Hidden Hydration Structure of Halide Ions: an Insight into the

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The Hidden Hydration Structure of Halide Ions: an Insight into the Importance of Lone Pairs Francesco Sessa, Paola D'Angelo, Leonardo Guidoni, and Valentina Migliorati J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b10636 • Publication Date (Web): 02 Dec 2015 Downloaded from http://pubs.acs.org on December 6, 2015

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The Journal of Physical Chemistry

The Hidden Hydration Structure of Halide Ions: an Insight into the Importance of Lone Pairs Francesco Sessa,† Paola D’Angelo,† Leonardo Guidoni,‡ and Valentina Migliorati∗,† Dipartimento di Chimica, “La Sapienza” Università di Roma, P.le Aldo Moro 5, 00185 Rome., and Dipartimento di Scienze Fisiche e Chimiche, Università degli Studi Dell’Aquila,Via Vetoio, L’Aquila. E-mail: [email protected]

Introduction

An elusive tetrahedral hydration structure for bromide in aqueous solution has been unveiled through the use of ab initio Molecular Dynamics. It has been revealed that a subset of first shell water molecules has a preferential strong interaction with the ion lone pairs, giving rise to a tetrahedral short lived complex. Through the use of a new geometric descriptor correlated to the ion-water pair interaction energy, we managed to divide the conventional first hydration shell into a tight first shell and a loose first shell, highlighting their different structural and dynamic behaviour. This picture suggests the mandatory role of lone pairs in the study of highly disordered systems where hydrogen bond is the most relevant interaction in the solvation process, such as weakly hydrated anions. This interaction-based approach leads to an improvement in the description of halide hydration given up to now by radial models.

Halide hydration plays a major role in a vast array of chemical reactions and biological processes. Solvent effects in a protic environment can heavily alter the reactivity of such species. 1 Reaching a thorough knowledge of solute-solvents interactions is therefore mandatory to fully understand the chemistry of hydrated halides. The chemical properties of halide ions in aqueous solution are very important also to provide a molecular level understanding of the Hofmeister series, in which anions and cations are ordered on the basis of their effect on protein stability. 2 This empirical ordering has been found to persist in many phenomena, such as the surface tension of electrolytes and colloid stability, 3 and there emerged a belief that the Hofmeister series reflects specific ion effects on the long-range structure of water, bringing to the classification of ions as kosmotropes (structure makers) and chaotropes (structure breaker). 2 However, spectroscopic investigations suggest that the structure maker/breaker concept applies very locally and that hydrated ions do not produce any long-range effects on water. 4,5 In this context, an accurate description of the local structure around an halide ions can be of great help also to explain on a molecular basis the Hofmeister series. 3

Keywords: Car Parrinello Molecular Dynamics, solvation chemistry, bromide ion, aqueous solution, tetrahedral coordination, hydrogen bond function, maximally localized Wannier functions ∗ To

whom correspondence should be addressed di Chimica, “La Sapienza” Università di

† Dipartimento

Roma. ‡ Dipartimento di Scienze Fisiche e Chimiche, Università degli Studi Dell’Aquila.

A plethora of theoretical and experimental studies have been conducted with the purpose of unraveling halide hydration. 6–12 The results of these ACS Paragon Plus Environment

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works are surprisingly widespread, lacking a unified picture for the solvation of these ions. Both first shell distances and coordination numbers reported in the literature are significantly scattered. For example bromide-water mean distance is reported between 3.19 and 3.40 Å and bromide hydration number is given in the range 4.2-8.9. 13 There are opposing results in the literature also about halide hydration geometries: according to some authors there are identifiable, albeit very flexible, solvation geometries for hydrated halides, 14 while in other investigations no well defined structures could be found. 7,15 This is due to the difficulty of defining halide coordination shells because of their diffuse character, and of the fast water exchange between the first and second hydration shells. 13 The residence times of water molecules in halide first solvation shells are estimated to be in the picosecond time scale. 16 Up to now ion solvation is described through the schematic model introduced by Wen 17 and Gurney, 18 where the solvent media around the ion is divided into several concentric spheres. Each sphere is a region of space containing solvent molecules equally interacting with the solute. Solvation structures are inherently described in terms of radial distribution functions by experimental methods such as X-ray or neutron diffraction and X-ray absorption spectroscopy, that are the only techniques able to provide structural information on liquid media. This is due to the fact that both the S(q) and χ (k) functions are mainly sensitive to pair distribution functions, while they are not able to distinguish the nature of intermolecular interactions. A simple radial model is effective in describing the first solvation shell for more “ordered” systems, such as aqueous solutions of transition-metal ions, where very stable complexes are formed, and solvent molecules that strongly or weakly interact with the ion belong to two neatly separable space regions. However, it does not necessarily hold true for more disordered systems, such as halides aqueous solutions, where this separation is not so neat. This is an issue not only for experimental techniques, but also for any theoretical approach trying to describe solvation in terms of radial distribution functions. We show in this work that solvation in halide aqueous solutions can be better described by taking account

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of more than just solute-solvent distances. Dealing with halides in a protic environment, like an aqueous media, the dominant solute-solvent interaction is hydrogen bonding. This is a well-known linear interaction between the X-H vector of the donor molecule and the electron density of the acceptor atom, often described by considering the orientation of said X-H vector towards the acceptor atom (i.e. the anion, in our system). Numerous efforts have been made in the literature to find an adequate geometric descriptor able to distinguish whether a molecule is hydrogen bonded to another or not, especially for neat liquid water. 19–24 However, due to the partly covalent nature of hydrogen bond, 25,26 a correct description would require taking account of how the electron density of the acceptor atom orients itself towards the “bound” hydrogen atom. Reiher et al. 21 have shown that the energy involved in a hydrogen bond can be approximated as a linear function of the shared electron number between the hydrogen atom and the acceptor atom, giving further evidence of how relevant electronic effects are when describing hydrogen bond. 27 A simple and convenient way to include electron density in the theoretical analyses is to consider the lone pairs of the acceptor atom and how they are oriented towards the hydrogen atoms. In the framework of ab initio approaches, a representation of the ion lone pairs can be obtained through the computation of Maximally Localized Wannier Functions Centers. 28,29 To study halide hydration we performed an “ab initio” Molecular Dynamics (AIMD) 30 simulation choosing as an example system a bromide aqueous solution. By using the ion lone pairs in the investigation we managed to identify a rather elusive hydration structure for the bromide ion. In fact, such a structure has not been previously observed by any theoretical or experimental techniques because of it being hidden by the inherent disorder of these systems.

Computational methods An AIMD simulation of the bromide ion in aqueous solution has been performed through the Car-Parrinello Molecular Dynamics (CPMD) approach 31 using the Kohn-Sham Density Func-

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tional Theory method 32 for electronic calculations. The system consists of one bromide ion solvated by 90 water molecules in a periodic cubic box with a 14 Å edge. The simulation has been carried out using the CPMD package. 33 BLYP functional 34,35 has been chosen as exchange-correlation functional. As it is known that BLYP functional has the tendency to give an overstructured description of liquid water 36 due to a poor estimation of Van der Waals interactions, Dispertion-Corrected Atom-Centered Pseudopotentials (DCACP 37 ) have been used to describe core electrons of oxygen and hydrogen atoms. The combined use of BLYP functional and DCACP pseudopotentials has been shown to overcome this problem and to provide an improved description of liquid water. 38 Bromide core electrons are treated instead with a Troullier-Martins pseudopotential. 39 An energy cutoff of 70 Ry has been selected for the plane-wave basis set. A fictitious mass of 400 a.u. has been associated to the electronic degrees of freedom. The equilibration has been carried out for 2 ps in the NVT ensemble at 300 K, using the Nosé-Hoover thermostat (coupling frequency of 1500 cm−1 ). The time step of the simulation is of 4 a.u., for a total production time in the NVE ensemble of 9.6 ps. A homogeneous background charge has been used to compensate for the bromide negative charge. 40 Note that we did not observe any relevant drift for the fictitious electronic kinetic energy throughout the CPMD simulation. A representation of the bromide ion lone pairs has been obtained by calculating the Maximally Localized Wannier Functions and their centers for each time step. The spatial distribution functions have been computed with an utility of the GROMACS package 41 and the isosurface value used is 4.17. All of the other analyses have been performed using in-house developed codes. The bromide ion-water pair interaction energies of 1000 configurations extracted from the simulation have been computed by DFT calculations on the isolated pairs. The pairs were randomly chosen in the space region of interest by selecting water molecules at a distance shorter than 4.75 Å from the ion. The setup adopted for the DFT wavefunction optimizations was the same as that used in the AIMD simulation.

Figure 1: (a) Spatial distribution functions of oxygen (red surfaces) and hydrogen (green surfaces) atoms around the bromide ion. Lone pairs of the bromide ion are reported in mauve. (b) Radial distribution functions (g(r)) between bromide lone pairs (LP) and water molecules, either with the closest hydrogen atom (H1, green line), the oxygen atom (red line) or the farthest hydrogen atom (H2, blue line).

Results and discussion In a previous work we showed that a CPMD simulation with the same setup of the one presented here correctly reproduces the Extended X-ray Absorption Fine Structure (EXAFS) spectrum of bromide ion in water, hence reproducing the correct local structure around the ion. 42 However, conventional radial and angular analysis performed on the trajectory did not highlight any defined solvation geometry for the six water molecules in the bromide hydration shell. By including here the use of lone pairs in our analysis we obtain a substantially different result, showing evidence of a tetrahedral coordination around the bromide ion. Figure 1a shows the spatial distribution functions (SDFs) for oxygen and hydrogen around the bromide ion. In this analysis the lone pairs are used to define the internal reference system in which the SDF are computed. The result reveals four high probability regions tetrahedrally arranged along the directions of bromide lone pairs. The use of an internal coordinate system is mandatory to remove the overall rotation of the solvent around the solute, which would give artificially rise to a spherical SDF. Use of a coordinate system based on the bromide lone pairs has allowed us to highlight a strong correlation between the instantaneous positions of lone


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To validate the use of such a descriptor we extracted the coordinates of 1000 ion-water pairs from the trajectory and evaluated their interaction energy by means of DFT calculations on the isolated pairs. We then calculated the η value for each ion-water pair by using the new ion lone pair positions obtained from the DFT calculations. Finally, we plotted the interaction energy against η . The result is shown in Figure 3. A correlation can be clearly seen between the pair interaction energy and η , as highlighted by the result of a linear regression performed on the data set. The correlation coefficient (R2 ) between data set and fit is 0.79, with an RMSD of 5.4 kJ/mol. As stated by Reiher et al. in a theoretical investigation on hydrogen bonds, from an optimistic point of view, the error in the calculation of bond dissociation energies of small compounds by using quantum chemical methods is expected to be of the order of 5 kJ/mol. 21 Therefore, such an error is a very good target when designing a descriptor which aims at estimating the strenght of an interaction.

molecule. The aim of this geometric descriptor is to give an approximate estimate of how strong the two species interact. The η value of a solvent molecule is a weighted distance between the hydrogen atom closest to the anion and the anion lone pair interacting with it. The angular weights take into account the orientation of the lone pair towards the hydrogen atom (angle αi j ) and the orientation of the O-H vector towards the lone pair (angle βi j ). For a given ion-water pair, the farther the geometry is from linearity, the higher the value of the angular weights. The geometric quantities involved are defined in Figure 2. Such definition of η is based on the concept that usually, for non-bonding interactions, the shorter is the distance, the stronger the interaction, as long as the distance is beyond Pauli’s repulsive region. Moreover, for linear interactions, the better is the orientation (low αi j and βi j values) and the stronger should be the interaction. A lower η value should therefore mean a stronger interaction. If this holds true η should correlate with the pair interaction energy between the two species. It should be noted that as each bromide ion has four lone pairs and each water molecule has two hydrogen atoms, for any given ion-water pair there will be eight possible lone pair-hydrogen combinations, and therefore eight possible η values. The η value of a given ion-water pair is taken as the minimum among the eight values, corresponding to the effectively interacting lone pair-hydrogen pair. 10

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Figure 4: η probability distribution P(η ) calculated for all ion-water pairs in the simulation.

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After testing the descriptor on isolated ion-water pairs by means of ab initio calculations, we tested its performance in solution by calculating the η probability distribution P(η ) for all ion-water pairs in the box throughout the simulation. The curve, displayed in Figure 4, has remarkable similarities with halide-water energy pair distribution function obtained from classical Molecular Dynamics (MD) by Jorgensen et al. 20,45 Both Jorgensen’s and our distribution functions show a small first peak at low energies (low η ) corresponding to

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first shell water molecules, and a wide peak at high energies for all other molecules. Furthermore, by converting η values in estimated energies through the fitting equation obtained for the isolated pairs (see Figure 3), we find that the first peak maximum is located at about -46 kJ/mol, which is in agreement with the experimental value of bromide-water binding energy reported by Hiraoka et al. 46

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conventional first hydration shell defined by a distance cutoff should be separated into a “tight” first shell, composed of solvent molecules strongly interacting with the ion, and a “loose” first shell, composed of the remaining and less perturbed first shell solvent molecules. To describe the tight first hydration shell we chose an η cutoff of 2.75 Å, which is the intersection between the 1st − 4th and 5th − 6th probability distributions (Figure 5a). This is in order to leave out the nearest first shell molecules when they are weakly interacting with the bromide ion, and to include the 5th and 6th water molecules when they are strongly interacting with it. It is very interesting that for an η value of 2.75 Å there is a change in slope in the first peak of the η distribution calculated for all the ion-water pairs (Figure 4). Such a change in slope suggests a diversity in how the water molecules interact with the ion, thus we believe that this η value can discern between tight and loose solvation shells. The distribution of bromide-water instantaneous coordination numbers up to this η value, displayed in Figure 5b, shows a dominant four-coordinated configuration and a mean coordination number of 3.7, in agreement with the existence of a tetrahedral solvation structure.

Figure 5: (a) η value probability distributions for two selections of water molecules: 1st to 4th closest to the ion (red line), and 5th and 6th closest to the ion (blue line). (b) Instantaneous coordination number probability distribution for the bromide ion evaluated selecting water molecules with an η value lower than 2.75 Å. Figure 5a shows the probability distributions of η for two different selections of water molecules: the four molecules with shortest Br-O distance (red line) and the 5th and 6th molecules in terms of Br-O distance (blue line). The four nearest molecules to the bromide ion give rise to a sharp peak, with a maximum at 1.96 Å, and the probability is zero beyond 4.5 Å. Conversely, the distribution of the 5th and 6th nearest molecules shows a very broad low-intensity peak and has non negligible values beyond 5 Å. The peak is also shifted towards larger η values. These features indicate that outer molecules of the first hydration shell interact much more weakly with the halide ion. However, the overlap between the 1st − 4th and 5th − 6th distributions suggests that the 5th and 6th water molecules can occasionally form stronger interactions with the ion than closer molecules. On account of these results, our hypothesis is that the

Figure 6: η = 2.75 Å isosurface, representing triplets of cutoff values on lone pair-hydrogen distance (rLP−H ), α angle and β angle. It should be noted that the use of a cutoff on the η value can be considered as a continuous extension of multiple cutoffs on the geometric


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important, for a correct desciption of such interactions, the use of geometric quantities involving lone pair and hydrogen positions. We then evaluated the same CDFs for the tight shell molecules only (Figure 7 b, e) and for loose shell and bulk molecules (Figure 7 c, f). The interesting result is that by selecting the tight shell molecules with a cutoff on the η value, we have managed to include in the tight shell description water molecules for which the geometric parameters are highly correlated, while leaving out those molecules with a low correlation for those parameters. After assessing the potentialities and limitations of our descriptor, we evaluated separately the contributions to the total Br-O g(r) of the tight shell molecules (Figure 8a) and of the remaining solvent molecules (loose shell and bulk, Figure 8b). More specifically we tested two different criteria to identify the tight first hydration shell: first by selecting the 4 molecules closest to the bromide ion (Figure 8, red lines) and second by selecting molecules inside the above mentioned 2.75 Å η cut-off value (Figure 8, blue lines). With both criteria we obtain for the tight shell a sharp peak with a maximum located at 3.33 Å, the same distance as that of the total Br-O g(r). It is notable that for the η cut-off criterion the g(r) goes to zero smoothly at 4.0 Å and does not result in a truncation, even if a 2.75 Å cut-off on η could include water molecules up to a Br-O distance of about 4.21 Å. Furthermore, if such a hidden tetrahedral structure was not present, it would be unlikely to see such a structured peak for the η selection. Major differences can be observed in the results obtained with the two criteria for the loose solvation shell of the bromide ion. If we consider all water molecules beyond the 4th in terms of BrO distance, we obtain a g(r) with a defined first peak located at 3.65 Å, indicating that at least part of the water molecules in this selection are still noticeably structured by the ion. Instead, the g(r) obtained for the η > 2.75 Å selection shows no discernible peaks in the first solvation shell range (2.90 - 3.93 Å), suggesting that all the water molecules “strongly” interacting with the ion have been included in the tight solvation shell description with this criterion. Still regarding the g(r) for the η selection of loose shell and bulk molecules, the overall shape suggests that bulk

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Figure 8: Br-O radial distribution functions evaluated on different water molecule selections: (a) comparison among all molecules (black line), 1st to 4th molecules closest to the ion (red line) and molecules inside the 2.75 Å η cut-off (blue line); (b) comparison among all molecules (black line), molecules beyond the 4th in terms of Br-O distance (red line) and molecules outside the 2.75 Å η cutoff value (blue line). water molecules enter the space region of the first hydration shell without being significantly structured by the ion, as pointed out by the plateau in the region between 3.47 and 3.86 Å. It is important to stress that these molecules can be also found very close to the ion, even if they are weakly interacting with it. This strongly evidences how halide hydration is ill-described in terms of distances only. As a last structural analysis we have calculated a CDF combining the Br-O g(r) and the distribution function of the O-Br-O* angle, labelled as θ , where O and O* are the oxygen atoms of water molecules inside the first shell cutoff distance of 3.93 Å. The CDF calculated for all the first shell water molecules is shown in Figure 9a. It can be seen that for distances in the range 3.2-3.5 Å a very broad distribution for angles between 60 and 180◦ is found, indicating a very disordered arrangement of water molecules around the ion. We have then evaluated separately the CDF involving the tight shell molecules only (Figure 9b) and the loose shell only (Figure 9c). Note that tight and loose shells are defined by using the above mentioned η cutoff criterion. As concerns the tight


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change events involving the shell. The evaluated NMRT for the tight hydration shell of the bromide ion is 0.16(0.01) ps, while the NMRT for the loose hydration shell is 0.09(0.02) ps. Overall the results of these analyses highlight a slightly different dynamic behavior between water molecules belonging to the tight hydration shell and molecules in the loose hydration shell: loose shell water molecules have slightly more diffusional freedom leading to a more labile shell than the tight one. Due to fast interconversion between the tightly and loosely bound water molecules, the overall water structure around the bromide ion, as expected, is very disordered. Moreover, exchange frequencies show that it is very unlikely for a tight water molecule to diffuse into the bulk directly, and this exchange process usually requires the water molecule to pass through the loose hydration shell. Since tight shell molecules are not always the closest to the ion, this is an interesting further evidence of their structured nature, showing that they have to “free” themselves from the ion before being able to diffuse into the bulk.

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Even if for the Cl− aqua ion some authors suggested the existence of instantaneous tetrahedral structures 44 while for F− a tetrahedral fist shell hydration complex was obtained from DFT-based MD simulations, 3 a tetrahedral coordination for the bromide ion in aqueous solution has not been reported in the literature so far, to the best of our knowledge. We believe that this is due to the intrinsic characteristics of structural experimental techniques such as X-ray or neutron diffration and X-ray absorption spectroscopy. These techniques provide a description of solvation mainly in terms of radial distribution functions. Moreover, in the case of disordered systems such as ionic solutions, the uncertainty in the coordination numbers determined by these experimental methods is usually too large for a conclusive determination of the hydration complex geometry. This is due to the strong correlation among the structural parameters involved in the fitting of the experimental data. When dealing with halide aqueous solutions, further complications come from the difficulty of defining halide coordination shells because of their diffuse character and of the fast water exchange between the first and second hydration shells. 13 This has led to a surprisingly widespread range of bromide hydration number reported in the literature (4.2-8.9) and to the lack of a unequivocal structural result on the bromide hydration geometry. As shown in this work, an improvement in the description of halide hydration given up to now by radial models can be obtained by taking explicitly account of the ion lone pairs in the analyses. Such an inference is based on the strong correlation between the placement of water molecules beloging to the tight first hydration shell and the arrangement of the ion lone pairs. Even though AIMD simulations can produce a representation of the lone pairs by means of the Wannier function centers, to the best of our knowledge these representations have never been used to investigate and identify solvation structures as done in this work. In classical MD it is not possible to have rigorous representations of the lone pairs. However, they could be introduced approximately through the use of negatively charged dummy atoms. This approach has been already applied to water models, 49,50 showing notable structural improvements, and to alkaline-earth and transition-

Overview and Conclusions In the present paper we have carried out a thorough investigation of bromide hydration by means of AIMD simulations and developing novel analysis procedures which include the use of the ion lone pairs. Bearing in mind the results presented, we come to several important conclusions in the topic of halide hydration. The first result is that the bromide ion has a tetrahedral solvation geometry in aqueous solution. Although bromide aqueous solutions are indeed disordered systems, they are not as disordered as depicted thus far. They do possess a tight first hydration shell, forming a short-lived complex with a defined tetrahedral geometry, composed by water molecules strongly interacting with the ion via hydrogen bonds. The remaining water molecules contributing to the g(r) first peaks (loose hydration shell) are unstructured water molecules penetrating into the first shell region from the bulk, and they are only mildly perturbed by the electrostatic charge of the ion. A pictorial example of this concept is shown in Figure 10.


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Figure 10: Pictorial example displaying how the conventional first hydration shell (left panel) is divided into a tetrahedral tight hydration shell and a loose hydration shell of undefined geometry (right panel) if the ion lone pairs are taken into account. metal ions, 51,52 but not for halide ions. The presence of lone pair representations, being it through Wannier function centers or dummy atoms, can be very important for monoatomic or diatomic anions. For these ions the explicit presence of lone pairs can supply an internal coordinate system for the analyses, which can be very important when dealing with disordered systems. Finally, in this work we introduced a geometrical descriptor, η , that can effectively describe a linear interaction between two species. Ab initio calculations have proven a neat correlation between ion-water pair interactions energies and the η descriptor. Furthermore, it has been shown that η is able to distinguish which water molecules form strong interactions with the bromide ion, and which do not. Beside the capability to discern between tight and loose water molecules, we believe that η can be used to determine the presence of an hydrogen bond between two species. It is known that the hydrogen bond energy is not a physical observable and hence can only be estimated and not measured. A huge effort has been made in the literature to develop hydrogen bond definitions based on energetic, geometric or wave function approaches. 19–24 The simplest approach to identify hydrogen bonds from MD trajectories or Monte

Carlo simulations is the use of geometrical criteria. However, any of these criteria will have some arbitrariness and will be approximate. A complete geometrical description of the configuration taken by two hydrogen bonded species would require several parameters (e.g. six parameters of the water dimer). For the sake of simplicity, only a subset of these parameters is usually chosen to define an hydrogen bond, and hence it is important to select the most significant ones. The significativity of the three geometric parameters chosen in this work (rLP−H , α and β ) is testified by the strong correlations between them that have been found. Furthermore, the use of sharp cutoffs on several geometric parameters can be a crude approximation in the description of properties that are instead continuous. A more desirable approach is the application of a cutoff on a function depending on all of the geometric parameters, as this would be a continuous extension of multiple cutoffs. In this framework, η has shown the further quality of having a strong correlation with the ion-water pair interaction energy, which is the most relevant quantity in defining an interaction. Thus, we suggest that the η function is a valid descriptor for the purpose of determining whether two species are hydrogen bonded or not.


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Acknowledgement This work was supported by the University of Rome “La Sapienza” (Progetto ateneo 2014, n.C26A14L7CX) and by the CINECA supercomputing center through the grant IscrCDIWA (n. HP10C2Q0F3).

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Anions from the Gas Phase to the Bulk Phase: The Case of the Halide Ions F− , Cl− , and Br− . J. Chem. Phys. 2012, 136, 044509. (10) Smith, J.; Saykally, R.; Geissler, P. The Effects of Dissolved Halide Anions on Hydrogen Bonding in Liquid Water. J. Am. Chem. Soc. 2007, 129, 13847–13856.

References (1) Reichardt, C.; Welton, T. Solvents and Solvent Effects in Organic Chemistry; WileyVCH, 2010.

(11) Chandra, A. Effects of Ion Atmosphere on Hydrogen-bond Dynamics in Aqueous Electrolyte Solutions. Phys. Rev. Lett. 2000, 85, 768–771.

(2) Tobias, D. J.; Hemminger, J. C. Getting Specific About Specific Ion Effects. Science 2008, 319, 1197–1198.

(12) Reif, M.; Hünenberger, P. Computation of Methodology-independent Single-ion Solvation Properties from Molecular Simulations. IV. Optimized Lennard-Jones Interaction Parameter Sets for the Alkali and Halide Ions in Water. J. Chem. Phys. 2011, 134, 144104.

(3) Baer, M. D.; Mundy, C. J. An Ab Initio Approach to Understanding the Specific Ion Effect. Faraday Discuss. 2013, 160, 89–101. (4) Smith, J. D.; Saykally, R. J.; Geissler, P. L. The Effects of Dissolved Halide Anions on Hydrogen Bonding in Liquid Water. J. Am. Chem. Soc. 2007, 129, 13847–13856.

(13) Ohtaki, H.; Radnai, T. Structure and Dynamics of Hydrated Ions. Chem. Rev. 1993, 93, 1157–1204. (14) Raugei, S.; Klein, M. L. An Ab Initio Study of Water Molecules in the Bromide Ion Solvation Shell. J. Chem. Phys. 2002, 116, 196– 202.

(5) Omta, A. W.; Kropman, M. F.; Woutersen, S.; Bakker, H. J. Negligible Effect of Ions on the Hydrogen-Bond Structure in Liquid Water. Science 2003, 301, 347–349. (6) Tongraar, A.; Rode, B. The Hydration Structures of F− and Cl− Investigated by Ab Initio QM/MM Molecular Dynamics Simulations. Phys. Chem. Chem. Phys. 2003, 5, 357–362.

(15) Tongraar, A.; Hannongbua, S.; Rode, B. M. QM/MM MD Simulations of Iodide Ion (I) in Aqueous Solution: A Delicate Balance between Ion-Water and Water-Water H-Bond Interactions. J. Phys. Chem. A 2010, 114, 4334–4339.

(7) Tongraar, A.; T-Thienprasert, J.; Rujirawat, S.; Limpijumnong, S. Structure of the Hydrated Ca2+ and Cl− : Combined X-ray Absorption Measurements and QM/MM MD Simulations Study. Phys. Chem. Chem. Phys. 2010, 12, 10876–10887.

(16) Bopp, P. The Physical Chemistry of Aqueous Solution; M.-C. Belliment-Funel and Neilson, G. W., Eds.; Reidel: Dordrecht, The Netherlands, 1987. (17) Frank, H.; Wen, W. Ion-Solvent Interaction. Structural Aspects of Ion-Solvent Interaction in Aqueous Solutions: A Suggested Picture of Water Structure. Discuss. Faraday Soc. 1957, 24, 133–140.

(8) Migliorati, V.; Sessa, F.; Aquilanti, G.; D’Angelo, P. Unraveling Halide Hydration: A High Dilution Approach. J. Chem. Phys. 2014, 141, 044509.

(18) Gurney, R. Ionic Processes In Solution; McGraw-Hill, 1953.

(9) Trumm, M.; Martínez, Y.; Réal, F.; Masella, M.; Vallet, V.; Schimmelpfennig, B. Modeling the Hydration of Mono-Atomic


12

Page 13 of 15

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


(19) Mezei, M.; Beveridge, D. Theoretical Studies of Hydrogen Bonding in Liquid Water and Dilute Aqueous Solutions. J. Chem. Phys. 1980, 74, 622–632.

(29) Silvestrelli, P.; Marzari, N.; Vanderbilt, D.; Parrinello, M. Maximally-Localized Wannier Functions for Disordered Systems: Application to Amorphous Silicon. Solid State Commun. 1998, 107, 7–11.

(20) Jorgensen, W.; Chandrasekhar, J.; Madura, J.; Impey, R.; Klein, M. Comparison of Simple Potential Functions for Simulating Liquid Water. J. Chem. Phys. 1983, 79, 926–935.

(30) Marx, D.; Hutter, J. Ab Initio Molecular Dynamics: Basic Theory and Advanced Methods; Cambridge University Press, 2009. (31) Car, R.; Parrinello, M. Unified Approach for Molecular Dynamics and Density-Functional Theory. Phys. Rev. Lett. 1985, 55, 2471.

(21) Reiher, M.; Sellmann, D.; Hess, B. Stabilization of Diazene in Fe(II)-Sulfur Model Complexes Relevant for Nitrogenase Activity. I. A New Approach to the Evaluation of Intramolecular Hydrogen Bond Energies. Theor. Chem. Acc. 2001, 106, 379–392.

(32) Kohn, W.; Sham, L. J. Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 1965, 140, A1133– A1138.

(22) Kumar, R.; Schmidt, J.; Skinner, J. Hydrogen Bonding Definitions and Dynamics in Liquid Water. J. Chem. Phys. 2007, 126, 204107.

(33) Hutter, J.; Alavi, A.; Deutch, T.; Bernasconi, M.; Goedecker, S.; Marx, D.; Tuckerman, M.; Parrinello, M. CPMD: MPI für Festkörperforschung and IBM Zurich Research Laboratory: Stuttgart, Germany 1995-1999,

(23) Wendler, K.; Thar, J.; Zahn, S.; Kirchner, B. Estimating the Hydrogen Bond Energy. J. Phys. Chem. A 2010, 114, 9529–9536. (24) Pan, Z.; Chen, J.; Lü, G.; Geng, Y.-Z.; Zhang, H.; Ji, Q. An Ab Initio Molecular Dynamics Study on Hydrogen Bonds between Water Molecules. J. Chem. Phys. 2012, 136, 164313.

(34) Becke, A. Density-Functional ExchangeEnergy Approximation with Correct Asymptotic Behavior. Phys. Rev. A 1988, 38, 3098. (35) Lee, C.; Yang, W.; Parr, R. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785.

(25) Isaacs, E.; Shukla, A.; Platzman, P.; Hamann, D.; Barbiellini, B.; Tulk, C. Covalency of the Hydrogen Bond in Ice: A Direct X-Ray Measurement. Phys. Rev. Lett. 1999, 82, 600–603.

(36) Schmidt, J.; VandeVondele, J.; Kuo, I.-F. W.; Sebastiani, D.; Siepmann, J. I.; Hutter, J.; Mundy, C. J. Isobaric-Isothermal Molecular Dynamics Simulations Utilizing Density Functional Theory: An Assessment of the Structure and Density of Water at NearAmbient Conditions. J. Phys. Chem. B 2009, 113, 11959–11964.

(26) Cordier, F.; Rogowski, M.; Grzesiek, S.; Bax, A. Observation of Through-HydrogenBond 2hJHC in a Perdeuterated Protein. J. Magn. Reson. 1999, 140, 510–512. (27) Zhang, C.; Khaliullin, R. Z.; Bovi, D.; Guidoni, L.; Kühne, T. D. Vibrational Signature of Water Molecules in Asymmetric Hydrogen Bonding Environments. J. Phys. Chem. Lett. 2013, 4, 3245–3250.

(37) Von Lilienfeld, O.; Tavernelli, I.; Rothlisberger, U.; Sebastiani, D. Optimization of Effective Atom Centered Potentials for London Dispersion Forces in Density Functional Theory. Phys. Rev. Lett. 2004, 93, 153004.

(28) Marzari, N.; Vanderbilt, D. Maximally Localized Generalized Wannier Functions for Composite Energy Bands. Phys. Rev. B 1997, 56, 12847–12865.

(38) Lin, I.-C.; Seitsonen, A.; Coutinho-Neto, M.; Tavernelli, I.; Rothlisberger, U. Importance


13


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

Page 14 of 15

(47) Migliorati, V.; Mancini, G.; Chillemi, G.; Zitolo, A.; D’Angelo, P. Effect of the Zn2+ and Hg2+ Ions on the Structure of Liquid Water. J. Phys. Chem. A 2011, 115, 4798–4803.

of van der Waals Interactions in Liquid Water. J. Phys. Chem. B 2009, 113, 1127–1131. (39) Troullier, N.; Martins, J. L. Efficient Pseudopotentials for Plane-Wave Calculations. Phys. Rev. B 1991, 43, 1993–2006.

(48) Hofer, T.; Tran, H.; Schwenk, C.; Rode, B. Characterization of Dynamics and Reactivities of Solvated Ions by Ab Initio Simulations. J. Comput. Chem. 2004, 25, 211–217.

(40) Hummer, G.; Pratt, L.; García, A. Molecular Theories and Simulation of Ions and Polar Molecules in Water. J. Phys. Chem. A 1998, 102, 7885–7895.

(49) Stillinger, F.; Rahman, A. Improved Simulation of Liquid Water by Molecular Dynamics. J. Chem. Phys. 1974, 60, 1528–1532.

(41) Berendsen, H. J. C.; van der Spoel, D.; van Drunen, R. GROMACS: A Message-Passing Parallel Molecular Dynamics Implementation. Comput. Phys. Commun. 1995, 91, 43– 56.

(50) Mahoney, M.; Jorgensen, W. A Five-Site Model for Liquid Water and the Reproduction of the Density Anomaly by Rigid, Nonpolarizable Potential Functions. J. Chem. Phys. 2000, 112, 8910–8922.

(42) D’Angelo, P.; Migliorati, V.; Guidoni, L. Hydration Properties of the Bromide Aqua Ion: the Interplay of First Principle and Classical Molecular Dynamics, and X-ray Absorption Spectroscopy. Inorg. Chem. 2010, 49, 4224– 4231.

(51) Duarte, F.; Bauer, P.; Barrozo, A.; Amrein, B. A.; Purg, M.; Åqvist, J.; Kamerlin, S. C. L. Force Field Independent Metal Parameters Using a Nonbonded Dummy Model. J. Phys. Chem. B 2014, 118, 4351–4362.

(43) Fulton, J. L.; Schenter, G. K.; Baer, M. D.; Mundy, C. J.; Dang, L. X.; Balasubramanian, M. Probing the Hydration Structure of Polarizable Halides: A Multiedge XAFS and Molecular Dynamics Study of the Iodide Anion. J. Phys. Chem. B 2010, 114, 12926– 12937.

(52) Saxena, A.; García, A. E. Multisite Ion Model in Concentrated Solutions of Divalent Cations (MgCl2 and CaCl2 ): Osmotic Pressure Calculations. J. Phys. Chem. B 2015, 119, 219–227.

(44) Bankura, A.; Santra, B.; Jr., R. A. D.; Swartz, C. W.; Klein, M. L.; Wu, X. A Systematic Study of Chloride Ion Solvation in Water Using van der Waals Inclusive Hybrid Density Functional Theory. Mol. Phys. 2015, 113, 2842–2854. (45) Chandrasekhar, J.; Spellmeyer, D.; Jorgensen, W. Energy Component Analysis for Dilute Aqueous Solutions of Li+ , Na+ , F− , and Cl− ions. J. Am. Chem. Soc. 1984, 106, 903–910. (46) Hiraoka, K.; Mizuse, S.; Yamabe, S. Solvation of Halide Ions with H2 O and CH3 CN in the Gas Phase. J. Phys. Chem. 1988, 92, 3943–3952.


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