ARTICLE pubs.acs.org/JPCA
Effect of the Zn2þ and Hg2þ Ions on the Structure of Liquid Water Valentina Migliorati,*,† Giordano Mancini,‡ Giovanni Chillemi,‡ Andrea Zitolo,† and Paola D’Angelo*,† † ‡
Dipartimento di Chimica, Universita di Roma “La Sapienza”, Piazzale Aldo Moro 5, 00185 Roma, Italy CASPUR, Inter-University Consortium for Supercomputing in Research, via dei Tizii 6b, 00185 Roma, Italy
bS Supporting Information ABSTRACT: The effect of ions on the structure of liquid water is still not completely understood, despite extensive experimental and theoretical studies. A combined XANES and molecular dynamics investigation on diluted Zn2þ and Hg2þ aqueous solutions reveals that the influence of a single ion on the bonding pattern of water molecules is strongly dependent on the nature of the ion. While the structure of water is not altered by the presence of the Zn2þ ion, the Hg2þ cation has a strong impact on the hydrogen-bond network of water that extends beyond the first coordination shell.
1. INTRODUCTION The essential role of water in physical, chemical, and biological processes is closely connected to the hydrogen-bond (HB) networks of the liquid, and modification of the tetrahedral structure of water induced by ions has been the subject of ongoing debate. Aqueous solutions of ions have been studied in great detail using both experimental and theoretical methods.19 Many of the experimental investigations, mainly based on neutron diffraction, have provided evidence that ions strongly affect the structure of liquid water and the perturbation extends beyond the first hydration shell.13 These findings are opposite to what was obtained from femtosecond pumpprobe spectroscopy, i.e., that addition of ions has no influence on the rotational dynamics of water molecules outside the first solvation shell.46 Several molecular dynamics (MD) simulations are present in the literature dealing with this issue, and some studies support the idea that the water structure is significantly influenced by the presence of ions,7,8 but others do not confirm this picture.9 Thus far, the complexity of the hydration phenomenon and the lack of experimental techniques able to provide detailed microscopic information on liquid systems have hampered the development of a unified picture on the extent of perturbation induced by ions in water. Most of the experimental results on ion hydration refer to highly concentrated solutions (0.5 M being usually the lowest limit) where the structure of the solvation shells is perturbed by surrounding ions of opposite sign. Experimental studies on the modification induced by a single ion on the water structure, in the hypothetical state of infinite dilution, are lacking. Here, we studied the ion-induced modification of the water structure by means of X-ray absorption near edge structure (XANES) spectroscopy and molecular dynamics simulations. This combined approach provides an excellent opportunity to study the effect of a single ion on the HB structure of water as XANES is the only technique so far that gives three-dimensional r 2011 American Chemical Society
structural information on very diluted solutions (down to the millimolar concentration range).10 In particular, we performed XANES measurements and calculated theoretical spectra averaging over a statistically significant ensemble of MD snapshots. XANES has been found to be sensitive not only to ionO firstand second-shell distributions but also to the relative location of the first- and second-shell oxygen atoms around the ion. The reliability of the MD simulations has been assessed by comparison with the XANES experimental data, and analysis of the MD trajectories allowed us to determine the change in the H-bonding properties of water in the presence of two different cations.
2. MATERIALS AND METHODS 2.1. Molecular Dynamics Details. MD simulations of Zn2þ
and Hg2þ in aqueous solution have been carried out using an effective two-body potential obtained by fitting the parameters of a suitable analytical function on an ab initio potential energy surface (PES). The ab initio PES was generated taking into account scalar relativistic effects using an effective core potential and including many-body effects by means of the polarizable continuum model (PCM).1113 A thorough description of the procedure used to obtain the ab initio PES can be found in refs 14 and 15 for the Zn2þ and Hg2þ ions, respectively. As far as the waterwater interactions are concerned, the SPC/E water model16 has been employed, since it provides a very good description of the structural and dynamic properties of liquid water.17 Moreover, two simulations of Hg2þ in aqueous solution with the SPC/ E and TIP5P18 water models produced very similar structural and dynamical results.19 The simulations were performed using the Received: February 1, 2011 Revised: April 8, 2011 Published: April 19, 2011 4798
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GROMACS package version 3.2.1,20 modified in order to include our ionwater effective two-body potential functions, by means of the following expression V ðrÞ ¼
qi qo Ao Bo Co Do þ 4 þ 6 þ 8 þ 12 þ Eo eFo rio rio rio rio rio rio qi qh Ah Bh Ch Dh þ þ 4 þ 6 þ 8 þ 12 rih rih rih rih ih ¼ ih1, ih2 rih
∑
ð1Þ
where rio, rih1, and rih2 are the ionwater distances, qi, qo, and qh are the electrostatic charges of the ion and the oxygen and hydrogen atoms, respectively (2, 0.8476, and 0.4238 au), and Ao, ..., Fo and Ah, ..., Dh are the parameters obtained by the fitting procedure. The systems including one ion and 819 water molecules have been simulated enforcing periodic boundary conditions in the NVT ensemble with 29.02 and 29.08 Å box lengths for Zn2þ and Hg2þ, respectively. The box dimensions were chosen as to reproduce the experimental densities of the solutions. All details on the MD simulations can be found in refs 14 and 15 for the Zn2þ and Hg2þ ions, respectively. The MD simulation of pure water has been carried out using the same protocol employed for the ionic solutions. The system was composed of 819 water molecules in a cubic box using periodic boundary conditions. A cutoff of 9 Å has been used for the nonbonded interactions, employing the particle-mesh Ewald method to correct for the long-range electrostatic effects.21,22 The temperature was kept fixed at 300 K by weak coupling to an external temperature bath using Berendsen’s method23 with a coupling constant of 0.1 ps. The simulation has been carried out for 15 ns, with a time step of 1 fs. The first 5 ns have been used to equilibrate the system and discarded in all analyses. Note that the OO g(r) of pure water is identical to that of bulk water molecules of the Zn2þ and Hg2þ solutions (see Figure 1S, Supporting Information, as an example). 2.2. X-ray Absorption Measurements. Zn2þ and Hg2þ aqueous solutions (0.05 M) were obtained by dissolving the appropriate amount of Zn(NO3)2 and Hg(ClO4)2 in water, respectively. The solutions were acified to about pH = 1 by adding HNO3 and HClO4, respectively, in order to prevent hydrolysis. XAS spectra at the Zn K edge were recorded using the EMBL spectrometer at DESY.24 Hg L3 XAS spectra were obtained at the X-ray absorption spectrometer BM29 of the European Synchrotron Radiation Facility.25 Spectra were recorded in transmission mode using a Si(311) double-crystal monochromator detuned to 50%. For each sample three spectra were recorded and averaged. The solutions were kept in a cell with Kapton film windows and Teflon spacers of 2 mm for Zn and 4 mm for Hg. 2.3. XANES Data Analysis and Computational Procedure. The XANES spectra of Zn2þ and Hg2þ in aqueous solution have been analyzed starting from the microscopic description of the system derived from the MD simulations. In the first step, the XANES spectrum associated with the MD trajectories has been calculated with the MXAN program26 using only the real part of the HedinLundqvist potential, that is, theoretical spectra do not account for any intrinsic and extrinsic inelastic process, while the damping associated with the experimental resolution is accounted for by convolution with a Gaussian function with full width at half-maximum of 1.7 and 2 eV for Zn and Hg, respectively. In the second step, to perform a comparison with the experimental data the damping associated with the inelastic processes has to be included in the calculation. To this purpose we used an
in-house modified version of the MXAN program that reads an external theoretical spectrum (the configurational averaged calculated data) and performs a minimization in the nonstructural parameter space only. In particular, the inelastic processes are accounted for by convolution with a broadening Lorentzian function having an energydependent width of the form Γ(E) = Γc þ Γmfp(E). The constant part Γc accounts for the core-hole lifetime, while the energydependent term represents all the intrinsic and extrinsic inelastic processes.26 The Γmfp(E) function is zero below an energy onset Es (which in extended systems corresponds to the plasmon excitation energy) and starts increasing from a given value A, following the universal functional form of the mean free path in solids. Both the onset energy Es and the jump A are introduced in the Γmfp(E) function via an arctangent functional form to avoid discontinuities. Least-squares fits of the XANES experimental data have been performed by minimizing the Rsq function.26 To calculate the spectrum associated with the Zn2þ and Hg2þ MD simulations, we extracted from the total MD trajectory for each ion two trajectories, the former containing only the ion and its first hydration shell and the latter containing both the first and the second hydration shells. In particular, for the second shell we considered all of the water molecules separated from the cation by a distance shorter than 5.2 Å since water molecules at larger distance have been found to provide a negligible contribution. From this trajectory we extracted 200 snapshots saved every 12.5 ps. Each snapshot has been used to generate the XANES associated with the corresponding instantaneous geometry, and the averaged theoretical spectrum has been obtained by summing all the spectra and dividing by the total number of MD snapshots used. An important question when dealing with the computation of spectra from MD simulations is to determine the total sampling length that is necessary to have a statistically significant average. To this end we carried out a statistical analysis of the errors associated with the averaged theoretical spectra. In particular, an averaged XANES spectrum over N configurations is affected by a variance σ2(Ei)/N, so for any N it is possible to calculate an energy-dependent error bar defined as ( σ(Ei)/N1/2.
3. RESULTS AND DISCUSSION To develop new understanding of the effect of ions on the microscopic structure of water we carried out an improved MD analysis together with a new simulation of pure SPC/E water. Due to the strong ionwater interactions, the water structure in the first coordination shell is inevitably perturbed by the presence of an ion, and this is particularly true for Zn2þ and Hg2þ, which form very stable and symmetric hydration complexes.14,15,27,28 Conversely, the perturbation induced by an ion outside the first hydration shell is more difficult to be predicted and is clearly dependent on the nature of the ion under investigation. We chose these two ions as Zn2þ forms a stable and regular octahedral hydration complex, while Hg2þ has a more disordered coordination sphere comprising seven water molecules.15,27 Hence, the extent of perturbation induced by these two ions on the water structure could be different. The validity of our MD simulations has been assessed using the XANES spectroscopy. We collected XANES spectra of two aqueous solutions containing 0.05 M Zn(NO3)2 and Hg(ClO4)2. In these systems each ion is surrounded by more than 1100 water molecules, thus reproducing the simulation conditions of 1 ion per 819 solvent molecules. In this concentration range the interaction 4799
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Figure 1. Comparison between the average XANES theoretical spectrum calculated from 200 MD snapshots including only the first (black line) and first plus second hydration shells (red line) for Zn2þ (A) and Hg2þ (B) in aqueous solution.
Figure 2. Comparison between the average XANES theoretical spectrum calculated from 200 MD snapshots including only the first hydration shell and the experimental data for Zn2þ (A) and Hg2þ (B) in aqueous solution.
between the cation and the counterions is completely missing, and it is therefore possible to single out the effect of a single ion on the water structure. XANES theoretical spectra have been calculated starting from the MD trajectories. For an individual atomic configuration, a spectrum was obtained by considering all of the water molecules belonging to either the first or the first plus second hydration shells of the ion and the XANES theoretical spectrum results from an average over 200 atomic configurations, equally spaced in time (see Figures 2S and 3S, Supporting Information). Given the extremely high sensitivity of XANES spectroscopy toward angular distributions and to the second hydration shell,2832 this technique is uniquely able to characterize the HB structure of the first- and second-shell hydration regions. Figure 1 shows the comparison between the Zn2þ and the Hg2þ XANES average theoretical spectra obtained from the MD configurations, including either the first hydration shell only or the first plus second hydration shells. Error bars have been reported for each energy point, and they were calculated as explained in section 2.3. Significant differences appear in the low-energy region up to about 60 eV from the threshold. In particular, for both Zn2þ and Hg2þ the edge intensity is lower in the spectrum containing only the first-shell water molecules, and the Zn2þ spectrum obtained from the first- plus second-shell clusters shows a clear additional feature in the energy region between 10 and 30 eV. Note that the differences between the first-shell-only and the first- plus second-shell spectra are much bigger than the error bars associated with the averaged theoretical XANES. These results confirm the sensitivity of XANES toward the structural arrangement of the second coordination sphere around the ion also at very low concentration. One interesting point that deserves deeper investigation regards the sensitivity of XANES toward the angular distribution of water molecules in the second hydration shell with respect to the first-shell ones. XANES probes the water structure seen from the ion, and the dominant effects from the angular distribution are associated with the multiple scattering (MS) paths within the first hydration shell. The difference in the calculation including the first and first plus second shells shown in Figure 1 is a clear indication that a second-shell signal is present, but it is not clear if it is only due to the single-scattering contribution associated with the ionOsecond shell g(r) or if it is also a signature of the ionO1O2 angular distribution (where O1 is an oxygen atom of the first hydration shell and O2 is an oxygen atom of the second hydration shell). A way to explore the sensitivity of XANES toward ionO1O2 angular distribution is to calculate the XANES spectrum including only the second-shell water molecules. To this end a trajectory containing the water
molecules within a distance range between 3.5 and 5.2 Å from the Zn2þ ion has been extracted from the MD simulation, and an average XANES signal has been obtained from 200 theoretical spectra including the second-shell only. The results of this analysis are shown in Figure 4S, Supporting Information. The difference (XANESfirstþsecond XANESfirst only XANESsecond only þ 2) corresponds the MS contributions associated with ZnO1O2 paths and is representative of the sensitivity of XANES toward the ionO1O2 angular distribution. As shown in Figure 5S, Supporting Information, the difference spectrum shows a clear oscillating behavior up to 50 eV above the edge, thus demonstrating that XANES spectroscopy probes not only the secondshell radial distribution function but also its angular distribution with respect to the first hydration shell. To assess the reliability of our theoretical results it is necessary to compare the total averaged XANES spectra with the experimental data. To this end all inelastic processes have been accounted for by convoluting the theoretical averaged spectra with a broadening Lorentzian function, and the corresponding nonstructural parameters have been optimized. In panel A of Figure 2, the experimental XANES data of Zn2þ in water are compared with the averaged theoretical spectrum including the first-shell clusters as derived from MD simulations. The overall agreement of the theoretical and experimental spectra is not good. In particular, the theoretical spectrum does not reproduce the feature in the region between 10 and 30 eV, which is the fingerprint of the second hydration shell contribution. For Hg2þ the main spectral differences between the theoretical spectrum including only the first hydration shell and experimental spectrum are detectable in the energy region between 20 and 60 eV. Also, in this case the agreement between experiment and theory is not good (Figure 2B), highlighting that inclusion of the second coordination sphere is mandatory to account for all of the details of the XANES experimental data. In Figure 3 the Zn2þ and Hg2þ averaged theoretical spectra including both the first and the second hydration shells are compared with the experimental data. In this case the agreement between experiment and theory is excellent, the intensity of the white line is recovered, and the shape of the first minimum region is very similar to the experimental data. It is important to remark that the XANES spectra have been calculated using the structural information obtained from the MD simulations without carrying out any minimization in the structural parameter space. Because of the high sensitivity of the XANES technique toward the structural environment of the photoabsorber this approach is a very strict test on the quality of the potentials used in the MD simulations. Therefore, the correspondence between the XANES 4800
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Figure 3. Comparison between the average XANES theoretical spectra calculated from 200 MD snapshots including the first and second hydration shells and the experimental data for Zn2þ (A) and Hg2þ (B) in aqueous solution.
experimental data and the theoretical spectra calculated from the MD microscopic configurations unambiguously proves the reliability of the water bonding pattern determined from our MD simulations for these two systems. To shed light on the perturbation induced by cations on the water structure we compared the structural properties of the second hydration shells of Zn2þ and Hg2þ obtained from MD simulations with those of pure water. To this end we calculated the oxygenoxygen radial distribution function between the oxygen atom of a water molecule in the cation second coordination shell and the oxygen atoms of all of the other water molecules (gOO2-shell(r)). From this analysis it is possible to have a direct picture of the structural environment seen by a water molecule belonging to the ion second coordination sphere. If the effect due to the presence of ions in solution on the solvent structure were limited to the first hydration shell, gOO2-shell(r) should closely resemble that of pure water. In the upper panel of Figure 4 we show the OO g(r) of pure water obtained from a MD simulation of 819 water molecules in a cubic box, compared with the gOO2-shell(r) of Zn2þ. Apart from a small peak at about 5.5 Å, the Zn2þ gOO2-shell(r) is almost identical to that of pure water, showing that the water structure beyond the first hydration shell is substantially unaffected by the presence of the ion. Insights into the source of the peak at 5.5 Å have been gained by recalculating the Zn2þ gOO2-shell(r) not including water molecules belonging to the cation first hydration shell. As shown in the upper panel of Figure 4 the peak completely disappears, revealing that its origin stems from the presence of the first-shell water molecules that are strongly oriented by the ion. The lower intensity of the oxygen oxygen g(r)’s obtained not including the first-shell water molecules is due to the fact that we did not apply any corrections for the excluded volume effects caused by removal of the first hydration shell. As far as the Hg2þ ion is concerned the gOO2-shell(r) is quite different from that of the pure water, meaning that ioninduced modification of the tetrahedral structure of water extends beyond the first hydration shell (lower panel of Figure 4). The main difference is the position of the g(r) second peak that is traditionally regarded as a signature of the H-bond network in water, which has a different shape and moves outward in the Hg2þ second shell as compared to pure water. This result is opposite to what was obtained from neutron diffraction studies13 in which a shift toward shorter distances of the OO g(r) second peak was observed with increasing salt concentration. Clues to the structural transformations of the water HB network upon addition of ions emerge from analysis of the bond angle O1OO2 that a central oxygen forms with the oxygen
Figure 4. Comparison of the oxygenoxygen g(r)’s obtained for pure water (black line) and for the second coordination shell of Zn2þ and Hg2þ including (red dashed line) or not including (blue dot-dashed line) water molecules belonging to the cation first hydration shell.
atoms within a distance of 3.3 Å (Figure 5A). This value corresponds to the first minimum of the pure water gOO(r) and can be considered as the cutoff distance of the first hydration shell. In pure water, where there is a quite regular HB network, the distribution of this angle is highly peaked at about 107° (1 cos(O1OO2) = 1.29), corresponding to a tetrahedral arrangement, and an additional low-intensity peak is detected at about 55° (1 cos(O1OO2) = 0.42). For Zn2þ and Hg2þ we calculated an O1OsecondO2 angle distribution in which the intervening oxygen belongs to the ion second hydration sphere and O1 and O2 are oxygen atoms within a cutoff distance of 3.3 Å (Figure 5B). Comparison with the O1OO2 distribution of pure water allows one to verify if the oxygen atoms of the Zn2þ and Hg2þ second coordination sphere are involved in a HB network having the same tetrahedral geometry of pure water. As far as the Zn2þ ion is concerned, the O1OsecondO2 distribution is very similar to that of pure water, proving that the Zn2þ second-shell water molecules are involved in a HB network having the same pattern of pure water and reinforcing the finding that the perturbation induced by the ion is limited to the first coordination sphere. Conversely, remarkable differences are found for the Hg2þ ion (Figure 5A): the main peak of the O1OsecondO2 distribution is shifted toward larger angles, while the intensity of the peak at 55° significantly increases. The higher coordination number and the C2 symmetry of the Hg2þ first shell complex15 induce a large number of second-shell water molecules (29%) to coordinate two water molecules belonging to the ion first hydration shell. The percentage of these “bridge” molecules is much smaller in the second shell of both water (6%) and Zn2þ (9%). The presence of such a large number of bridge molecules in the Hg2þ second shell is responsible for both the higher intensity of the peak at 55° and the shift toward larger angles of the peak at 107° in the angle distribution, as compared to the Zn2þ solution and pure water. As a result, the tetrahedral arrangement of water molecules in the Hg2þ second shell is highly distorted, thus disrupting the HB 4801
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Figure 6. O1OO2 angle distributions obtained from the MD simulation of Hg2þ in aqueous solution. In the O1OO2 notation O is the oxygen atom of water molecules belonging to different coordination spheres around the Hg2þ ion and O1 and O2 are oxygen atoms of water molecules belonging to a coordination sphere of 3.3 Å radius around the central oxygen O.
Table 1. Percentages of Water Molecules FHBn Having n WaterWater HBs and Average Number of HBs Per Water Molecule (n), Calculated for Pure Water and for the Second Coordination Shell of Zn2þ and Hg2þ in Aqueous Solutions
Figure 5. (A) O1OO2 angle distributions obtained from the MD simulations. In the O1OO2 notation O is the central oxygen while O1 and O2 are oxygen atoms of water molecules belonging to a coordination sphere of 3.3 Å radius around the central oxygen. (B) Representation of the O1OsecondO2 angle used in the angular distribution. The octahedral complex typical of the Zn2þ hydration sphere is shown as an example.
network usually found in the bulk. In order to shed light into the extent of perturbation induced by the Hg2þ ion on the water structure, we calculated the O1OO2 distribution where O is the oxygen atom of water molecules belonging to different coordination spheres around the Hg2þ ion and O1 and O2 are oxygen atoms of water molecules belonging to a coordination sphere of 3.3 Å around the central oxygen O. In particular, the second coordination shell has been defined considering the water molecules in the distance range 3.755.4 Å from the Hg2þ ion, and higher distance spheres were considered with a 2 Å thickness. The results of this analysis are shown in Figure 6, and they indicate that only the second hydration shell is strongly perturbed by the Hg2þ ion, while for distances longer than 5.4 Å from the cation the water bonding pattern is very similar to that of pure water. To better characterize the effect of these structural transformations on the HB network, we calculated the percentage FHBn of water molecules that engage in n HBs and the HB average number per water molecule in pure water and in the second shell of Zn2þ and Hg2þ (Table 1). We adopted a configurational criterion where two water molecules are hydrogen bonded only if their inter-oxygen distance is lower than 3.5 Å and simultaneously the hydrogenoxygen distance is lower than 2.45 Å and
FHB1
FHB2
FHB3
FHB4
FHB5
n
pure water Zn2þ
1.0 1.3
9.0 12.0
33.4 39.8
50.6 44.5
6.0 2.4
3.52 3.35
Hg2þ
2.5
17.2
43.6
35.4
1.3
3.16
the oxygenoxygenhydrogen angle is less than 30°.7 For Zn2þ a slight decrease of FHB4 and increase of FHB3 are found, as compared to pure water. However, both pure water and the Zn2þ second shell are dominated by molecules that form four HBs, while in the Hg2þ second coordination sphere a dominant percentage of water molecules forms only three HBs, leading to a decrease of the average number of HBs per water molecule. In conclusion, our results show that the influence of a single ion on the HB network of liquid water is highly dependent on its nature. If the ion is strongly hydrated and forms a stable and symmetric hydration complex the tetrahedral structure of the HB network of the outer shells is not significantly altered. However, in the presence of a more disordered first coordination shell and weaker hydration structure modification of the waterwater HB environment induced by the presence of the ion extends beyond the first coordination shell. The use of XANES spectroscopy in combination with MD simulations, on the one hand, gave us the unique opportunity to study the effect of a single ion on the water structure in a concentration range where all possible ionic pair effects have been removed and, on the other hand, allowed us to obtain very accurate information on the bonding pattern of water molecules belonging to the ion second coordination sphere.
’ ASSOCIATED CONTENT
bS
Supporting Information. Further details are given in Figures 1S to 5S. This material is available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (V.M.),
[email protected] (P.D.).
’ ACKNOWLEDGMENT This work was supported by CASPUR with the standard HPC Grant 2010 entitled “A combined X-ray absorption spectroscopy, MD simulations and QM calculation procedure for the structural characterization of ill-defined systems”. We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities. ’ REFERENCES (1) Botti, A; Bruni, F.; Imberti, S.; Ricci, M. A.; Soper, A. K. Ions in water: the microscopic structure of concentrated NaOH solutions. J. Chem. Phys. 2004, 120, 10154–10162. (2) Botti, A; Bruni, F.; Imberti, S.; Ricci, M. A.; Soper, A. K. Ions in water: the microscopic structure of a concentrated HCl solution. J. Chem. Phys. 2004, 121, 7840–7848. (3) Mancinelli, R.; Botti, A.; Bruni, F.; Ricci, M. A.; Soper, A. K. Perturbation of water structure due to monovalent ions in solution. Phys. Chem. Chem. Phys. 2007, 9, 2959–2967. (4) Moilanen, D. E.; Wong, D.; Rosenfeld, D. E.; Fenn, E. E.; Fayer, M. D. Ion-water hydrogen-bond switching observed with 2D IR vibrational echo chemical exchange spectroscopy. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 375–380. (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) Park, S.; Fayer, M. D. Hydrogen bond dynamics in aqueous NaBr solutions. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 16731–16738. (7) Chandra, A. Effects of ion atmosphere on hydrogen-bond dynamics in aqueous electrolyte solutions. Phys. Rev. Lett. 2000, 85, 768–771. (8) Chialvo, A. A.; Simonson, J. M. The effect of salt concentration on the structure of water in CaCl2 aqueous solutions. J. Mol. Liq. 2004, 112, 99–105. (9) Lyubartsev, A. P.; Laasonen, K.; Laaksonen, A. Hydration of Liþ ion. An ab-initio molecular dynamics simulation. J. Chem. Phys. 2001, 114, 3120–3126. (10) D’Angelo, P.; Benfatto, M.; Della Longa, S.; Pavel, N. V. Combined XANES and EXAFS analysis of Co2þ, Ni2þ, and Zn2þ aqueous solutions. Phys. Rev. B 2002, 66, 064209. (11) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum mechanical continuum solvation models. Chem. Rev. 2005, 105, 2999–3093. (12) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. New developments in the polarizable continuum model for quantum mechanical and classical calculations on molecules in solution. J. Chem. Phys. 2002, 117, 43–54. (13) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. Energies, structures, and electronic properties of molecules in solution with the C-PCM solvation model. J. Comput. Chem. 2003, 24, 669–681. (14) Chillemi, G.; D’Angelo, P.; Pavel, N. V.; Sanna, N.; Barone, V. Development and validation of an integrated computational approach for the study of ionic species in solution by means of effective two-body potentials. The case of Zn2þ, Ni2þ, and Co2þ in aqueous solutions. J. Am. Chem. Soc. 2002, 124, 1968–1976. (15) Chillemi, G.; Mancini, G.; Sanna, N.; Barone, V.; Della Longa, S.; Benfatto, M.; Pavel, N. V.; D’Angelo, P. Evidence for Sevenfold Coordination in the First Solvation Shell of Hg(II) Aqua Ion. J. Am. Chem. Soc. 2007, 129, 5430–5436. (16) Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. The missing term in effective pair potentials. J. Phys. Chem. 1987, 91, 6269–6271.
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