3-

Pure H2O

ery OAs,H atoms, in all cases, donates almost one H-bond to waters. However, the number of H-bonds accepted by these sites increases across the series, from H3 AsO4 to HAsO42 – . This can be attributed to the increasing charge and availability of more configuration space for hydrogens of waters as compared to H3 AsO4 . The H-bonds accepted by the OAs sites also increases with the charge of the species, from H3 AsO4 to AsO43 – . The AsO43 – is found to accept the maximum number of H-bonds of about 2.8 per site. Overall, the total numbers of H-bonds formed (accepted and donated) by H3 AsO4 , H2 AsO4 – , HAsO42 – and AsO43 – add up to be 6.8, 8.7, 10.3 and 11.2 respectively. These estimates are in good quantitative agreement with the RDF and SDD discussed earlier. Shown with blue bars in Fig.4, the H2 O−H2 O H-bonds in solutions, for all the cases, are found to be lesser in numbers compared to pure waters. This predicts a strong affinity of As−V species in aqueous environment, likely to extend beyond the first hydration shells. This increase in solute-solvent H-bonds are by and large at the expense of the H-bonds between water molecules. H-bond correlation function defined in Eq.1-2 are useful to assess nature of the H-bonds between different sites based on the dynamical

2

0

H2AsO4

6

(c)

3

H3AsO4

Figure 4: Number of H-bonds (computed based H-bonding criteria described in section 2) formed by the solute species are shown, along with those for bulk waters. H-bonds donated by OAs,H are shown as black bars, accepted ones as red, while those accepted by OAs are shown as green colored bars. H2 O · · · H2 O H-bonds are shown as blue bars.

As

-H

3

2.5 (b)

3

gO-O(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2.5

3

3.5

4

4.5

5

5.5

6

r (Å)

Figure 3: Radial distribution functions (RDF) (a) between Oxygens of the solute (OAs and OAs,H ) and water, and (b) Hydrogens of the solute (H) and water, are compared to the O−O and O−H RDFs for pure water, from CPMD trajectories at 315 K. (c) shows the O−O RDFs for waters dissolving the different As−V species. The H-bonds formed by the As−V species in water can be of three types- (a) donated by OAs,H (OAs,H −H · · · O), (b) accepted by OAs,H (OAs,H · · · H−O) and (c) accepted by OAs (OAs · · · H−O), except for AsO43 – , where only the third type of H-bonds are formed. A quantitative number of the different types of H-bonds, calculated based on the H-bonding criteria discussed in section 2, is presented in Fig.4. Ev-


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(a)

0

OAs,H-H---O of H3AsO4 -

OAs,H-H---O of H2AsO4

0

2-

OAs,H-H---O of HAsO4

-

OAs,H---H-O of H2AsO4 OAs,H---H-O of

2HAsO4

OAs---H-O of H3AsO4

-

OAs---H-O of H2AsO4

-2

OAs---H-O of OAs---H-O of

2HAsO4 3AsO4

Pure H2O

-3

-1

ln(SHB(t)) & ln(CHB(t))

ln SHB(t)

(b)

OAs,H---H-O of H3AsO4

-1

-2

-3 H3AsO4

-

H2AsO4

2-

HAsO4

-4

3-

AsO4

-4

0

0.5

1

1.5

2

Pure H2O

2.5 -5

Correlation time, t(ps)

0

0.5

1

1.5

2

2.5

Correlation time, t(ps)

Figure 5: (a) Comparison of SHB (t) for different types of H-bonds formed by different sites of the solute species in aqueous environment, along with that for pure water. (b) Comparison of SHB (t) and CHB (t) for bulk water H-bonds of different types of solution environments under study, along with that for pure waters. SHB (t)s are shown in continuous lines, as specified in legends, while CHB (t)s are represented by broken lines of respective colors. 0

dred fs, presumably because of librational motions of water molecules. The long-time behavior is nearly exponential, thus the characteristic time constant (τs ) provides a simple qualitative measure of the lifetimes of H-bonds. We shall however note that some authors have followed more rigorous approaches, such as fitting with multiple exponents, integrating the area under the graphs up to infinity, etc. (see the article by Antipova 48 and references therein). We are adopting a rather simple approach, 48 since only qualitative behavior is of significance. The average lifetimes, τs , of the H-bond between different solute-water sites, fitting to the long time (t > 0.5 ps) behavior of SHB (t) are presented in table 2. It shall be noted that these values are liable to large statistical errors due to the fewer number of solute-solvent bonds in the system, despite the fact that we have utilized maximum available time origins from the trajectories. Thus, these values serve largely as are quantitative “indicators” of the nature of H-bonds. From Fig.5(a) and table 2, it is seen that, as a donor, H3 AsO4 forms the long-lived H-bonds (OAs,H −H · · · O) with water among all As−V species. Consistent with its low pKa1 , and the few deprotonation events noted during the MD

(t)) OH

ln (C2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.5

H3AsO4

-

H2AsO4

2HAsO4 3AsO4

-1 0

Pure H2O 0.5

1

1.5

2

2.5

3

3.5

Correlation time, t (ps)

Figure 6: Orientational correlation functions, calculated for H2 O molecules along the vector ˜ OH. trajectory obtained from molecular dynamics simulations. SHB (t), known as continuous Hbond correlation function, as defined above, is sensitive to the intermediate H-bond breaking, hence provide a qualitative measure of the lifetimes of H-bonds. In Fig.5, we have presented various types of solute-solvent H-bond correlation plots, along with the one for pure waters (in logarithmic scale for the sake of clarity in visualization). These functions decay faster than a simple exponential for the first few hun-


7


run, H3 AsO4 should be forming strongest of H-bonds in the series. The high τs of the Hbonds donated by OAs,H can be attributed to these strong H-bonds. In fact, excluding its few short-lived deprotonated states, the water molecule receiving its H-bonds hardly ever got exchanged during the course of time. On the contrary, the H-bonds accepted by OAs sites (OAs · · · H−O) are most long-lived for AsO43 – . The life-times of H-bonds accepted by OAs show a systematic increase with the charge of the species. Though the same was expected of the OAs,H sites, the life-times of accepted and donated by the OAs,H in H2 AsO4 – and HAsO42 – doesn’t quite fit in to this pattern. This possibly owes to the statistical errors given the few number of solute-solvent H-bonds. It is very useful to note that the H-bonds accepted by the OAs,H (OAs,H · · · H−O) are slightly short-lived, or weaker, than pure waters, while those accepted by the OAs (OAs · · · H−O) are long-lived, or stronger, in all cases. The H-bonds donated by the species (OAs,H −H · · · O) are significantly stronger than those between water molecules. In Fig. 5(b), the continuous and intermittent H-bond correlation functions, SHB (t) and CHB (t) for water dissolving the various As−V species as well as that for the pure water are shown. The corresponding characteristic decay constants τs and τc , fitting to a single exponential are presented in table 3. By and large, the characteristic time constants of H-bonds of water dissolving H2 AsO4 – and HAsO42 – are closer to those of pure water, while for the solutions of H3 AsO4 and AsO43 – the corresponding values are somewhat lower. The orientational correlation functions for different solvent-water molecules, C2 (t), defined as in Eq.3, with the vector ~v chosen to be ~ vectors of H2 O molecules, are shown the OH in Fig.6. These plots exhibit an initial rapid decay (owing to the librational motions of the H2 O molecules), followed by slower decay that characterize the orientatonal relaxation of the water molecules. As for the H-bond correlations, CHB (t) , the characteristic decay constant are estimated by fitting to single exponentials over the region between 0.5-3.5 ps, for sake of simplicity. The relaxation times of solvent wa-

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ter molecules for the different As−V species are also tabulated in table3, in the 4th column. The experimental values of H2 O orientational relaxation lies around 1.7-2.6 ps, 49–51 while MD simulations have generally over-estimated the values. 39,42,43,46,50–52 Previous literature suggests that these values are sensitive to the computational techniques adopted, fitting procedure, statistics, etc. 39,46,50–52 Fig. 6 and table 3 suggest that waters dissolving As−V species exhibit somewhat slower orientational relaxation compared to pure water, the slowest being noted for the case of H2 AsO4 – . It has been noted earlier that these relaxation are sensitive to the presence of ions in the environment, particularly the dipole moment and polarizabilities of the solutes, in addition to external influences, such as temperature and pressure. 39,44,53,54,54–59 Bursulaya et. al. 55 showed that the increase of dipole moment of the solutes slows down the orientational relaxation of solutes. The dipole moments for the various As−V species in gas phase, calculated in this study, are 2.95 D, 3.80 D, 2.90 D and 0.04 D, respectively for H3 AsO4 , H2 AsO4 – , HAsO42 – and AsO43 – . In agreement with Bursulaya et. al., 55 in the present case as well the orientational relaxation (not shown) of the solute molecules themselves (taking the vectors along As−OAs and As−OAs,H bonds) were slowest for H2 AsO4 – among all the As−V species. This could in-turn slow down the dynamics of the solvents molecules. Thus, factors such as the higher dipole moment of H2 AsO4 – and its slightly over-structured solvent (noted earlier in the O−O RDF in Fig.3(c)), could be responsible for the slow orientational relaxation of water dissolving H2 AsO4 – .

Vibrational Density of States The vibrational density of states (VDOS) or power spectra, calculated as the Fourier transformation of the velocity auto-correlation function (VACF), are presented in Fig.7. The top panel of the figure shows the spectra for the hydrogens of H3 AsO4 (black) and of pure water (orange). Also shown, above the plots, as down triangles, are the gas phase normal modes fre-


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Table 2: Lifetimes (τs ) of H-bonds computed by exponential fitting of SHB (t) graphs in Fig.5(a). The total numbers of solvent sites are also listed, along with the statistically averaged number of different H-bonds, based on Fig.4. Species H3 AsO4 H2 AsO4 – HAsO42 – AsO43 –

H-bond type OAs,H − H · · · O OAs,H · · · H − O OAs · · · H − O OAs,H − H · · · O OAs,H · · · H − O OAs · · · H − O OAs,H − H · · · O OAs,H · · · H − O OAs · · · H − O O−H−O

τs (ps) 6.23 0.24 0.96 3.00 0.43 1.58 4.02 0.34 1.60 2.90

Total number 2.93(3 sites) 1.72 (3 sites) 2.12 (1 site) 1.98 (2 sites) 1.60 (2 sites) 5.14 (2 sites) 0.98 (1 site) 1.07 (1 site) 8.24 (3 sites) 11.25 (4 sites)

Table 3: The estimated lifetimes and structural relaxation times of the H-bonds, τs and τc of the solvent water molecules, respectively from SHB (t) and CHB (t) in Fig.5(b). The 4th column provides the orientational relaxation times, τ2OH of H2 O molecules. Solute H3 AsO4 H2 AsO4 – HAsO42 – AsO43 – Pure H2 O

τs (ps) 0.76 1.00 0.92 0.77 0.88

τc (ps) 3.71 6.04 5.51 4.86 5.42

τ2OH (ps) 5.30 7.53 5.56 5.48 4.86

Table 4: Gas phase normal mode frequencies of H3 AsO4 along with a qualitative description of vibrational modes, recognized from visualization. Sr. No. Frequency (cm – 1 ) 1-8 97, 173, 204, 303, 317, 332, 336, 342 9-11 704, 724, 745 12-14 961, 968, 988 15 1048 16 3682 17 3689 18 3695

Description of the mode Various low frequency modes (torsion, twisting, rocking, wagging, etc.) As−OAs,H bond stretching modes. Various As−OAs,H −H bending modes. Asymmetric stretching of As−OAs bond w.r.t. the As−OAs,H bonds. Asymmetric stretching of OAs,H −H bonds, involving two of them. Asymmetric stretching of OAs,H −H bonds, involving all three of them. Symmetric stretching of OAs,H −H bonds involving all three. of 1400 cm−1 are reproduced also in the lower panel for ease of comparison. In aqueous environments, the OAs,H −H and O−H vibration modes (top panel of Fig.7) gets highly red-shifted (by about 1000 cm – 1 ) owing to H-bonding. The As−OAs,H −H bending modes observed in the range of 950 -1000 cm−1 (see Table 4) in the gas-phase appears to have blue-shifted to 1000 − 1300 cm−1 with significant dispersion (see H-spectra (red) on top-

quencies of H3 AsO4 molecule. The gas-phase frequencies are listed in table4 along with qualitative descriptions of the modes, based on the visualization of the Eigen vectors of the Hessian. In the bottom panel, power spectra of OAs (as solid lines) and OAs,H (as dotted lines) for the different As−V species, namely, H3 AsO4 (black), H2 AsO4 – (red), HAsO42 – (green) and AsO43 – (blue), are shown. The normal frequencies of gas phase H3 AsO4 , up to the range


9


Intensity 0

500

1000

1500

2000

2500

Parrinello scheme employs heavy electronic mass to propagate the electronic wave-functions efficiently, causing a “drag effect” on the nuclear degrees of freedom, thus resulting in an artificial red-shift in the vibration frequencies of the atomic species. In order to estimate this artifact we have carried out a short BornOppenheimer Molecular Dynamics (BOMD) simulation of H3 AsO4 in water, at an identical theoretical level. Please refer to the supplementary material for a comparison of VDOS calculated from CPMD and BOMD simulations. It shall be noted that CPMD simulations over estimates the red-shift by about 25%. Thus a quantitative assessment of frequency shifts may account for this.

(a)

H3AsO4 gas phase H of H3AsO4 H of H2O

3000

3500

4000

(b)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0

OAs, H of H3AsO4 OAs, H of H2AsO4 OAs, H of HAsO24 OAs of H3AsO4 OAs of H2AsO4 OAs of HAsO24 OAs of AsO34 200

400

600

800

1000

Frequency (cm 1)

1200

Page 10 of 15

1400

Conclusion

Figure 7: Vibrational density of states (VDOS) calculated from velocity autocorrelation function (VACF) for OAs,H (dotted lines) and OAs (solid lines) of H3 AsO4 (black), H2 AsO4 – (red), HAsO42 – (green) and AsO43 – (blue) are shown. The triangles, with respective colors, represents the gas phase optimized harmonic modes of vibrations for the species. See table 4 for descriptions of these modes.

We have carried out extensive ab initio MD simulations on inorganic As−V species, namely H3 AsO4 , H2 AsO4 – , HAsO42 – and AsO43 – , in aqueous environment, and the nature of hydration and H-bonding are investigated in details. It is seen that the compactness of the hydration shells as well as the total number of H-bonds formed with water increase in across the series, from H3 AsO4 to AsO43 – , with the net effective charge of the solute. The H-bonds donated by the As−V species are found to be considerably longer lived compared to those accepted by the species, and those of pure water, and follows an overall systematic behavior. Moreover, the nature of the solvent-water itself is found to be sensitive to the solute species it dissolves, impacting the H-bond life-times as well as structural and orientational relaxation times of the solvent-water molecules. It is proposed that the dipole moments of the solute could also be influencing the orientational relaxation of the surrounding water molecules. These observations are also found to be consistent with the calculated vibrational density of states. It is hoped that the spectroscopic insights presented here will be useful in the identification and estimation of arsenic species in more complex environments. Further, the microscopic details related to the hydration struc-

panel of Fig.7) in aqueous environment. It shall also be noted that the As−OAs,H −H bending modes are significantly lower than the H−O−H bending modes (shown in black on top-panel of Fig.7) that measures around 1550 cm−1 . The As−OAs stretching mode of H3 AsO4 observed at 1048 cm−1 in gas-phase are red-shited through about 150 cm−1 in the solution due to hydrogen bonding. The As−OAs and As−OAs,H stretching modes observed respectively at 1048 cm−1 and over 700-750 cm−1 for the gas-phase H3 AsO4 , shows systematic softening across the series, from H3 AsO4 to AsO43 – , with the net effective charges of the species. This is consistent with the increase in the bond-lengths of As−OAs and As−OAs given in table 4. Overall nature of the spectra is in agreement with previous experimental and simulation results in literature. 49,60–62 We shall, however, note that the Car-


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ture and H-bonding detailed in this work could serve the benchmark for developing force-fields of various As−V species in water. Forcefield based approaches permit MD simulations on larger systems and over much longer time scales, thus facilitating investigation of these species in more complex environments, such as their adsorption on mineral surfaces, membranes etc., that is of interest for water remediation strategies. Advanced spectroscopic techniques, such as Large Angle X-ray Scattering (LAXS), Extended X-ray Absorption Spectroscopy (EXAFS), 47 Femtosecond Fluorescence Spectroscopy, 63 UV/Visible absorption and emission, 64 permits experimental verification of these results. Such prospective investigations would be very important for evolving a comprehensive understanding of these species in a complementary fashion.

(4) Tossell, J. Theoretical studies on arsenic oxide and hydroxide species in minerals and in aqueous solution. Geochim. Cosmochim. Acta 1997, 61, 1613–1623. (5) Ahuja, S. Arsenic contamination of groundwater: mechanism, analysis, and remediation; John Wiley & Sons, 2008. (6) Leist, M.; Casey, R.; Caridi, D. The management of arsenic wastes: problems and prospects. J.Hazard. Mater. 2000, 76, 125–138. (7) Choong, T. S.; Chuah, T.; Robiah, Y.; Koay, F. G.; Azni, I. Arsenic toxicity, health hazards and removal techniques from water: an overview. Desalination 2007, 217, 139–166. (8) Nickson, R.; McArthur, J.; Burgess, W.; Ahmed, K. M.; Ravenscroft, P.; Rahmann, M. Arsenic poisoning of Bangladesh groundwater. Nature 1998, 395, 338.

Acknowledgement We thank the Center for Development of Advanced Computing (CDAC), Pune, for generous allowance of CPU hours, and Department of Science and Technology (DST), New Delhi for financial support (through grant no.: SR/FST/PSII-020/2009). The use of CPMD 25 software package for MD simulations, VMD 65 for the visualization of MD trajectories and TRAVIS 66 for the calculation of spatial density distribution (SDD) plots are acknowledged.

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Supporting Information Available: Comparison of VDOS calculated from CPMD and BOMD simulations. This material is available free of charge via the Internet at http://pubs.acs.org/.

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