Influence of Counterions on the Hydration Structure of Lanthanide Ions

Subscriber access provided by MT ROYAL COLLEGE

Article

Influence of Counterions on the Hydration Structure of Lanthanide Ions in Dilute Aqueous Solutions Valentina Migliorati, Alessandra Serva, Francesco Sessa, Andrea Lapi, and Paola D'Angelo J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b12571 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22 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

The Journal of Physical Chemistry

Influence of Counterions on the Hydration Structure of Lanthanide Ions in Dilute Aqueous Solutions Valentina Migliorati,∗,† Alessandra Serva,† Francesco Sessa,† Andrea Lapi,†,‡ and Paola D’Angelo∗,† Department of Chemistry, University of Rome “La Sapienza”, P.le Aldo Moro 5, 00185 Rome, Italy, and Istituto CNR di Metodologie Chimiche-IMC, Sezione Meccanismi di Reazione c/o Department of Chemistry, University of Rome “La Sapienza”, P.le Aldo Moro 5, 00185 Rome, Italy E-mail: [email protected]; [email protected]

Abstract

tion complex formed in solution. The MD results have been confirmed by comparison with the Ln K-edge XAS experimental data and the quantitative analysis of the extended X-ray absorption fine structure (EXAFS) spectra of the three salt solutions has provided a definite proof of the accuracy of the force field employed in the simulations and of the MD structural result. The anion-water and water-water hydrogen bonds lifetimes have been analyzed highlighting the slow down effect of the triflate, nitrate and Tf2 N− anions on the hydrogen bonds dynamics in the Ln3+ first solvation shell, with the effect being stronger in the Dy3+ solutions, due to the higher charge density of the Dy3+ ion as compared to La3+ .

A synergic approach combining Molecular Dynamics (MD) simulations and X-ray absorption spectroscopy (XAS) has been used to investigate diluted (0.1 M) aqueous solutions of two lanthanides ions (Ln3+ ), namely La3+ and Dy3+ , with triflate, nitrate and bis(trifluoromethylsulfonyl)imide (Tf2 N− ) as counterions. The different complexing ability of the three anions has been highlighted by the analysis of the MD simulations: Tf2 N− does not form inner-sphere complexes while a small amount of triflate coordinates both the La3+ and Dy3+ cations in their first solvation shell. On the other hand, the nitrate ion is almost absent in the La3+ first coordination sphere, while forming contact ion pairs with Dy3+ . Both lanthanide ions are found to preferentially interact with the water molecules and the total number of oxygen atoms coordinating the Ln3+ cations in their first solvation sphere is the same in all of the solutions, regardless they belong to water molecules or to the counterion. The presence of counterions in the cation first or second shell changes neither the first shell distance nor the symmetry of the hydra-

Introduction Understanding the hydration structure of metal cations in aqueous solution is very important to address their solvation properties and chemical reactions. Several experimental and theoretical studies on lanthanide (Ln) ions in water have been carried out over the past years and the hydration structure of these ions is now well established. 1–12 With the exception of Ce in aqueous solution lanthanides exist exclusively in the highly charged trivalent state Ln3+ which makes them strongly hydrated. The average distance between the Ln3+ ions and the water molecules of the first hydration shell de-

∗ To

whom correspondence should be addressed of Chemistry, University of Rome “La Sapienza”. ‡ Istituto CNR di Metodologie Chimiche-IMC,Sezione Meccanismi di Reazione † Department

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 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 2 of 22

hydration properties of Cm3+ and Th4+ has been provided by recent ab initio MD simulations. 21,22 In these works it was shown that the presence of the counterions induces a strenghtening of the cation primary hydration shell as a consequence of an altered dynamics of the hydrogen bonds between the water-water and water-counterion pairs. MD simulations have been frequently used to shed light into the structural and dynamical properties of Ln3+ aqueous solutions and most of these studies have been carried out in the socalled infinite dilution approximation with a single ion immersed in bulk water and with neutrality achieved by a uniformly distributed charge of opposite sign. 1–8 However, counterions can be inserted in the simulation to neutralize the net charge and this allows one to shed light into the formation of inner or outer-sphere complexes with the cation. Several MD calculations have been carried out using parametrized polarizable force fields to account for the polarization induced on the water molecules by the 3+ charge of the Ln cations. 1,5–7,38,39 However, it can be quite challenging for polarizable force fields to reproduce the fine balance among the different interactions taking place in the presence of the counterion, especially considering the long simulation time that is required to accurately sample the phase space visited by these systems. On the other hand, in a recent MD simulation study of the whole lanthanide series a force field without explicit polarization has been developed and it has been found to properly reproduce the structural properties of the Ln3+ hydration complexes in water, 40 and in ethyl ammonium nitrate. 41 Here, this force field has been used to provide a microscopic description of the Ln3+ ions in aqueous solutions including counterions having different complexing properties. From an experimental point of view, Ln3+ complexation with perchlorate, halide, nitrate and triflate has been studied with different techniques such as EXAFS, 15,16,26 17 O and 19 F NMR, 18,42 Raman, 19,20 UV-Visible 17 and time resolved laserinduced fluorescence spectroscopies. 27 However, although these techniques provide reliable structural or dynamic information, a thorough microscopic description of the short- and medium-range structure of Ln3+ salts in water is still lacking.

creases across the series and the coordination number goes from nine for the lighter elements to eight for the heavier ones. 1,2,4,5,9–11,13 An important topic that is under debate in the literature is the influence of counterions on the coordination structure and dynamics of metal ions. 14–20 In highly concentrated solutions (> 1 M) contact ion pairs do form due to the low number of water molecules that are not sufficient to complete the first and second hydration shells of the ions. For more dilute solutions the effect of the counterions strongly depends on their chemical nature. Perchlorate (ClO− 4 ) and triflate (trifluoromethanesulfonate, TfO− ) are considered weakly complex-forming anions and it is well established that they do not enter the Ln3+ first hydration shell. 15–22 Conversely, nitrate anions coordinate Ln3+ ions in their first coordination sphere, assuming either a bidentate or monodentate coordination geometry, 17,19,23–25 while for chloride ions an inner-sphere complexation has been found to occur only at high salt concentrations. 17,19,20,26–29 In this work, we have carried out a combined Molecular Dynamics (MD) and extended X-ray absorption fine structure (EXAFS) investigation on aqueous solutions of two Ln3+ ions (La3+ and Dy3+ ) with triflate, nitrate and bis(trifluoromethylsulfonyl)imide (Tf2 N− ) as counterions in dilute conditions (0.1 M). The schematic molecular structure of the counterions investigated in this work is shown in Figure 1. Tf2 N− and nitrate anions have been chosen since they are the most common anions in imidazoliumbased and protic ionic liquids that are very promising solvents for Ln3+ separation processes from aqueous media. 30–36 The triflate anion has been studied as it is a weakly-complexing species with a structure more similar to the bulky Tf2 N− anion as compared to perchlorate. On the other hand, La3+ and Dy3+ have been chosen as they are placed at the beginning and at the middle of the lanthanide series and the Ln3+ hydration properties are known to change across the series. In previous works it was found that the first shell coordination number of lanthanide and actinide ions decreases or increases depending on whether the counterions are located in the primary or secondary shell of the cation, respectively. 37 A different picture of the influence of counterions on the


2

Page 3 of 22 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


K for a total time of 20 ns, after a 3 ns equilibration run, using a 1 fs time step. The length of the simulation box edges are 44.14 Å, 44.60 Å and 43.87 Å for the Ln(TfO)3 , Ln(Tf2 N)3 and Ln(NO3 )3 salt solutions, respectively. The NoséHoover thermostat, 59,60 with a relaxation constant of 0.5 ps, was used to control the system temperature. Long-range interactions have been evaluated by the particle-mesh Ewald method, 61 while the cut-off of the nonbonded interactions was set to 9 Å. Periodic boundary conditions have been applied in order to simulate bulk material.

Figure 1: Molecular structure of the triflate (TfO− ) (a), bis(trifluoromethylsulfonyl)imide (Tf2 N− ) (b) and nitrate (NO− 3 ) (c) anions. Here, MD simulations have been used in combination with the EXAFS spectroscopy that provides accurate short range structural information around a selected ion, 43–54 to clarify the Ln3+ complexation with anions in dilute aqueous solutions and to obtain a complete picture of the hydration structure of both the cations and anions in water.

X-ray absorption measurements La(TfO)3 , Dy(TfO)3 , La(NO3 )3 ·6H2 O and Dy(NO3 )3 ·xH2 O were purchased from SigmaAldrich, all with a stated purity of >99% and further purification was not carried out. La(Tf2 N)3 and Dy(Tf2 N)3 were synthesized by reaction of the corresponding oxides (La2 O3 and Dy2 O3 , SigmaAldrich) with bis(trifluoromethanesulfonyl)imide following the procedure reported by Ferry et al. in ref. 62 0.1 M aqueous solutions of Ln(TfO)3 , Ln(Tf2 N)3 and Ln(NO3 )3 were made by dissolving a weighed amount of salt in freshly distilled water. La and Dy K-edge spectra were collected at room temperature at ESRF, on the bending magnet X-ray absorption spectroscopy beam line BM23, in transmission geometry. The spectra were collected by using a Si(311) double-crystal monochromator with the second crystal detuned by 20% for harmonic rejection. The aqueous solutions were kept in cells with Kapton film windows and Teflon spacers of 2 cm.

Methods Molecular Dynamics Classical MD simulations of 0.1 M aqueous solutions containing lanthanide salts (Ln(TfO)3 , Ln(Tf2 N)3 and Ln(NO3 )3 , with Ln3+ = La3+ and Dy3+ ) have been performed by means of the Gromacs package. 55 The SPC/E model 56 was adopted for water molecules, while the force field parameters for the TfO− , Tf2 N− and NO− 3 anions were taken from Lopes and Padua. 57,58 As regards the La3+ and Dy3+ cations, we used standard point charge 3+ and Lennard-Jones parameters (σLnLn and εLnLn ) developed by us in order to obtain accurate structural properties of La3+ and Dy3+ in pure water. 40 All the Lennard-Jones cross terms between different atom types have been calculated using the Lorentz-Berthelot combining rules, with the exception of the interaction between the Ln3+ cation and the O atoms of the NO− 3 anions. The Lennard-Jones parameters for this interaction have been obtained by combining σLnLn and εLnLn with the σOO and εOO values taken from the SPC/E water model. 56 The systems were composed of 5 Ln3+ cations, 15 anions and 2775 water molecules in a cubic box, created by randomly assigning initial positions to all ions and molecules and then equilibrated under NPT conditions in order to obtain the starting configuration. The simulations have been carried out in the NVT ensemble at 300

EXAFS data analysis The analysis of EXAFS data has been carried out with the GNXAS program, and all details on the theoretical framework cand be found in Ref. 63 Phase shifts have been calculated using muffintin potentials starting from the coordination structure obtained from the MD simulations. The exchange and correlation parts of the potential are determined on the basis of the local-density approximation of the self-energy of the excited photoelectron using an appropriate complex optical potential. The real part of self-energy has been


3


calculated by using the Hedin-Lundqvist energydependent potential. Inelastic losses of the photoelectron in the final state have been accounted for intrinsically by the complex potential. Previous investigations on the solvation structure of lanthanide ions, 13 metal ions 64–66 and halides, 67–69 have shown that in the case of solutions a correct approach to compute the χ(k) signal uses radial distribution functions g(r) according to the equation: Z ∞

χ(k) =

Page 4 of 22

lations. For this reason, we have simulated large boxes containing 5 Ln3+ cations (together with 15 anions and 2775 water molecules) starting from randomly assigned initial positions of the ions and water molecules; after an NPT equilibration the systems were additionally equilibrated for 3 ns in the NVT ensemble and production runs were then collected for a simulation time of 20 ns. In order to investigate the complexation properties of lanthanide salts in water we have calculated the g(r)’s between the La3+ and Dy3+ cations and the anions (Ln-X g(r)’s), that are shown in Figure 2. In particular, we have chosen the sulfur atom, S, as observed atom for the TfO− anion, and the nitrogen atom, N, for the Tf2 N− and NO− 3 anions. The g(r) peak positions r and the coordination numbers CN are listed in Table 1 for all of the solutions.

dr 4πρr2 g(r)A(k, r) sin [2kr + φ (k, r)]

0

(1) where A(k, r) and φ (k, r) are the amplitude and phase functions, respectively, and ρ is the density of the scattering atoms. Given the short-range sensitivity of the EXAFS tecnique, the total theoretical χ(k) signal has been calculated starting from the MD Ln-O and Ln-H g(r)’s and only those associated with the water molecules have been found to provide a detectable contribution. Least-squares fits of the EXAFS raw spectra have been carried out keeping fixed the structural parameters while two non structural parameters have been optimized, namely E0 (core ionization threshold) and S20 (many body amplitude reduction function).

Lanthanide triflates The triflate anions are considered as weakly complex-forming species and are often used as counterions for structural studies of the hydration properties of lanthanide ions in water. 9,10,12 The g(r)’s between the La3+ and Dy3+ cations and the triflate anions calculated from our MD simulations of the lanthanide triflate aqueous solutions show a first shell peak with maxima at 3.90 and 3.80 Å for the La3+ and Dy3+ ions, respectively, followed by a second shell peak and some lessdefined oscillations beyond. However, the Ln-S first shell coordination numbers, obtained by integrating the g(r) first peak up to the first minimum, are very low for both ions (0.2) indicating that the lanthanide triflate salts are substantially dissociated in water. This result is in agreement with the common assumption that triflate does not form ion pairs with lanthanides in aqueous solution, and also with two NMR investigations, the former carried out on Dy(TfO)3 aqueous solutions 18 and the latter on Ln(TfO)3 in water for the entire lanthanide series, 42 where it was found that the triflate anion does not enter the Ln3+ first coordination sphere. The same result has also been obtained for the La3+ ion in a recent combined Raman scattering and DFT study. 19 In order to get additional insights into the salt association process and to evaluate the sampling of the system phase space, it is useful to plot the evo-

Results and discussion Lanthanide salts: inner- versus outersphere complexation The first question that we address here is whether lanthanide (La3+ and Dy3+ ) triflate, Tf2 N− and nitrate salts form inner-sphere complexes in water. To this end we have carried out MD simulations of aqueous solutions containing the lanthanide salts, as described in detail in the Methods section. When dealing with salt association processes in solution it is very important to properly sample the phase space visited by the systems, as a poor statistics could result in completely erroneous results. In particular, if the salt dissociation/association process is slow and the simulation time is too short, the initial conditions can bias the results that are obtained from the calcu-


4

Page 5 of 22

Ln-X TfO

20

Lanthanide Tf2 N−

3+

La 3+ Dy

10 0 4

The analysis of our MD simulations shows that lanthanide Tf2 N− salts are fully dissociated in water. Indeed, the g(r)’s between the La3+ and Dy3+ cations and the Tf2 N− anions show only a broad low intensity peak at long Ln-N distances (Figure 2). Inspection of the Ln-N time profiles (panels c and d of Figure 3 and Figures S3 and S4 of Supporting Information) reveals that the Tf2 N− anions approach the La3+ and Dy3+ cations (Ln-N distances in the range 4.5-5.0 Å) many times in the course of the simulations, but they never enter the Ln3+ first solvation shell.

-

Tf2N

3 2

3+

La 3+ Dy

1 0 60

-

NO3

40

Lanthanide nitrates 3+

The g(r)’s between the La3+ and Dy3+ cations and the nitrate anions calculated from our MD simulations show different trends: for the La3+ ion a low intensity first shell peak with maximum at 3.80 Å is found, while for Dy3+ the first shell peak with maximum position of 3.61 Å is much more intense (see Figure 2). This result is at variance with our findings on lanthanide triflate and Tf2 N− salts in aqueous solutions, where the two Ln3+ cations have shown very similar structural properties. In the nitrate salt solutions, the Ln-nitrate first shell coordination number obtained for La3+ is very low, while in the case of the Dy3+ ion a larger number of nitrate ions enter the cation first coordination sphere (first shell coordination number of 0.5). These findings are consistent with the low association constant values of Dy3+ and other Ln3+ nitrate salts determined by microcalorimetry and solvent extraction techniques. 70,71 Moreover, the coordination number obtained for the La3+ ion is in line with the results of a recent Raman and DFT investigation, where it was shown that, in dilute La3+ nitrate aqueous solutions, a very small percentage of nitrate ions coordinates La3+ in its first coordination sphere. 19 On the other hand, in a combined MD and UV/VIS spectroscopy study, nitrate anions were observed in the Dy3+ first coordination shell only for aqueous solutions with a salt concentration above 2.0 mol Kg−1 . 17

La 3+ Dy

20 0

ions remain in the Dy3+ first coordination shell for longer times as compared to the La3+ ion.

-

30

g(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


3

4

5

6

7

8

9

10

r (Å) Figure 2: Ln-X radial distribution functions, g(r)’s, calculated from the MD simulations of 0.1 M Ln(TfO)3 , Ln(Tf2 N)3 and Ln(NO3 )3 aqueous solutions. Note that X = S for TfO− and X = N for Tf2 N− and NO− 3 anions. lution of the Ln-S distances for each single Ln3+ cation in the course of the simulations. The time evolutions of the Ln-distances for a single Ln3+ ion taken as an example are shown in panels a and b of Figure 3, while the time evolutions for all of the Ln3+ ion are shown in Figures S1 and S2 of Supporting Information for La3+ and Dy3+ ions, respectively. As concerns the La3+ ion, at the end of the NVT equilibration there are two cations with one triflate anion in their first solvation shell and then during the simulation many different anions enter, for short simulation times, the La3+ first coordination sphere. Conversely, a lower number of triflate anions approaches the Dy3+ cation in the course of the simulation, but sometimes the an-


5


(a)

(b)

(c)

(d)

(e)

(f)

Page 6 of 22

Figure 3: Time evolution of the Ln-X distances of one Ln3+ cation calculated from the MD simulation of 0.1 M La(TfO)3 (a), Dy(TfO)3 (b), La(Tf2 N)3 (c), Dy(Tf2 N)3 (d), La(NO3 )3 (e) and Dy(NO3 )3 (f) aqueous solutions. Note that X = S for TfO− and X = N for Tf2 N− and NO− 3 anions. The plotted distance ranges have been chosen to highlight possible exchange events between the first and the second solvation shells of the Ln3+ ions. Each different color indicates that the X atom belongs to a different counterion present in the solution. An open question concerning nitrate coordination of lanthanide ions is whether the nitrate ions act as monodentate or bidentate ligands. In solid state structures the dominant coordination of nitrate is the bidentate one, 72 while quantum chemical calculations have suggested that increase of the number of water molecules in the Ln3+ first coordination shell promotes the monodentate coordination mode of nitrate with respect to the Ln3+ ion. 25 The presence of a single peak in our Ln-N g(r)’s indicates that a single nitrate coordination mode takes place in both La3+ and Dy3+ nitrate aqueous solutions, i.e. the monodentate one. This finding is in agreement with the results obtained by Duvail et al. from MD simulations of Nd3+ and Dy3+ nitrates in water. 17 As mentioned above, La3+ and Dy3+ nitrate salts show different structural properties, and a different dynamic behavior of such salts can also be appreciated by looking at the time profile of the Ln-N distances (panels e and f of Figure 3 and Figures S5 and S6 of Supporting Information): for La3+ the nitrate anions enter the first solvation

shell only for short simulation times (the longest lifetime of the anions in the La3+ first shell is about 0.2 ns), while in the case of Dy3+ the nitrate can durably coordinate the cation also for longer time intervals (longest lifetime of 7.7 ns). The slow dynamics of he Dy3+ nitrate salt has been previously pointed out by Duvail et al. who did not observe any salt dissociation in their 2 ns MD simulation of a dilute Dy3+ nitrate aqueous solution. 17

Ln3+ first coordination shell La3+ salts The results of our MD simulations of La3+ salts in water have highlighted a different behaviour of the counterions also in dilute conditions. In particular, the Tf2 N− ion does not form inner-sphere complexes, while a very small amount of triflate and nitrate anions coordinate the La3+ cation in its first coordination shell. In order to shed light into the first shell solvation properties of the La3+ ion


6

Page 7 of 22 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


Table 1: Structural properties calculated from the MD simulations of Ln3+ salt aqueous solutions, together with those of Ln3+ ions in pure water. 40

La3+

system

(1)

(1)

CNOW

rLa−OW

9.5 9.3 9.5 9.5

2.57 2.57 2.57 2.57

pure water TfO− Tf2 N− NO− 3

(1)

(1)

CNO

rLa−O

0.2

2.53

0.04

2.65

(2)

(2)

CNOW

rLa−OW

17.9 16.8 17.1 17.0

4.71 4.71 4.71 4.71

(1)

(1)

(2)

(2)

CNX

rLa−X

CNX

rLa−X

0.2

3.90

0.04

3.80

0.4 0.5 0.8

5.80 7.00 5.20

Dy3+

system

(1)

(1)

CNOW

rDy−OW

9.0 8.8 9.0 8.5

2.39 2.39 2.39 2.39


(1)

(1)

CNO

rDy−O

0.2

2.35

0.5

2.43

(2)

(2)

CNOW

rDy−OW

16.6 15.6 15.9 15.3

4.54 4.53 4.52 4.54

(1)

(1)

(1)

(2)

(2)

CNX

rDy−X

CNX

rDy−X

0.2

3.80

0.5

3.61

0.4 0.5 0.6

5.65 6.90 5.00

(2)

Coordination numbers of the Ln3+ first (CNOW ) and second (CNOW ) hydration shells and position (Å) of the Ln-OW g(r) first (1)

(2)

(rLn−OW ) and second (rLn−OW ) peaks. Coordination numbers of oxygen atoms belonging to the anions that are in the Ln3+ first (1)

(1)

(CNO ) coordination shell and position (Å) of the Ln-O g(r) first (rLn−O ) peak. Coordination numbers of anions that are in the (1)

(1)

(2)

(2)

Ln3+ first (CNX ) and in the second (CNX ) coordination shells and position (Å) of the first (rLn−X ) and second (rLn−X ) Ln-X g(r) peaks (X = S for the TfO− anion and X = N for the Tf2 N− and NO− 3 anions).

we have computed the La-OW and La-HW g(r)’s that are reported in Figure 4 multiplied by the numerical density of the observed atoms ρ (OW and HW refer to the water oxygen and hydrogen atoms, respectively). When treating systems with different densities, the simple comparison of the g(r)’s can be misleading, as already pointed out in the literature. 73,74 The La-OW running coordination numbers are also reported in Figure 4. All of the g(r)ρ’s show very sharp and structured first shell peaks, as strong cation-water interactions take place and the neat separation between the LaOW and the La-HW g(r) first peaks indicates that the water molecules are strongly oriented due to the electrostatic field of the La3+ ion. Note that the Ln-water g(r)ρ’s computed for the three salts are very similar among each other and also to the Ln-

water g(r)ρ’s calculated from our previous MD simulation of La3+ ion in pure water, carried out in the infinite dilution approximation with a single La3+ ion immersed in bulk water and using the same interaction potential parameters adopted here (for additional details see Ref. 40 ). The structural parameters of the La-OW g(r)’s are listed in Table 1: the position of the La-OW g(r) first peak maxima is the same in all systems (2.57 Å), independent of the counterion. Since the cation-anion interactions take place between the lanthanide ion and the oxygen atoms of the anion (O), it is useful to calculate the La-O g(r)’s that are shown in Figures S7 of Supporting Information, while the corresponding structural parameters are listed in Table 1. Inspection of this table shows that the La3+ ion coordinates the oxygen atoms of the ni-


7


0.4

pure water TfO Tf2N NO3-

0.3 0.2 0.1 0.0

2.4

2.6

2.8

3.0

6 4

0.4

0.3 0.1

0

0.0

2.2

2.4

0.5

La-HW

g(r)·ρ

0.2 0.1

2.6

6 4 2 0

2.8

r (Å)

0.6

0.3

0.0

pure water TfO Tf2N NO3-

0.4 0.2

2

8

Dy-OW

0.5

r (Å)

0.5

10

0.6

8

La-OW

N(r) g(r)·ρ

g(r)·ρ

0.5

0.7

N(r)

10

0.6

g(r)·ρ

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 8 of 22

Dy-HW

0.4 0.3 0.2 0.1

3.0

3.2

3.4

3.6

0.0

3.8

2.8

r (Å)

3.0

3.2

3.4

3.6

r (Å)

Figure 4: Ln-OW and Ln-HW radial distribution functions, g(r)’s, multiplied by the numerical density of the observed atoms ρ and Ln-OW running coordination numbers obtained from the MD simulations of 0.1 M Ln(TfO)3 (green), Ln(Tf2 N)3 (blue) and Ln(NO3 )3 (red) aqueous solutions. OW and HW are the water oxygen and hydrogen atoms. The Ln-Water g(r) ρ’s (black) calculated from the MD simulations of Ln3+ ions in pure water (see Ref. 40 ) are also shown. and allows one to identify the first shell symmetry of an ion in solution. 76 For this analysis, we have extracted from the La3+ simulations configurations containing 9-fold coordinated La3+ ions only, and on these trajectories we have calculated the CDFs combining the distribution functions of the La-O distances and O-La-O angles, where O represents oxygen atoms belonging either to water molecules or to anions in the La3+ first coordination shell. The CDFs obtained for the salt solutions are very similar to each other and to the function calculated in pure water (Figure 5): two peaks are found and they are centered at a La-O distance of about 2.57 Å and O-La-O angles of 70◦ and 135◦ . In all the simulations the form of the distributions, with the characteristic L-shape of the low angle peak, is consistent with a tricapped trigonal prism (TTP) geometry of the first shell cluster, in agreement with the results reported in the literature for light Ln3+ ions. 2,5,77 The close similarity

trate and triflate anions at longer and shorter distances, respectively, as compared to the La-OW first shell distance. A longer distance of the nitrate oxygen atoms with respect to the water oxygen has been previously found in an EXAFS investigation of lanthanide nitrate salts in water. 75 Note that the total number of oxygen atoms in the La3+ first coordination shell is always the same (9.5) regardless they belong to water molecules or to the counterion. This finding suggests that the La3+ coordination is mainly driven by electrostatic forces and the first shell structure is preserved also when a small amount of ionic pair is formed, as in the case of the triflate solution. It is now interesting to check whether the geometry of the first hydration shell of oxygen atoms surrounding the La3+ ion is the same in the different solutions. To this end we have used a new tool developed by us that is based on the calculation of combined distribution functions (CDFs)


8

Page 9 of 22 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


nitrate counterions as the increase of the Dy3+ charge density determines the formation of contact pairs in the 0.1 M aqueous solution that are not formed in the case of the larger La3+ ion. The Dy-OW and Dy-HW g(r)ρ’s and Dy-OW running coordination numbers calculated for the three salt solutions are shown in Figure 4, together with the Dy-water g(r)ρ’s calculated in the infinite dilution approximation. 40 Also in this case the Dy-OW distance (2.39 Å) is identical in all of the simulations (see Table 1), and it is shorter as compared to the La-OW one, as a consequence of the lanthanide contraction. 2 The Dy-OW g(r)ρ’s of the Tf2 N− solution is identical to that of pure water while small differences can be observed in the case of the triflate and nitrate anions, as also evident from the Dy-OW running coordination numbers. The Dy-OW first shell hydration number calculated in the Tf2 N− solution is 9.0 as in pure water, and as expected it is slightly lower than that of La3+ ion, in agreement with the decrease of lanthanide hydration numbers going from the first to the last element of the series. 2 As concerns the Dy3+ triflate and nitrate salts, we observe a decrease in the Dy-OW first shell hydration number (8.8 and 8.5 for triflate and nitrate, respectively) but also in this case the sum of the coordination numbers of water molecules and anions is always 9.0 (Table 1). As observed for the La3+ ion, by looking at the structural parameters (Table 1) of the Dy-O g(r)’s with O belonging to the triflate and nitrate anion (Figures S7 of Supporting Information), a longer and shorter Dy-O distance is found, respectively, as compared to the distance between Dy3+ and the oxygem atoms of first shell water molecules. In order to shed light into the tree-dimensional arrangements of the oxygen atoms surrounding the Dy3+ ion we have calculated the CDF combining the Dy-O g(r) and the O-Dy-O angle distribution, where O are oxygen atoms belonging either to water molecules or to anions in the Dy3+ first coordination shell (see Figure S8 of Supporting Information). As observed for the La3+ salts, the form of the CDF, with two peaks centered at a distance of about 2.39 Å and O-Dy-O angles of 70◦ and 135◦ , is consistent with the existence of a TTP symmetry of the Dy3+ first coordination shell. Moreover, the strong similarity of the CDF obtained for the different systems shows that the geometry of the oxy-

Figure 5: Combined distribution functions (CDFs) between the La-O distances and the O-La-O angles evaluated for the 9-fold configurations only extracted from the MD simulations of La(TfO)3 , La(Tf2 N)3 and La(NO3 )3 aqueous solutions. The CDF calculated from the MD simulation of La3+ ion in pure water (see Ref. 40 ) is also shown. of the CDFs indicates that the three-dimensional arrangement of the oxygen atoms in the La3+ first coordination shell is the same in all of the salt solutions, regardless if all oxygen atoms belong to water molecules (pure water, nitrate and Tf2 N− solutions) or a small fraction of oxygen atoms belong to the anion (triflate solution). Dy3+ salts It is well known that the lanthanide ionic radius in water decreases along the series giving rise to an increase of the charge density of the Ln3+ ions with increasing atomic number. 2 This gives rise to different coordination properties of the lanthanide ions and can cause a different behaviour in the presence of counterions along the series. The diverse charge density between La3+ and Dy3+ has no effect on the interaction with the bulky Tf2 N− and triflate species for which identical complexation properties have been found for the two Ln3+ cations. In particular, Tf2 N− does not enter the Dy3+ first hydration shell while a small percentage of the triflate anion forms an inner-shell complex with the Dy3+ cation (see Figure 2). A different result has been obtained in the case of the


9


Page 10 of 22

Figure 6: MD snapshots showing the local environment of anions that are in the La3+ second coordination shell. The water molecules of the La3+ first and second coordination shells are colored blue and yellow, respectively, while those belonging to the anion first hydration shell are orange. In particular, the water molecules that are simultaneously in the first coordination shell of the cation and the anion are colored magenta, while those that belong to the second coordination shell of the cation and to first shell of the anion are cyan. gen atoms surrounding the Dy3+ ion is the same in all of the salt solutions. Note that the higher charge density of the Dy3+ ion as compared with La3+ gives rise to a first shell complexation mainly with the nitrate anion that, in turn, is the one with the highest charge density. However, the presence of counterions in the first or second shell changes neither the Ln-O first shell distance nor the symmetry of the hydration complex formed in solution.

ble 1). The La-X g(r) second peak is due to the formation of solvent-separated ion pairs between cation and anion, and it becomes wider and more − unstructured going from the NO− 3 to the TfO and − finally to the Tf2 N anion. The peculiar forms of the Ln-X g(r) second peaks suggest that they are the superposition of at least two peaks, originating from different structural arrangements. In the triflate and nitrate solutions, two possible configurations of the anions in the second shell of the La3+ ion are found, the former with two oxygen atoms of the anion pointing towards the cation, the latter with only one oxygen atom pointing towards La3+ . As far as the Tf2 N− anion is concerned, the large conformational freedom of this bulky anion results in a broad La-X g(r) second peak that extends over a large La-X distance range. Note that La-X distances below 6 Å are due to configurations where the anions approach the cation via the N atom, and not by means of interactions involving oxygen atoms. To give an idea of some possible anion orientations in the cation second coordination shell, we report in Figure 6 some representative MD snapshots extracted from the La3+ MD simulations. In these examples, one oxygen atom of the anion points towards the La3+ ion and it is coordinated with (i) two water molecules (magenta) belonging to the La3+ first hydration shell (see the

Ln3+ second coordination shell La3+ salts The second hydration shell of the La3+ ion is very similar in the aqueous solutions of the three La3+ salts (Figure S9 of Supporting Information), and resembles the La3+ second hydration shell found in pure water. In particular, the La-OW second shell distance is identical in all of the systems, while a slight decrease of the number of second shell water molecules is found with respect to the one obtained with no counterions in the simulation box (Table 1). This reduction is due to the presence of anions in the La3+ second coordination sphere, as revealed by the second peak of the (2) Ln-X g(r)’s shown in Figure 2 and by the CNX coordination numbers that increase from 0.4 to 0.8 going from triflate to nitrate salt solutions (see Ta-


10

Page 11 of 22 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


Table 2: Sructural and dynamic properties calculated from the MD simulations of Ln3+ salts aqueous solutions, together with those of Ln3+ ions in pure water. 40

La3+

(2)

system


(2)

(2)

Dy3+

(2)

(2)

tWAT

tX

n1HB

n2HB

n3HB

τ1

τ2

tWAT

tX

n1HB

n2HB

n3HB

τ1

τ2

11 13 15 13

25 23 41

3.5 3.4 3.4 3.4

3.5 3.5 3.5

5.7 5.0 6.9

1.0 1.9 1.9 1.9

1.2 0.7 1.5

14 15 15 16

27 22 57

3.5 3.4 3.4 3.4

3.5 3.5 3.5

5.9 5.1 6.5

1.2 2.2 2.2 2.3

1.2 0.7 1.5

(2)

tWAT and tX are the mean residence times (ps) of water molecules and anions in the Ln3+ second coordination shell, respectively. n1HB is the average number of water-water hydrogen bonds per water molecule, n2HB is the average number of hydrogen bonds per solvent molecule by including both water-water and anion-water hydrogen bonds, while n3HB is the average number of anion-water hydrogen bonds per anion. τ1 and τ2 are the lifetimes (ps) of the hydrogen bonds formed between the water molecules belonging to the first solvation shell of the Ln3+ ion and all the other species and formed between the anions and all the water molecules, respectively.

TfO− example), (ii) one water molecule (magenta) being in the La3+ first hydration shell (see the Tf2 N− example). Conversely, in the example reported for the nitrate ion, two anion oxygen atoms point towards the central cation and they interact with two distinct water molecules belonging to the La3+ first hydration shell. Note that only three examples have been shown, one for each anion, but we have found the same three possible configurations for all of the three anions. Moreover, the anions can also be coordinated by one or two water molecules (cyan) being in the La3+ second hydration shell. From a dynamic point of view, in the course of the simulations the water molecules, as well as the anions, frequently enter and leave the Ln3+ second coordination shell. The exchange process rate has been evaluated by computing the mean residence (2) times of both water molecules (tWAT ) and anions

of water molecules are quite similar for all the solutions and comparable to the value calculated for the second hydration shell of La3+ in pure water. These values are also of the same order of magnitude of those calculated in a previous MD investigation of Ln3+ ion in water, where a polarizable force field has been employed. 5 The residence times of anions are longer than those of water molecules, and the calculated values for triflate and Tf2 N− are very similar and shorter than the residence time of nitrate ions. Dy3+ salts As already observed for La3+ salts, the second hydration shell of Dy3+ in the salt solutions resembles the second hydration sphere found in pure water (Figure S9 of Supporting Information), with a decrease of the number of water molecules due to the presence of counterions (see Table 1). The DyX g(r) second peaks calculated for the different salt solutions are very similar to the ones obtained for the La3+ ion, apart from a shift towards shorter distances as a consequence of the lanthanide contraction (Figure 2). 2 The broad peak shape can be ascribed to the presence of several anion orientations in the Dy3+ second coordination shell, and a visual inspection of the Dy3+ MD simulations

(2)

(tX ) in the Ln3+ second coordination sphere (see Table 2), by means of the Impey method. 78 Following previous studies, 67,69,79–85 a t* value of 0.5 ps has been chosen to define a real exchange in the Impey procedure. All of the calculated residence times are of the order of magnitude of picoseconds, thus revealing the flexible nature of the La3+ second solvation shell. The residence times


11


La

Dy 1.0 -

TfO Tf2N NO3-

0.5

0.0 38900

39000

E (eV)

-

0.5

0.3

0.3

0.2

0.2

-1

0.1

TfO Tf2N NO3-

0.0

39100

χ(k)k (Å )

-1

Absorption

Absorption

1.0

χ(k)k (Å )

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 12 of 22

0.0 -0.1 -0.2

53800

53900

E (eV)

54000

0.1 0.0 -0.1 -0.2

2

3

4

5

6

-1

7

8

9 10

2

k (Å )

3

4

5

6

-1

7

8

9 10

k (Å )

Figure 7: Top panels: Ln K-edge X-ray absorption spectra of the 0.1 M Ln(TfO)3 (green), Ln(Tf2 N)3 (blue) and Ln(NO3 )3 (red) aqueous solutions. Bottom panels: corresponding EXAFS signals extracted with a three segmented cubic spline. has shown that the possible configurations of the anions are similar to those found in the La3+ systems. The flexible nature of the Dy3+ second coordination sphere is reflected in the residence times of water molecules and anions in such shell that are found in the picosecond time scale (Table 2). As observed for the La3+ ion, the exchange process of water molecules is faster than the anion one, and, among different anions, the residence time of nitrate is the longest one (57 ps). This value is in agreement with the results obtained in a MD study of Dy(NO3 )3 in water. 17

ever, the counterions have found to have no influence on the number of oxygen atoms coordinating the Ln3+ ions in the first solvation shell. In particular, both the Ln-O first shell distance and the geometry of the hydration complex is the same in all the investigated salt solutions, and they are equal to the ones found in pure water. To verify the reliability of these results we have collected XAS data of all of the investigated solutions. EXAFS is a short-range technique and the structural information achievable is limited to the first coordination shell around the photoabsorber in the case of disordered systems and aqueous solutions. The La3+ and Dy3+ raw K-edge XAS spectra are shown in the top panels of Figure 7. As expected, they are identical independently from the nature of the counterion thus confirming that the Ln3+ first hydration shell structure is not influenced by the counterion. These results are confirmed by the EXAFS spectra extracted with a three segmented

EXAFS results The MD simulations have shown that in dilute aqueous solutions of Ln3+ TfO− , Tf2 N− and NO− 3 salts the cation preferentially interacts with water molecules, and only a very small amount of TfO− and NO− 3 enter the first hydration shell. How-


12

Page 13 of 22 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


for the other systems. In particular, from the top the Ln-OW and Ln-HW theoretical two body signals are shown together with the total χ(k) curve compared with the experimental spectrum and the residual curves. It is important to stress that in this analysis the structural parameters have been kept fixed to those obtained from the MD simulations and only the non-structural parameters have been refined. The agreement between the experimental and theoretical data is quite good in all cases, even if it can be noticed the presence of an oscillation at k values higher than 10 Å−1 in the residual curve that is known to be due to a wrong shape of the radial distribution around the photoabsorber atom. 40 A perfect agreement is obtained when all of the structural parameters are refined as previously shown in the literature. 13 The lower panels of Figure 8 show the non-phase-shift corrected Fourier Transforms of the EXAFS experimental data compared with the theoretical signals and a quite good agreement is observed also in the reciprocal space. Note that the nice agreement between the theoretical and experimental data proves that the Lennard-Jones interaction potentials developed for the Ln3+ ions in pure water 40 are able to properly reproduce the solvation properties of the La3+ and Dy3+ ions also in aqueous solutions containing different counterions.

Figure 8: Upper panels: comparison of the EXAFS experimental data (dotted red line) of the 0.1 M Ln(TfO)3 aqueous solutions with the theoretical spectrum (solid blue line) calculated from the MD Ln-OW and Ln-HW g(r)’s. The residual curves are plotted in green. Lower panels: non-phaseshift corrected Fourier Transforms of the EXAFS experimental data (dotted red line) and of the theoretical signals (solid blue line). cubic spline, shown in the bottom panels of Figure 7. A definitive proof of the accuracy of the force fields used in the MD simulations has been gained by carrying out a quantitative analysis of the EXAFS spectra starting from the structural results obtained from the simulations. In particular, starting from the MD Ln-OW and Ln-HW g(r)’s we calculated for each solution a total χ(k) theoretical signal using eq. 1, and we compared it with the experimental EXAFS signal. Minimization procedures have been performed in the range k= 2.4-14.0 Å−1 . Note that the structural parameters have been kept fixed during the fitting procedures and in such way the first solvation shell structure obtained from the simulations can be directly compared with experimental data. The results of these analyses for the La3+ and Dy3+ triflate solutions are shown in Figure 8, as an example, and the same results have been obtained

Hydration properties of anions Our results have shown that in lanthanide triflate, Tf2 N− and nitrate solutions very little cation-anion first shell association occurs. On the other hand, the anions are solvated by water molecules, as evidenced by the anion-water g(r)’s calculated from our MD simulations and reported in Figure 9. In particular, the top and bottom panels of the figure show the O (anion)-HW and O (anion)-OW g(r)’s, respectively. The structural parameters of the anion-water g(r)’s are listed in Table S1 of Supporting Information. As evident from Figure 9, the interactions between the anions and the water molecules are formed via the water hydrogen atoms. In particular, in all cases the O (anion)HW (water) g(r)’s show a first distinct peak at shorter distances as compared to the O(anion)-OW g(r) first maximum, and a second and less defined double peak at longer distances, indicating


13


Page 14 of 22

Interestingly, the water molecules interacting with the Tf2 N− anion are found along the anion S-O direction, while in in the case of the TfO− and NO− 3 ions, the hydrogen and oxygen atoms are found in torus-shaped distributions around the S-O and NO vector directions, respectively.

Water-water and anion-water hydrogen bonds In all of the aqueous solutions containing the Ln3+ ions and triflate, Tf2 N− and nitrate counterions a network of hydrogen bonds is formed among the water molecules and also between the water molecules and the anions. To carry out the analysis of the water-water hydrogen bonds we adopted a configurational criterion where two water molecules are hydrogen bonded only if their interoxygen distance is shorter than 3.5 Å and simultaneously hydrogen-oxygen distance is shorter than 2.45 Å and the oxygen(acceptor)oxygen(donor)-hydrogen(donor) angle is lower than 30◦ . 89 The average number of water-water hydrogen bonds per solvent molecule (n1HB ) calculated from our MD simulations is identical in all of the systems containing either La3+ or Dy3+ ions and the different counterions (see Table 2). The value is slightly lower to that calculated in the simulations of Ln3+ in pure water, but if we calculate the average number of hydrogen bonds per solvent molecule by including both water-water and anion-water hydrogen bonds (n2HB ) we obtain exactly the same results in the solutions with and without counterions. This result indicates that the water molecules tend to form always the same number of hydrogen bonds with the surrounding species, namely with other water molecules or triflate, Tf2 N− and nitrate anions. Conversely, the average number of anion-water hydrogen bonds per anion (n3HB ) are rather similar in the solutions containing either La3+ or Dy3+ , while depending on the nature of the anion and increasing in the order Tf2 N− < triflate < nitrate (Table 2). Note that in order to define an anion-water hydrogen bond we have used as cutoff distances the first minima of the O(anion)-HW (water) and O(anion)-OW (water) g(r)’s listed in Table S1 of Supporting Information and the same angle value cutoff used for water (O(anion)-OW-HW angle lower than 30◦ ). 89

Figure 9: Anion-water radial distribution functions, g(r)’s, obtained from the MD simulations of 0.1 M La(TfO)3 (green), La(Tf2 N)3 (blue) and La(NO3 )3 (red) aqueous solutions: O(anion)-HW g(r)’s (top panel) and O(anion)-OW g(r)’s (bottom panel), where OW and HW are the oxygen and hydrogen atoms of the water molecules, respectively. that anion-water hydrogen bonds are found in all of the aqueous solutions. The strongest anionwater interactions are formed with the nitrate ions, as evidenced from higher and more defined anionwater g(r) first peaks. Note that the structural parameters of the nitrate-water g(r)’s obtained from our MD simulation are comparable with calculated 86,87 and experimental 88 data of NO− 3 in water. Additional insight into anion-water interactions can be gained by looking at the spatial distribution functions (SDFs) of water molecules surrounding the anions (Figure 10). Note that these functions have been calculated in an internal reference system integral with the nitrate plane for the nitrate ion and integral with one sulfonyl group for the triflate and Tf2 N− anions. The SDFs of water hydrogen and oxygen atoms are shown as iceblue and magenta surfaces, respectively. For all the three anions, the water-anion association is mainly driven by interactions of water hydrogen atoms with the anion oxygen atoms, remarking the ability of the anions to form hydrogen bonds with water.


14

Page 15 of 22 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


Figure 10: Spatial distribution functions (SDFs) of hydrogen (iceblue) and oxygen (magenta) atoms of − − water molecules around the TfO− , Tf2 N− and NO− 3 anions. Note that in the case of TfO and Tf2 N the atoms of one anion sulfonyl group have been used to build the internal reference system for the SDF calculation and the rest of the molecular structure has been shown for clarity, only. One of the fundamental dynamical processes occurring in aqueous solution is the formation and breaking of hydrogen bonds which generally occurs in the subpicosecond time scale in pure water. The hydrogen bond dynamics can be investigated by constructing a hydrogen bond population function h(t) which is unity when a tagged pair of water molecules is hydrogen bonded at time t according to an adopted definition, and zero otherwise. To investigate the hydrogen bond breaking dynamics, we calculated the time correlation function: 89 SHB (t) = hh(0)H(t)i/hhi

of the hydrogen bonds formed between the water molecules belonging to the first solvation shell of the Ln3+ ion and all the other species, i.e. water molecules and anions (τ1 ). The τ1 values calculated from our MD simulations are listed in Table 2. For a given Ln3+ ion, the τ1 value obtained for the system containing only the Ln3+ ion increases as compared to the value of bulk water, and it increases significantly in the aqueous solutions containing also the triflate, Tf2 N− and nitrate counterions. Therefore, the hydrogen bonded pairs in the ionic solutions are more strongly held as compared to bulk water molecules. This result is in agreement with previous findings of two ab initio MD studies of Atta-Fynn et al., 21,22 where counterions such as Cl− and Br− where found to slow down the hydrogen bond dynamics in the first solvation shell of Th4+ and Cm3+ . Moreover, the τ1 values obtained for the triflate, Tf2 N− and nitrate solution of a given Ln3+ ion are almost the same, indicating that the different counterions have the same impact on the hydrogen bond lifetimes. On the other hand, if we compare the results of La3+ and Dy3+ systems, we see that the hydrogen bond lifetimes in the Dy3+ solutions are higher than those of the La3+ ones, and this stronger slow-down effect could be due to the higher charge density of the Dy3+ ion as compared to La3+ . Finally, we have computed the lifetimes of the

(2)

where H(t) = 1 if the tagged pair of water molecules, for which h(0) is calculated, remains continuously hydrogen bonded till time t and zero otherwise, h...i denotes an average over all pairs. SHB describes the probability that an initially hydrogen bonded pair remains bonded at all times up to t. The associated relaxation time is obtained as the time integrals of SHB and can be interpreted as the average lifetime of a hydrogen bond. The average water-water hydrogen bond lifetime calculated from our MD simulations for bulk water molecules is 0.7 ps. Here, it is interesting to investigate the influence of the Ln3+ ion on the hydrogen bond dynamics by calculating the lifetimes


15


hydrogen bonds formed between the anions and all the water molecules (τ2 ) that are listed in Table 2. The τ2 values show a strong dependence on the counterion nature, as expected, with the longest and shortest lifetime found for nitrate and Tf2 N− , respectively, reflecting the different strenght of the hydrogen bonds formed by the different anions. Note that the τ2 values are identical in the solutions containing either La3+ or Dy3+ .

Page 16 of 22

previous NMR or Raman scattering spectroscopy studies which concluded that triflate does not form ion pairs with the Ln3+ ions, 18,19,42 Conversely, our results show that a very small amount of triflate enters the La3+ and Dy3+ first coordination sphere (Ln-triflate coordination number of 0.2). It is important to stress that the combined use of the EXAFS technique and MD simulations allowed us to provides direct structural information on Ln3+ first solvation shell in solution. As concerns the choice of the simulation method, among different theoretical approaches classical MD is the most proper choice to afford the long simulation time that is required to accurately sample the phase space visited by these systems. Indeed the ion pair dynamics can be very slow, as shown for example in the case of the Dy3+ nitrate salt in which the anion exchange processes can be observed only in long simulation time length (tens of nanoseconds). The use of ab initio simulations to predict the complexing ability of anions could therefore result in erroneous outcomes due to the limited simulation times (tens of picoseconds) achievable using these computationally demanding methods. Beside the investigation of the Ln3+ solvation structure and dynamics, we have also studied the hydration properties of the anions and the waterwater and water-anion hydrogen bond dynamics, showing that the hydrogen bonded pairs are more strongly held in the solutions as compared to pure water, with the hydrogen bond lifetimes being higher in the Dy3+ sytems as compared to the La3+ ones. In conclusion, the structural and dynamic properties of La3+ and Dy3+ triflate, Tf2 N− and nitrate aqueous solutions are dominated by electrostatic effects in which the differences found between La3+ and Dy3+ are mainly due to the higher charge density of the heavier ion.

Discussion and Conclusions In this work we have carried out a combined MD and EXAFS investigation of dilute aqueous solutions of Ln3+ (La3+ and Dy3+ ) salts with the triflate, nitrate and Tf2 N− anions. The results of our MD simulations highlighted the different complexing ability of the three counterions: Tf2 N− does not form inner-sphere complexes while a small amount of triflate coordinates both the La3+ and Dy3+ cations in their first solvation shell. On the other hand, the nitrate ion has a different behaviour in La3+ and Dy3+ salt solutions, as it is almost absent in the La3+ first solvation shell, while forming contact ion pairs with Dy3+ . In all of the investigated systems, the La3+ and Dy3+ cations preferentially interact with the water molecules and the counterions have no influence on the geometry of the Ln3+ solvation complex formed in solution. The total number of oxygen atoms coordinating the Ln3+ cations in their first solvation shell is the same in all of the solutions, regardless they belong to water molecules or to the counterion, suggesting that the cation coordination is mainly driven by electrostatic forces. These theoretical findings have been confirmed by comparison with the Ln K-edge XAS experimental data. Moreover, a quantitative analysis of the EXAFS spectra of the systems have been carried out starting from the structural results obtained from the simulations. The very good agreement found between theory and experiment has allowed us to definitely assess the reliability of the force field employed in the MD simulations. It is important to stress that determining the complexing ability of different counterions in solution is in general a very difficult task. As concerns the triflate ion our results are at variance with

Acknowledgement This work was supported by the University of Rome “La Sapienza” (Progetto ateneo 2015 n. C26H159F5B and C26N159PNB) and by the CINECA supercomputing center through the grant IscrC_DESTINIS (n. HP10CZTDIS).


16

Page 17 of 22 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


References

Dynamics Study. Inorg. Chem. 2009, 48, 10239–10248.

(1) Kowall, T.; Foglia, F.; Helm, L.; Merbach, A. E. Molecular Dynamics Simulations Study of Lanthanides Ions Ln3+ in Aqueous Solutions Including Water Polarization. Change in Coordination Number from 9 to 8 along the Series. J. Am. Chem. Soc. 1995, 117, 3790–3799.

(9) D’Angelo, P.; De Panfilis, S.; Filipponi, A.; Persson, I. High-Energy X-Ray Absorption Spectroscopy: A New Tool for Structural Investigations of Lanthanoids and Third-Row Transition Elements". Chem. Eur. J. 2008, 14, 3045–3055.

(2) D’Angelo, P.; Spezia, R. Hydration of Lanthanoids(III) and Actinoids(III): An Experimental/Theoretical Saga. Chem.- Eur. J. 2012, 18, 11162–11178.

(10) Persson, I.; D’Angelo, P.; De Panfilis, S.; Sandstrom, M.; Eriksson, L. Hydration of Lanthanoid(III) Ions in Aqueous Solution and Crystalline Hydrates Studied by EXAFS Spectroscopy and Crystallography: the Myth of the "Gadolinium Break". Chem. Eur. J. 2008, 14, 3056–3066.

(3) Clavaguéra, C.; Pollet, R.; Soudan, J. M.; Brenner, V.; Dognon, J. P. Molecular Dynamics Study of the Hydration of Lanthanum(III) and Europium(III) Including Many-Body Effects. J. Phys. Chem. B 2005, 109, 7614–7616.

(11) Cossy, C.; Helm, L.; Powell, D. H.; Merbach, A. E. A Change in Coordination Number from Nine to Eight Along the Lanthanide(III) Aqua Ion Series in Solution: a Neutron Diffraction Study. New J. Chem. 1995, 19, 27–35.

(4) Floris, F. M.; Tani, A. A Study of Aqueous Solutions of Lanthanide Ions by Molecular Dynamics Simulation with Ab Initio Effective Pair Potentials. J. Chem. Phys. 2001, 115, 4750–4765.

(12) D’Angelo, P.; Zitolo, A.; Migliorati, V.; Persson, I. Analysis of the Detailed Configuration of Hydrated Lanthanoid(III) Ions in Aqueous Solution and Crystalline Salts by Using Kand L3 -Edge XANES Spectroscopy. Chem.Eur. J. 2010, 16, 684–692.

(5) Duvail, M.; Vitorge, P.; Spezia, R. Building a Polarizable Pair Interaction Potential for Lanthanoids(III) in Liquid Water: a Molecular Dynamics Study of Structure and Dynamics of the Whole Series. J. Chem. Phys. 2009, 130, 104501.

(13) D’Angelo, P.; Zitolo, A.; Migliorati, V.; Chillemi, G.; Duvail, M.; Vitorge, P.; Abadie, S.; Spezia, R. Revised Ionic Radii of Lanthanoid(III) Ions in Aqueous Solution. Inorg. Chem. 2011, 50, 4572–4579.

(6) Villa, A.; Hess, B.; Saint-Martin, H. Dynamics and Structure of Ln(III)-Aqua Ions: A Comparative Molecular Dynamics Study Using ab Initio Based Flexible and Polarizable Model Potentials. J. Phys. Chem. B 2009, 113, 7270–7281.

(14) Annis, B. K.; Hahn, R. L.; Narten, A. H. Hydration of the Dy3+ Ion in Dysprosium Chloride Solutions Determined by Neutron Diffraction). J. Chem. Phys. 1985, 82, 2086– 2091.

(7) Duvail, M.; Souaille, M.; Spezia, R.; Cartailler, T.; Vitorge, P. Pair Interaction Potentials with Explicit Polarization for Molecular Dynamics Simulations of La3+ in Bulk Water. J. Chem. Phys. 2007, 127, 034503.

(15) Näslund, J.; Lindqvist-Reis, P.; Persson, I.; Sandström, M. Steric Effects Control the Structure of the Solvated Lanthanum(III) Ion in Aqueous, Dimethyl Sulfoxide, and N,N-Dimethylpropyleneurea Solution. An EXAFS and Large-Angle X-ray Scattering Study. Inorg. Chem. 2000, 39, 4006–4011.

(8) D’Angelo, P.; Zitolo, A.; Migliorati, V.; Mancini, G.; Persson, I.; Chillemi, G. Structural Investigation of Lanthanoid Coordination: a Combined XANES and Molecular


17


(16) Lindqvist-Reis, P.; Muõz Pàez, A.; DìazMoreno, S.; Pattanaik, S.; Persson, I.; Sandström, M. The Structure of the Hydrated Gallium(III), Indium(III), and Chromium(III) Ions in Aqueous Solution. A Large Angle X-ray Scattering and EXAFS Study. Inorg. Chem. 1998, 37, 6675–6683.

Page 18 of 22

atures: Thermodynamics and Coordination Modes. Inorg. Chem. 2009, 48, 964–970. (25) Dobler, M.; Guilbaud, P.; Dedieu, A.; Wipff, G. Interaction of Trivalent Lanthanide Cations with Nitrate Anions: a Quantum Chemical Investigation of Monodentate/Bidentate Binding Modes. New J. Chem. 2001, 25, 1458–1465.

(17) Duvail, M.; Ruas, A.; Venault, L.; Moisy, P.; Guilbaud, P. Molecular Dynamics Studies of Concentrated Binary Aqueous Solutions of Lanthanide Salts: Structures and Exchange Dynamics. Inorg. Chem. 2010, 49, 519.

(26) Allen, P. G.; Bucher, J. J.; Shuh, D. K.; Edelstein, N. M.; Craig, I. Coordination Chemistry of Trivalent Lanthanide and Actinide Ions in Dilute and Concentrated Chloride Solutions. Inorg. Chem. 2000, 39, 595–601.

(18) van Loon, A. M.; van Bekkum, H.; Peters, J. A. Structures of Dysprosium(III) Triflates in Water, Methanol, and 2-Propanol As Studied by 17 O and 19 F NMR Spectroscopy. Inorg. Chem. 1999, 38, 3080–3084. (19) Rudolph, W. W.; Irmer, G. Hydration and Ion Pair Formation in Common Aqueous La(III) Salt Solutions - a Raman Scattering and DFT Study. Dalton Trans. 2015, 44, 295–305.

(27) Ruas, A.; Guilbaud, P.; Auwer, C. D.; Moulin, C.; Simonin, J.-P.; Turq, P.; Moisy, P. Experimental and Molecular Dynamics Studies of Dysprosium(III) Salt Solutions for a Better Representation of the Microscopic Features Used within the Binding Mean Spherical Approximation Theory. J. Phys. Chem. A 2006, 110, 11770–11779.

(20) Rudolph, W. W.; Irmer, G. Raman Spectroscopic Characterization of Light Rare Earth Ions: La3+ , Ce3+ , Pr3+ , Nd3+ , and Sm3+ , - Hydration and Ion Pair Formation. Dalton Trans. 2017, 46, 4235–4244.

(28) Soderholm, L.; Skanthakumar, S.; Wilson, R. E. Structures and Energetics of Erbium Chloride Complexes in Aqueous Solution. J. Phys. Chem. A 2009, 113, 6391– 6397.

(21) Atta-Fynn, R.; Bylaska, E. J.; de Jong, W. A. Importance of Counteranions on the Hydration Structure of the Curium Ion. J. Phys. Chem. Lett. 2013, 4, 2166–2170.

(29) Beuchat, C.; Hagberg, D.; Spezia, R.; Gagliardi, L. Hydration of Lanthanide Chloride Salts: A Quantum Chemical and Classical Molecular Dynamics Simulation Study. J. Phys. Chem. B 2010, 114, 15590–15597.

(22) Atta-Fynn, R.; Bylaska, E. J.; de Jong, W. A. Strengthening of the Coordination Shell by Counter Ions in Aqueous Th4+ Solutions. J. Phys. Chem. A 2016, 120, 10216–10222.

(30) Binnemans, K. Lanthanides and Actinides in Ionic Liquids. Chem. Rev. 2007, 107, 2592– 2614. (31) Migliorati, V.; Ballirano, P.; Gontrani, L.; Caminiti, R. Crystal Polymorphism of Hexylammonium Chloride and Structural Properties of Its Mixtures with Water. J. Phys. Chem. B 2012, 116, 2104–2113.

(23) Bonal, C.; Morel, J.-P.; Morel-Desrosiers, N. Interactions Between Lanthanide Cations and Nitrate Anions in Water. Part 1.-Effect of the Ionic Strength on the Gibbs Energy, Enthalpy and Entropy of Complexation of the Neodymium Cation. J. Chem. Soc., Faraday Trans. 1996, 92, 4957–4963.

(32) Billard, I.; Ouadi, A.; Gaillard, C. LiquidLiquid Extraction of Actinides, Lanthanides, and Fission Products by Use of Ionic Liquids: from Discovery to Understanding. Anal. Bioanal. Chem. 2011, 400, 1555–1566.

(24) Rao, L.; Tian, G. Complexation of Lanthanides with Nitrate at Variable Temper-


18

Page 19 of 22 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


(33) Migliorati, V.; Ballirano, P.; Gontrani, L.; Materazzi, S.; Ceccacci, F.; Caminiti, R. A Combined Theoretical and Experimental Study of Solid Octyl and Decylammonium Chlorides and of Their Aqueous Solutions. J. Phys. Chem. B 2013, 117, 7806–7818.

(42) Djanashvili, K.; Platas-Iglesias, C.; Peters, J. A. The Structure of the Lanthanide Aquo Ions in Solution as Studied by 17 O NMR spectroscopy and DFT calculations. Dalton Trans. 2008, 602–607. (43) Serva, A.; Migliorati, V.; Lapi, A.; Aquilanti, G.; Arcovito, A.; D’Angelo, P. Structural Properties of Geminal Dicationic Ionic Liquid/Water Mixtures: a Theoretical and Experimental Insight. Phys. Chem. Chem. Phys. 2016, 18, 16544–16554.

(34) Plechkova, N. V.; Seddon, K. R. Applications of Ionic Liquids in the Chemical Industry. Chem. Soc. Rev. 2008, 37, 123–150. (35) Nakashima, K.; Kubota, F.; Maruyama, T.; Goto, M. Feasibility of Ionic Liquids as Alternative Separation Media for Industrial Solvent Extraction Processes. Ind. Eng. Chem. Res. 2005, 44, 4368–4372.

(44) Fulton, J. L.; Kathmann, S. M.; Schenter, G. K.; Balasubramanian, M. Hydrated Structure of Ag(I) Ion from SymmetryDependent, K- and L-Edge XAFS Multiple Scattering and Molecular Dynamics Simulations. J. Phys. Chem. A 2009, 113, 13976– 13984.

(36) Sun, X.; Ji, Y.; Guo, L.; Chen, J.; Li, D. A Novel Ammonium Ionic Liquid Based Extraction Strategy for Separating Scandium from Yttrium and Lanthanides. Sep. Purif. Technol. 2011, 81, 25–30.

(45) Dang, L. X.; Schenter, G. K.; Glezakou, V.A.; Fulton, J. L. Molecular Simulation Analysis and X-ray Absorption Measurement of Ca2+ , K+ and Cl− Ions in Solution. J. Phys. Chem. B 2006, 110, 23644–23654.

(37) Choppin, G. R.; Peterman, D. R. Applications of Lanthanide Luminescence Spectroscopy to Solution Studies of Coordination Chemistry. Coord. Chem. Rev. 1998, 174, 283–299.

(46) Migliorati, V.; Filipponi, A.; Di Cicco, A.; De Panfilis, S.; D’Angelo, P. Structure of Water in Zn2+ Aqueous Solutions from Ambient Conditions up to the Gigapascal Pressure Range: A XANES and Molecular Dynamics Study. Inorg. Chem. 2017, 56, 14013–14022.

(38) Duvail, M.; Vitorge, P.; Spezia, R. Temperature Influence on Lanthanoids (III) Hydration from Molecular Dynamics Simulations. Chem. Phys. Lett. 2010, 498, 90–96. (39) Duvail, M.; Spezia, R.; Vitorge, P. A Dynamic Model to Explain Hydration Behaviour along the Lanthanide Series. ChemPhysChem 2008, 9, 693–696.

(47) Merkling, P. J.; Muñoz Páez, A.; Sánchez Marcos, E. Exploring the Capabilities of X-ray Absorption Spectroscopy for Determining the Structure of Electrolyte Solutions: Computed Spectra for Cr3+ or Rh3+ in Water Based on Molecular Dynamics. J. Am. Chem. Soc. 2002, 124, 10911–10920.

(40) Migliorati, V.; Serva, A.; Terenzio, F. M.; D’Angelo, P. Development of Lennard-Jones and Buckingham Potentials for Lanthanoid Ions in Water. Inorg. Chem. 2017, 56, 6214– 6224.

(48) Migliorati, V.; Chillemi, G.; D’Angelo, P. On the Solvation of the Zn2+ Ion in Methanol: A Combined Quantum Mechanics, Molecular Dynamics, and EXAFS Approach. Inorg. Chem. 2011, 50, 8509–8515.

(41) Serva, A.; Migliorati, V.; Spezia, R.; D’Angelo, P. How Does CeIII Nitrate Dissolve in a Protic Ionic Liquid? A Combined Molecular Dynamics and EXAFS Study. Chem.-Eur. J. 2017, 23, 8424–8433.

(49) Merkling, P. J.; Martínez, J. M.;


19

Muñoz Páez, A.; Pappalardo, R. R.;


Sánchez Marcos, E. Molecular-DynamicsBased Investigation of Scattering Path Contributions to the EXAFS Spectrum: the Cr3+ Aqueous Solution Case. Phys. Rev. B 2001, 64, 092201.

Page 20 of 22

(56) Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. The Missing Term in Effective Pair Potentials. J. Phys. Chem. 1987, 91, 6269–6271. (57) Canongia Lopes, J. N.; Deschamps, J.; Pádua, A. A. H. Modeling Ionic Liquids Using a Systematic All-Atom Force Field. J. Phys. Chem. B 2004, 108, 2038–2047.

(50) D’Angelo, P.; Migliorati, V.; Mancini, G.; Chillemi, G. A Coupled Molecular Dynamics and XANES Data Analysis Investigation of Aqueous Cadmium(II). J. Phys. Chem. A 2008, 112, 11833–11841.

(58) Canongia Lopes, J. N.; Pádua, A. A. H. Molecular Force Field for Ionic Liquids Composed of Triflate or Bistriflylimide Anions. J. Phys. Chem. B 2004, 108, 16893– 16898.

(51) Carrera, F.; Torrico, F.; Richens, D. T.; Muñoz Páez, A.; Martínez, J. M.; Pappalardo, R. R.; Marcos, E. S. Combined Experimental and Theoretical Approach to the Study of Structure and Dynamics of the Most Inert Aqua Ion [Ir(H2 O)6 ]3+ in Aqueous Solution. J. Phys. Chem. B 2007, 111, 8223– 8233.

(59) Nosé, S. A Unified Formulation of the Constant Temperature Molecular Dynamics Methods. J. Chem. Phys. 1984, 81, 511–519. (60) Evans, D. J.; Holian, B. L. The Nosé-Hoover Thermostat. J. Chem. Phys. 1985, 83, 4069– 4074.

(52) Migliorati, V.; Serva, A.; Aquilanti, G.; Olivi, L.; Pascarelli, S.; Mathon, O.; D’Angelo, P. Combining EXAFS Spectroscopy and Molecular Ddynamics Simulations to Understand the Structural and Dynamic Properties of an Imidazolium Iodide Ionic Liquid. Phys. Chem. Chem. Phys. 2015, 17, 2464–2474.

(61) Essmann, U.; Perera, L.; Berkowitz, M. L.; Darden, T.; Lee, H.; Pedersen, L. G. A Smooth Particle Mesh Ewald Method. J. Chem. Phys. 1995, 103, 8577–8593. (62) Ferry, A.; Furlani, M.; Franke, A.; Jacobsson, P.; Mellander, B.-E. Spectroscopic Studies of Luminescent and Ionically Conducting Eu[N(CF3 SO2 )2 ]3 -PPG4000 Complexes. J. Chem. Phys. 1998, 109, 2921–2928.

(53) Migliorati, V.; Zitolo, A.; D’Angelo, P. Using a Combined Theoretical and Experimental Approach to Understand the Structure and Dynamics of Imidazolium Based Ionic Liquids/Water Mixtures. 1. MD Simulations. J. Phys. Chem. B 2013, 117, 12505–12515.

(63) Filipponi, A.; Di Cicco, A. X-ray-absorption Spectroscopy and n-body Distribution Functions in Condensed Matter. II. Data Analysis and Applications. Phys. Rev. B 1995, 52, 15135–15149.

(54) D’Angelo, P.; Zitolo, A.; Aquilanti, G.; Migliorati, V. Using a Combined Theoretical and Experimental Approach to Understand the Structure and Dynamics of ImidazoliumBased Ionic Liquids/Water Mixtures. 2. EXAFS Spectroscopy. J. Phys. Chem. B 2013, 117, 12516–12524.

(64) Chillemi, G.; Barone, V.; D’Angelo, P.; Mancini, G.; Persson, I.; Sanna, N. Computational Evidence for a Variable First Shell Coordination of the Cadmium(II) Ion in Aqueous Solution. J. Phys. Chem. B 2005, 109, 9186–9193.

(55) 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.

(65) D’Angelo, P.; Migliorati, V.; Mancini, G.; Barone, V.; Chillemi, G. Integrated Experimental and Theoretical Approach for the


20

Page 21 of 22 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


Structural Characterization of Hg2+ Aqueous Solutions. J. Chem. Phys. 2008, 128, 084502.

(73) D’Angelo, P.; Serva, A.; Aquilanti, G.; Pascarelli, S.; Migliorati, V. Structural Properties and Aggregation Behavior of 1-Hexyl-3Methylimidazolium Iodide in Aqueous Solutions. J. Phys. Chem. B 2015, 119, 14515– 14526, PMID: 26479331.

(66) D’Angelo, P.; Pavel, N. V.; Roccatano, D.; Nolting, H.-F. Multielectron Excitations at the L Edges of Barium in Aqueous Solution. Phys. Rev. B 1996, 54, 12129–12138.

(74) Migliorati, V.; Serva, A.; Aquilanti, G.; Pascarelli, S.; D’Angelo, P. Local Order and Long Range Correlations in Imidazolium Halide Ionic Liquids: a Combined Molecular Dynamics and XAS Study. Phys. Chem. Chem. Phys. 2015, 17, 16443–16453.

(67) 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.

(75) Yaita, T.; Narita, H.; Suzuki, S.; Tachimori, S.; Motohashi, H.; Shiwaku, H. Structural Study of Lanthanides(III) in Aqueous Nitrate and Chloride Solutions by EXAFS. J. Radioanal. Nucl. Chem. 1999, 239, 371–375.

(68) Pham, V.; Tavernelli, I.; Milne, C.; van der Veen, R.; D’Angelo, P.; Bressler, C.; Chergui, M. The Solvent Shell Structure of Aqueous Iodide: X-Ray Absorption Spectroscopy and Classical, Hybrid QM/MM and Full Quantum Molecular Dynamics Simulations. Chem. Phys. 2010, 371, 24 – 29.

(76) Sessa, F.; D’Angelo, P.; Migliorati, V. Combined Distribution Functions: a Powerful Tool to Identify Cation Coordination Geometries in Liquid Systems. Chem. Phys. Lett. 2018, 691, 437–443.

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

(77) Kowall, T.; Foglia, F.; Helm, L.; Merbach, A. E. Molecular Dynamics Simulations Study of Lanthanides Ions Ln3+ in Aqueous Solutions. Analysis of the Structure of the First Hydration Shell and of the Origin of Symmetry Fluctuations. J. Phys. Chem. 1995, 99, 13078–13087.

(70) Bonal, C.; Morel, J.-P.; Morel-Desrosiers, N. Interactions Between Lanthanide Cations and Nitrate Anions in Water Part 2: Microcalorimetric Determination of the Gibbs Energies, Enthalpies and Entropies of Complexation of Y3+ and Trivalent Lanthanide Cations. J. Chem. Soc., Faraday Trans. 1998, 94, 1431–1436.

(78) Impey, R. W.; Madden, P. A.; McDonald, I. R. Hydration and Mobility of Ions in Solution. J. Chem. Phys. 1983, 87. (79) Migliorati, V.; D’Angelo, P. Unraveling the Sc3+ Hydration Geometry: The Strange Case of the Far-Coordinated Water Molecule. Inorg. Chem. 2016, 55, 6703–6711.

(71) Andersson, S.; Eberhardt, K.; Ekberg, C.; Liljenzin, J.-O.; Nilsson, M.; Skarnemark, G. Determination of Stability Constants of Lanthanide Nitrate Complex Formation Using a Solvent Extraction Technique. Radiochim. Acta 2006, 94, 469–474.

(80) Migliorati, V.; Mancini, G.; Tatoli, S.; Zitolo, A.; Filipponi, A.; De Panfilis, S.; Di Cicco, A.; D’Angelo, P. Hydration Properties of the Zn2+ Ion in Water at High Pressure. Inorg. Chem. 2013, 52, 1141–1150.

(72) Lees, A. M.; Kresinski, R. A.; Platt, A. W. Preparation, Structure, Solid State and Gas Phase Stability of the Mixed Neodymium Nitrate-Chloride Complex NdCl(NO3 )2 [(MeO)2 PO]2 C(OH)tBu2 . Inorg. Chim. Acta 2006, 359, 1329–1334.

(81) Hofer, T. S.; Tran, H. T.; Schwenk, C. F.; Rode, B. M. Characterization of Dynamics


21


Table of Contents

and Reactivities of Solvated Ions by Ab Initio Simulations. J. Comput. Chem. 2004, 25, 211–217. (82) Migliorati, V.; D’Angelo, P. A Quantum Mechanics, Molecular Dynamics and EXAFS Investigation into the Hg2+ Ion Solvation Properties in Methanol Solution. RSC Adv. 2013, 3, 21118–21126. (83) D’Angelo, P.; Migliorati, V.; Sessa, F.; Mancini, G.; Persson, I. XANES Reveals the Flexible Nature of Hydrated Strontium in Aqueous Solution. J. Phys. Chem. B 2016, 120, 4114–4124. (84) Spezia, R.; Migliorati, V.; D’Angelo, P. On the Development of Polarizable and Lennard-Jones Force Fields to Study Hydration Structure and Dynamics of Actinide(III) Ions Based on Effective Ionic Radii. J. Chem. Phys. 2017, 147, 161707. (85) Sessa, F.; D’Angelo, P.; Guidoni, L.; Migliorati, V. Hidden Hydration Structure of Halide Ions: an Insight into the Importance of Lone Pairs. J. Phys. Chem. B 2015, 119, 15729– 15737. (86) Dang, L. X.; Chang, T.-M.; Roeselova, M.; Garrett, B. C.; Tobias, D. J. On NO− 3 -H2 O Interactions in Aqueous Solutions and at Interfaces. J. Chem. Phys. 2006, 124, 066101. (87) Thøgersen, J.; Réhault, J.; Odelius, M.; Ogden, T.; Jena, N. K.; Jensen, S. J. K.; Keiding, S. R.; Helbing, J. Hydration Dynamics of Aqueous Nitrate. J. Phys. Chem. B 2013, 117, 3376–3388. (88) Caminiti, R.; Licheri, G.; Piccaluga, G.; Pinna, G. On NO− 3 -H2 O Interactions in Aqueous Solutions. J. Chem. Phys. 1978, 68, 1967–1970. (89) Chandra, A. Effects of Ion Atmosphere on Hydrogen-Bond Dynamics in Aqueous Electrolyte Solutions. Phys. Rev. Lett. 85, 768– 771.


22

Page 22 of 22

Influence of Counterions on the Hydration Structure of Lanthanide Ions

Recommend Documents