Using a Combined Theoretical and Experimental Approach to

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Using A Combined Theoretical and Experimental Approach to Understand the Structure and Dynamics of Imidazolium based Ionic Liquids/Water Mixtures. 2. EXAFS Spectroscopy Paola D'Angelo, Andrea Zitolo, Giuliana Aquilanti, and Valentina Migliorati J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp404868a • Publication Date (Web): 10 Sep 2013 Downloaded from http://pubs.acs.org on September 16, 2013

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The Journal of Physical Chemistry

Using a Combined Theoretical and Experimental Approach to Understand the Structure and Dynamics of Imidazolium Based Ionic Liquids/Water Mixtures. 2. EXAFS Spectroscopy

‡

Paola D’Angelo∗,‡ , Andrea Zitolo‡ , Giuliana Aquilanti† , Valentina Migliorati∗,‡ Dipartimento di Chimica, Università di Roma “La Sapienza”, P.le A. Moro 5, 00185 Roma, Italy † Elettra-Sincrotrone Trieste S.C.p.A s.s. 14, km 163.5, I-34149 Basovizza, Trieste, Italy ∗ [email protected], [email protected] July 4, 2013 Abstract Extended X-ray absorption fine structure (EXAFS) spectroscopy is employed, in conjunction with Molecular Dynamics (MD) simulations, to investigate the interaction of water with the Br− ion in an imidazolium-based ionic liquid (IL). 1-butyl-3-methylimidazolium bromide/water mixtures with molar ratios ranging from 1:3 to 1:200 have been analysed and a clear picture of the structural arrangements of the water molecules inside the IL has been obtained from the synergic interpretation of the EXAFS and MD data. At the lowest investigated water content, the presence of water is mainly detected around the Br− anion. Upon increasing the water fraction, more water molecules enter the Br− first coordination shell but always in a lower number than what is needed to saturate the inner sphere. This suggests that interactions exist also between water and the imidazolium cation. The

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existence of tight ion pairs has been evidenced even when water is present in the mixtures in great excess. Keywords: EXAFS, Ionic Liquids, Molecular Dynamics, Water Mixtures, Imidazolium, Bromide

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1

Introduction

Ionic liquids (ILs) are an emerging class of solvents that are fluid at room temperature, and consist entirely of ionic species. They are typically built up by organic cations and inorganic anions, have negligible vapor pressure and thus are nonvolatile, and they are nonflammable under ambient conditions. These properties have led to an increasing interest on ILs from both a scientific and technological/industrial point of view. 1–5 The enormous versatility of this class of materials derives from the wide combination of inorganic and organic building blocks which can be used for their preparation. The achievement of the desired functional and/or structural properties is ruled by the obtainment of the desired composition, morphology, micro- and nanostructure in the final composite materials. Consequently, a primary concern becomes a sound and thorough characterization of the materials and the determination of reliable structure/property relationships. Among the various IL families, 1-alkyl-3-methylimidazolium [Cn mim] salts are the most widely used and the nature of the interaction between the imidazolium cation and the anion, or of the molecular organization in the case of mixtures with other molecular compounds is of very fundamental importance. 6 In the literature much attention has been paid to IL/water mixtures due to their potential applications in industrial processes and as green solvents. 7–20 The influence of water on the chemo-physical properties of [Cn mim] ILs depends on different factors such as the nature of the anion, the length of the alkyl chain and the aggregation behaviour of the IL. Previous investigations mainly carried out with 1 H and 13 C NMR spectroscopy have been devoted to the characterization of the interaction between water molecules and the hydrogen atoms of the imidazolium cation. 9,21,22 The interaction between water and the IL anion has been experimentally addressed using ATR-IR spectroscopy and in ILs consisting of anions with a strong ability to form hydrogen bonds, such as [NO3 ]− and [CF3 CO2 ]− , water molecules were found to be mainly hydrogen bonded to the anion. 8 Despite these studies no direct structural information on the interaction between the water molecules and the anion in IL/water mixtures has been gained up to now, due to the lack of suitable experimental methods. One key experimental technique that can furnish useful information on the anion environment when dealing with halide ILs is the X-ray absorption spectroscopy (XAS). Extended X-ray absorption fine structure (EXAFS) is one of the very few experimental techniques that provides direct clues about 3



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Figure 1: Chemical structure of the [C4 mim]+ cation.

the structural properties of disordered systems, 23–31 thus making it an invaluable tool for structural characterization of IL/water mixtures. 32 In this work we have applied this experimental technique to the study the Br− local structure in [C4 mim]Br/water mixtures (see Figure 1), in combination with Molecular Dynamics (MD) simulations. 33 The importance of combining XAS with MD is motivated by the fact that the study of the structural and dynamic properties of disordered systems is a complex task, and it is very difficult to obtain accurate information, especially when using a single method of investigation. The combined MD-XAS approach allows one on the one hand to have reliable structural models to be used in the analysis of the EXAFS data, on the other hand to test the potential functions used in the classical MD simulations. In this study, the synergic use of MD and EXAFS allowed us to have a global structural picture of the complex molecular organization of [C4 mim]Br/water mixtures both in proximity of the Br− anion and in the longer distance range.

2 2.1

Methods Molecular Dynamics Details

In order to properly choose the Br− -water interaction potential to be used in the MD simulations of the [C4 mim]Br/water mixtures, three MD simulations of the Br− ion in aqueous solution have been performed using the SPC/E 4


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water model, 34 and different Br− -water interaction potentials present in the literature. 35–37 The MD simulations of the Br− ion in water have been carried out using the DL POLY package. 38 The systems were composed by one Br− ion and 819 water molecules in a cubic box, using periodic boundary conditions. A homogeneous background charge has been used to compensate for the presence of the Br− ion. The simulation protocol is the same as that used for the simulations of the [C4 mim]Br/water mixtures described in Ref. 33 . The systems were equilibrated for 3 ns and the production runs were carried out in the NVT ensemble for 6 ns, with a timestep of 1 fs and saving a configuration every 100 timesteps. The temperature was kept constant at 300 K using the Nosé-Hoover thermostat 39,40 with a relaxation constant of 0.5 ps. A cut-off of 12 ˚ A was used to deal with non-bonded interactions, with the Ewald summation method to treat long-range electrostatic effects. 41 The structural properties obtained from the simulations have been described in terms of radial distribution functions, gAB (r): NA X NB X 1 δ(rij − r) hρB (r)i = gAB (r) = hρB ilocal NA hρB ilocal i=1 j=1 4πr2

(1)

where hρB (r)i is the particle density of type B at distance r around type A, and hρB ilocal is the particle density of type B averaged over all spheres around particle A with radius rmax . The Br-O and Br-H g(r)’s of the [C4 mim]Br/water mixtures were calculated starting from the MD simulations described in Ref. 33 . To directly compare the MD and EXAFS structural results the Br-O and Br-H g(r)’s are modelled with gamma-like distributions defined as: p−1 1 1 1 r−R r−R p2 exp −p − p+ p2 p2 f (r) = N σΓ(p) σ σ

(2)

where Γ(p) is the Euler’s Gamma function for the parameter p, and N is the coordination number providing the correct normalization, R is the mean distance, σ is the standard deviation and β is the asymmetry index 1 (third cumulant divided by σ 3 ) β = 2p− 2 . Note that the R values are the average distances of the distributions that are shifted toward larger values with respect to the maximum of the g(r)’s due to the asymmetry. 5



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2.2

X-absorption measurements

The [C4 mim]Br was purchased from Iolitec GmbH with a stated purity of > 99% and it was dried in vacuum for about 36 h. The water content was then determined by Karl-Fischer tritation and was found to be between 150 and 300 ppm for the different samples. Further purification was not carried out. [C4 mim]Br/water mixtures with different IL/water molar ratios (1:3, 1:6, 1:16, 1:70 and 1:200) were prepared by adding the proper amount of bidistilled water to [C4 mim]Br. A 0.1 M RbBr aqueous solution was also prepared. The Br K-edge XAS spectra were collected at RT in transmission mode at the Elettra Synchrotron (Trieste, Italy) on the beamline 11.1. 42 The first inflection point of the spectra was found at 13472 eV and the data were collected up to 14300 eV. The storage ring was operating at 2 GeV with an optimal storage beam current between 300 and 130 mA.

2.3

EXAFS data analysis

The EXAFS theoretical signals have been calculated by means of the GNXAS program and a thorough description of the theoretical framework can be found in Ref. 43 . Phase shifts, A(k,r) and φ(k,r), have been calculated starting from one of the MD configurations, by using muffin-tin potentials and advanced models for the exchange-correlation self-energy (Hedin-Lundqvist). The values of the muffin-tin radii are 0.2 ˚ A, 0.9 ˚ A, and 2.3 ˚ A for hydrogen, oxygen, and bromine, respectively. These radii correspond to an overlap between the bromine and hydrogen atoms of about 10 %. Inelastic losses of the photoelectron in the final state have been accounted for intrinsically by complex potential. The imaginary part also includes a constant factor accounting for the core-hole width. In the first step of the analysis the EXAFS signal has been modelled as a function of the radial distribution function g(r) as: Z ∞ χ(k) = dr 4πρr2 g(r)A(k, r) sin [2kr + φ(k, r)] (3) 0

where A(k, r) and φ(k, r) are the amplitude and phase functions, respectively, and ρ is the density of the scattering atoms. χ(k) theoretical signals have been calculated by introducing in Eq. 3 the model radial distribution functions obtained from the MD simulations. Comparison between the theoretical and experimental χ(k) signals allows the reliability of the g(r)’s, and 6


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consequently of the potential functions used in the simulations, to be checked. It is well known that the atomic background of several elements contains important contributions associated with the opening of multi-electron excitation channels. Here the background function used to extract the χ(k) experimental signals has been modeled by means of step-shaped functions accounting for the 1s4p, 1s3d, and 1s4p double-electron resonances, as determined for HBr and TMABr. 44–46 Both the MD Br-O and Br-H g(r)’s of the water molecules obtained from the simulations have been used to calculate the single scattering first shell χ(k) theoretical signal. Conversely, for the [C4 mim]Br/water mixtures, the carbon, nitrogen and hydrogen atoms of the cation have been found to provide a negligible contribution to the total χ(k) signal. Least-squares fits of the EXAFS raw experimental data have been performed and two non structural parameters, namely E0 (core ionization threshold energy) and S02 have been optimized. In the second step, the analysis of the [C4 mim]Br/water mixtures has been carried out by modeling the Br-H and Br-O first coordination shells with gamma-like distribution curves with mean distance R, coordination number N , standard deviation σ, and asymmetry index β. Least-squares fits of the EXAFS raw experimental data have been performed to optimize both the structural and non structural parameters. The results of this analysis have been compared with the MD structural determinations.

3 3.1

Results Choice of the Br-water potential

In order to perform MD simulations of [C4 mim]Br/water mixtures a proper choice of the force field parameters is mandatory for a correct description of the systems. As fully described in Ref. 33 the force field parameters for the [C4 mim]Br IL were taken from Lopes and Padua, 37,47 while the SPC/E 34 model was used for water.Fig1.pdf The Lennard-Jones parameters for all of the different atoms were obtained from the Lorentz-Berthelot combining rules. As both IL and water force fields were optimized for the pure components, the Lennard-Jones parameters for the Br-water cross interactions in IL/water mixtures do not necessarily follow a simple set of ”mixing rules”. For this reason, it is important to check the capability of the Br-water poten7



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tial obtained by combining the Lopes and Padua (LP) and SPC/E force fields in properly deriving the structural properties of a Br− water solution. To this aim we have carried out a MD simulation of Br− in aqueous solution using the LP potential for the Br− ion and the SPC/E model for water. Starting from the MD trajectory we have calculated the Br-H and Br-O g(r)’s that are shown in Figure 2. For the bromide ion the hydration number as determined by X-ray and neutron diffraction and X-ray absorption is in the range 6.0-7.4 48–52 while the Br-O first shell average bond length determined experimentally is in the range 3.19-3.40 ˚ A. 48 In a recent investigation combining Car-Parrinello MD simulations and EXAFS experimental data the first hydration shell has been found to be composed by 6.5 water molecules with a Br-O distance of 3.33 ˚ A. 53 Looking at the Br-O g(r) obtained with the LP potential it is evident that the Br-O first shell distance is about 0.2 ˚ A too short as compared with the Car-Parrinello determination. 53 To have a direct proof of the reliability of the simulation the MD structural results can be directly compared with the EXAFS experimental data. χ(k) theoretical signals are calculated starting from the MD Br-H and Br-O g(r)’s. In the top-left panels of Figure 3, the comparison between the experimental signal and the theoretical curves is reported. The first two curves from the top are the Br-H and Br-O first shell contributions calculated by inserting in Eq. 3 the MD g(r)’s without any adjustable parameter, while the reminder of the figures shows the total theoretical signal compared with the experimental spectrum. The agreement between the calculated and experimental EXAFS spectra is quite poor and a clear mismatch of the main frequency can be observed. This behavior is due to the short value of the average Br-H and Br-O first shell distances as compared to the experimental results. This finding is reinforced by the Fourier transform (FT) moduli of the EXAFS χ(k) theoretical and experimental signals shown in the lower panel of Figure 3. The FT’s have been calculated in the k-range 3.0-11.0 ˚ A−1 with no phase shift correction applied. The theoretical first-neighbor peak obtained from the simulation is found to be broader and shifted towards shorter distances than predicted by the experiment. Given the disagreement between the experimental and theoretical structural results we decided to use different Lennard-Jones parameters for the Br− ion in the MD simulation. In particular, we carried out two additional simulations using either the OPLS 35 or the Pálinkás 36 (PAL) Lennard-Jones parameters for the Br− ion in combination with the SPC/E water model. 8


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Br-H

g(r)

4 3 2 1 0

2

2.5

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3.5

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r (Å) 5

Br-O

4

g(r)

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3 2 1 0

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2.5

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4.5

5

r (Å) Figure 2: Upper panel: Br-H radial distribution functions obtained from MD simulations using the PL (black line), OPLS (red line) and PAL (blue line) Lennard-Jones parameters. Lower panel: Br-O radial distribution functions obtained from MD simulations using the PL (black line), OPLS (red line) and PAL (blue line) Lennard-Jones parameters.

The Br-O and Br-H g(r)’s obtained using the OPLS and PAL force fields are shown in Figure 2, together with the LP ones. Inspection of this figure reveals that use of different interaction parameters strongly affects the first shell position of both the Br-H and B-O g(r)’s. In particular, the Br-O first shell maxima are found at 3.12, 3.33 and 3.48 ˚ A for the LP, OPLS and 9



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Figure 3: Upper panels: Comparison between the EXAFS experimental data (dotted red line) and the theoretical signals (solid blue line) calculated from the LP (left panel), OPLS (middle panel) and PAL (right panel) Br-O and Br-H g(r)’s. Lower panels: Nonphase-shifted corrected Fourier transforms of the experimental data (dotted red line) and of the theoretical signals (solid blue line).

10


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PAL potentials, respectively. Also in this case a direct comparison between the MD and EXAFS structural results has been carried out and the results are shown in the middle and right panels of Figure 3. The theoretical χ(k) signal calculated from the OPLS g(r)’s matches the experimental data very well and a good agreement is found also looking at the FT’s. Conversely a clear disagreement is found with the PAL parameters due to the long value of the Br-O first shell distance. The FT’s reported in the lower panels of Figure 3 confirm these findings. Starting from these results we resorted to adopt the OPLS parameters to describe the Br-water interactions in the simulations of the [C4 mim]Br/water mixtures and a thorough description of the MD protocol and results can be found in Ref 33 .

3.2

EXAFS analysis of the[C4 mim]Br/water mixtures

The Br K-edge EXAFS spectra of the [C4 mim]Br/water mixtures with IL/water molar ratios of 1:3, 1:6, 1:16, 1:70 and 1:200 are shown in Figure 4, together with the experimental data of pure [C4 mim]Br and Br− in aqueous solution. The EXAFS spectra have been extracted with a three segmented cubic spline and the corresponding Fourier transform calculated in the interval k=1.5-8.0 ˚ A−1 are shown in the lower panel of Figure 4. The first remarkable observation is that also when only three water molecules per ion pair are added to the pure IL, the EXAFS spectrum is strongly modified. In particular, the amplitude of the experimental signal of pure [C4 mim]Br is lower as compared with the 1:3 IL/water solution and a clear shift of the main oscillation occurs. Also the FT of pure [C4 mim]Br is characterized by a lower intensity first shell peak located at slightly longer distances as compared with the solutions. This result indicates that the local coordination around the Br− ion is significantly affected by the presence of the water molecules also at very low water content, thus meaning that the water molecules tend to preferentially interact with the Br− ion, in agreement with prevailing concepts. 7,8,14 When looking at the EXAFS data of the IL/water mixtures, the frequencies of the oscillations reveal a structural trend with increasing water content. Higher EXAFS amplitudes are detected at higher water concentrations indicating a progressive crowding of the Br− ion coordination environment. Moreover, the Br-O first shell seems to become more asymmetric going from the 1:3 to the 1:200 IL/water molar ratio looking at the frequencies of the EXAFS signals. These findings are confirmed by the FT’s whose intensity increases at higher dilution. Another interesting observation is that also in the 11



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Figure 4: Upper panel: experimental EXAFS spectra of [C4 mim]Br/water mixtures with molar ratios of 1:3, 1:6, 1:16, 1:70 and 1:200 together with the EXAFS spectra of pure [C4 mim]Br and Br− in water. Lower panel: Nonphase-shifted corrected Fourier transforms of the experimental data reported in the upper panel.

12


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1:16 IL/water solution the Br− environment is different from that of pure water, even if 16 water molecules are sufficient to saturate the Br− inner hydration shell. To rationalize these results we have carried out MD simulations of the [C4 mim]Br/water mixtures and a detailed description of the MD results can be found in Ref. 33 . In particular, a direct comparison between the local coordination geometry of the Br− ion obtained from the MD calculations and EXAFS data allows on the one hand to properly interpret the experimental spectra, on the other to assess the reliability of the simulations. The Br-H and Br-O radial distribution functions obtained from the MD trajectories of the IL/water mixtures are shown in Figure 5. In all cases a well defined first peak followed by a depletion zone can be observed, indicating the existence of a stable first hydration shell. To better visualize the dilution effects on the Br-water radial distribution functions, we report in panels B and D of Figure 5 the corresponding radial densities n(r) = 4πr2 ρg(r), where ρ is the density of hydrogen or oxygen atoms in the solutions. In such a case of strong ρ variations this quantity is better suited to appreciate the actual variation of the first shell coordination numbers, i.e. the integral of the first peak, and of the actual peak broadening and shift, with increasing water content. Note that when the water concentration becomes higher the Br− first hydration shell becomes more populated. Moreover, the Br-H and Br-O first shell peaks are more asymmetrical with increasing dilution indicating a more disordered environment around the Br− ion. In order to gain a more quantitative description of the structural properties of the Br− first hydration shell in the different mixtures, the g(r) first peaks have been modelled with the gamma-like peaks of Eq. 2, whose parameters are fitted to the MD distributions. The structural parameters obtained from the fitting of the MD Br-O g(r)’s are collected in Table 1 for all the investigated samples. Inspection of this table reveals that the Br− hydration number increases from 2.9 to 6.9 in going from the 1:3 to the 1:200 mixture. Correspondingly, the Br-O distances becomes slightly longer and the variance of the distribution experiences an enhancement indicating a more disordered local environment. When comparing these values with those reported in Table 2 of Ref. 33 , one has to consider that here R represents the mean value of the distribution that, for asymmetric shells, is always shifted toward larger distances as compared with the maximum of the g(r). Note that while the maximum position does not change, the Br-O average distance increases with diluition due to the increase of the asymmetry of the hydration shell. 13



A

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3.5

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r (Å)

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r (Å)

Figure 5: Panels A and B: Br-H radial distribution functions g(r)’s and radial densities n(r) = 4πr2 ρg(r), obtained from the MD simulations carried out for the 1:3 (green line), 1:6 (magenta line), 1:16 (blue line), 1:70 (black line) and 1:200 (cyan line) [C4 mim]Br/water mixtures. Panels C and D: Br-O radial distribution functions g(r)’s and radial densities n(r)’s, obtained from the MD simulations carried out for the 1:3 (green line), 1:6 (magenta line), 1:16 (blue line), 1:70 (black line) and 1:200 (cyan line) [C4 mim]Br/water mixtures.

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A remark we would like to make concerns the number of water molecules that enter the Br− first hydration shell, as compared to the total number of water molecules present in the IL solution. At low water content (1:3) almost all of the water molecules tend to interact with the anion, while for higher concentrations the number of coordinating water molecules is systematically lower than in pure water. This finding suggests that some water molecules interact with the imidazolium cation as also found from the MD simulations (see Ref. 33 ). Direct comparison of the MD results with the EXAFS experimental data allows one to unambiguously assess the accuracy of the simulations. χ(k) theoretical signals have been calculated using Eq. 3, starting from the Br-H and Br-O g(r) distributions calculated from the MD trajectories for all the investigated mixtures. We have also calculated the chi(k) theoretical signals associated with the carbon, nitrogen and hydrogen atoms of the cation and these atoms have been found to provide a negligible contribution to the total EXAFS signal for all of the investigated [C4 mim]Br/water mixtures. The MD structural parameters were kept fixed during the analysis. The EXAFS analysis for the 1:16 [C4 mim]Br/water mixture is shown in Figure 6 as an example. The first two curves from the top are the Br-H and Br-O theoretical signals, and the reminder of the figure shows the total theoretical contribution, in comparison with the experimental spectrum and the resulting residuals. Overall, the calculated EXAFS spectrum matches the experimental data very well. The comparison between the EXAFS experimental data and the theoretical signals derived from the MD simulations for all of the [C4 mim]Br/water mixtures is shown in Figure 7. In all cases the agreement between theory and experiment is very good proving that the structural information derived from the MD simulations is reliable, and that the chosen force field is fully capable of providing a correct description of the systems. In the second step we have analysed the EXAFS spectra of the IL/water mixtures using asymmetric shells to model the Br-H and Br-O distributions. Least-squares fits of the EXAFS spectra were performed in the range k=2.3-12.0 ˚ A−1 and minimization procedures were applied to the whole set of structural and nonstructural parameters to improve, as far as possible, the agreement between calculated signals and experimental spectra. The Br-O structural parameters obtained from this analysis for the entire series are reported in Table 1. The error values reported in the table have been calculated using a statistical analysis in which two-dimensional contour plots 15



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Table 1: Best-fit parameters (Br-O coordination numbers N , average distances R, Debye-Waller factors σ 2 , and asymmetry parameter β) for the [C4 mim]Br/water mixtures as determined by the MD simulations and the EXAFS analysis.

1:3 MD 1:3 EXAFS 1:6 MD 1:6 EXAFS 1:16 MD 1:16 EXAFS 1:70 MD 1:70 EXAFS 1:200 MD 1:200 EXAFS Water MD Water EXAFS

N

R(˚ A)

σ 2 (˚ A2 )

β

2.9

3.40

0.031

0.8

2.5(5)

3.38(2)

0.032(2)

0.8

4.2

3.40

0.033

0.9

3.6(5)

3.39(2)

0.035(1)

0.9

5.2

3.42

0.037

0.9

4.8(5)

3.40(2)

0.036(1)

0.9

6.3

3.44

0.043

1.0

6.5(5)

3.40(2)

0.043(1)

1.0

6.8

3.44

0.046

1.0

6.9(5) 7.2

3.41(2) 0.043(1) 3.45

6.9(5)

0.048

3.42(1) 0.043(1)

16


1.0 1.0 1.0

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Figure 6: Upper panel: Comparison between the EXAFS experimental spectrum of the 1:16 [C4 mim]Br/water mixture (dotted red line) and theoretical signals (solid blue line) calculated from the Br-H and Br-O MD g(r)’s. Lower panel: Nonphase-shifted corrected Fourier transforms of the experimental data (dotted red line) and of the theoretical signals (solid blue line).

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Figure 7: Comparison between the experimental EXAFS spectra of [C4 mim]Br/water mixtures with molar ratios of 1:3, 1:6, 1:16, 1:70 and 1:200 (dotted red line) and the theoretical signals calculated from the MD Br-H and Br-O g(r)’s (solid blue line).

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of selected parameters have been generated. This analysis examines the correlations among fitting parameters and evaluates the statistical errors by following the procedure described in detail in Ref. 54 . Even if it is well known that in the case of disordered systems there is a strong correlation between DW factors and coordination numbers, as both parameters are related to the amplitude of the EXAFS oscillation, the error associated with the first shell coordination numbers is sufficiently small to furnish a clear picture of the coordination properties of the Br− ion. It is important to stress that this second analysis of the EXAFS data is completely independent from the former one starting from the MD g(r)’s. The values reported in Table 1 show a good agreement between the MD and EXAFS determinations thus confirming the validity of the obtained results. The substantial agreement between the MD and EXAFS results is also evident from the trend of the Br− hydration number as a function of the IL/water molar ratio shown in Figure 8. The number of water molecules that interact with the anion decreases with increasing IL/water as expected. To provide visual insights, four representative snapshots of the environment seen by the anion in the [C4 mim]Br/water mixtures are shown in Figure 9. It can be seen the evolution of the Br− environment that progressively loses [C4 mim]+ cations to accommodate more and more water molecules, going from low to high dilution conditions.

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Figure 8: Br-O first shell coordination numbers of the [C4 mim]Br/water mixtures as a function of the molar ratio as determined from the MD simulations (blue line) and EXAFS data analysis (magenta line).

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Figure 9: Br− first coordination sphere in the [C4 mim]Br/water mixtures with IL/water molar ratios of 1:3 (A), 1:6 (B), 1:16 (C) and 1:200 (D) as found in representative MD snapshots.

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3.3

Discussion and Conclusion

The present work was carried out with a view of investigating the interaction between water molecules and the Br− anion in [C4 mim]Br/water mixtures. To this aim solutions with IL/water ratios of 1:3, 1:6, 1:16, 1:70 and 1:200 have been studied by combining EXAFS experiments and MD simulations. The MD calculations have been carried out using an ”ad hoc” force field for the bromine-water interaction and a detailed description of the MD protocol and results is reported in Ref. 33 . The synergic experimental/theoretical procedure has been adopted at first to chose the best-performing Lennard-Jones parameters for the Br-water potential among several functions present in the literature. A Br− water solution has been used for the force field optimization and the transferability of the selected potential to the IL/water mixtures has been then proved by comparing the MD results with the EXAFS experimental data. It is well known in the literature that in IL/water mixtures the anionwater interactions play a major role, 8,13,14 and this is particularly true for the Br− ion that possesses a strong ability to form hydrogen bonds with water molecules. 53 Published results are mainly based on theoretical calculations or on experimental data that are not able to directly probe the anion local coordination. In this context, our approach is a unique tool as it uses the only experimental method able to provide direct structural information on the short-range environment of the Br− anion. The EXAFS spectrum of the 1:3 [C4 mim]Br/water mixture has been found to be significantly different from that of pure [C4 mim]Br. As a consequence also when the water content in the mixture is very low, water molecules tend to preferentially interact with the Br− anion, thus deeply altering its inner coordination sphere. Note that in this case almost all of the water molecules present in the solution are found in the proximity of the Br− ion. Upon increasing the water fraction, more water enters the Br− first coordination shell but there is always a lower number of molecules than needed to saturate the inner sphere, even when water in added in great excess. This suggests that interactions exist also between water and the imidazolium cation, as already found from the MD results (see Ref. 33 ). The presence of water-cation interactions in [Cn mim]Br IL/water mixtures has been previously detected by the analysis of H and 13 C NMR spectra. 9,21,22 At the lowest investigated water content (1:1), the interaction of water has been found to be specific and localized at the imidazolium ring hydrogen atoms that are 22


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capable of establishing hydrogen bonds with water. At higher water content interactions appear between water and all of the IL protons, thus passing from a regime of selective interaction to a less defined non selective solvation. 21 A last remark we would like to make concerns the existence of cationanion ion pairs in the investigated solutions. The results of previous experimental and theoretical studies on imidazolium based ILs in water solutions were interpreted by assuming the existence of tight ion pairs albeit the great excess of water present in the mixtures. 9,55 This behaviour is confirmed by our EXAFS and MD results showing the existence of a strong cation-anion correlation in all of the investigated [C4 mim]Br/water mixtures. In particular, by combining the MD and EXAFS findings we can conclude that the anion-cation interactions are often mediated through water molecules that act as a bridge between counterions. In conclusion, this combined experimental and theoretical work enabled us to gain a clear picture of the structural properties of [C4 mim]Br/water mixtures, both in the surrounding of the imidazolium cation and of the Br− anion. This new approach has been found to be particularly well suited to gain detailed information on IL solutions despite their complexity.

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Acknowledgments This work was supported by the University of Rome ”La Sapienza” (Progetto ateneo 2012, n. C26A129ZAY).

References [1] Freemantle, M. An Introduction to Ionic Liquids; RSC Publishing, 2009. [2] Rogers, R., Seddon, K., Eds. Ionic Liquids IIIA: Fundamentals, Progress, Challenges, and Opportunities: Properties and Structure; ACS Symp. Ser.; American Chemical Society, Washington D.C., 2005; Vol. 901. [3] Rogers, R., Seddon, K., Eds. Ionic Liquids IIIB: Fundamentals, Progress, Challenges, and Opportunities: Transformations and Processes; ACS Symp. Ser.; American Chemical Society, Washington D.C., 2005; Vol. 902. [4] Rogers, R. D.; Seddon, K. R. Ionic Liquids–Solvents of the Future? Science 2003, 302, 792–793. [5] Gontrani, L.; Bodo, E.; Triolo, A.; Leonelli, F.; D’Angelo, P.; Migliorati, V.; Caminiti, R. The Interpretation of Diffraction Patterns of Two Prototypical Protic Ionic Liquids: a Challenging Task for Classical Molecular Dynamics Simulations. J. Phys. Chem. B 2012, 116, 13024–13032. [6] Seddon, K. R.; Stark, A.; Torres, M.-J. Influence of Chloride, Water, and Organic Solvents on the Physical Properties of Ionic Liquids. Pure Appl. Chem. 2000, 72, 2275–2287. [7] Porter, A. R.; Liem, S. Y.; Popelier, P. L. A. Room temperature Ionic Liquids Containing Low Water Concentrations-a Molecular Dynamics Study. Phys. Chem. Chem. Phys. 2008, 10, 4240–4248. [8] Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Molecular States of Water in Room Temperature Ionic Liquids. Phys. Chem. Chem. Phys. 2001, 3, 5192–5200.

24


Page 24 of 30

Page 25 of 30

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


[9] Mele, A.; Tran, C. D.; De Paoli Lacerda, S. H. The Structure of a Room-Temperature Ionic Liquid with and without Trace Amounts of Water: The Role of C-H...O and C-H...F Interactions in 1-n-Butyl-3Methylimidazolium Tetrafluoroborate. Angew. Chem., Int. Ed. 2003, 42, 4364–4366. [10] Moreno, M.; Castiglione, F.; Mele, A.; Pasqui, C.; Raos, G. Interaction of Water with the Model Ionic Liquid [bmim][BF4]: Molecular Dynamics Simulations and Comparison with NMR Data. J. Phys. Chem. B 2008, 112, 7826–7836. [11] Miki, K.; Westh, P.; Nishikawa, K.; Koga, Y. Effect of an ”Ionic Liquid” Cation, 1-Butyl-3-methylimidazolium, on the Molecular Organization of H2 O. J. Phys. Chem. B 2005, 109, 9014–9019. [12] Schröder, C.; Rudas, T.; Neumayr, G.; Benkner, S.; Steinhauser, O. On the Collective Network of Ionic Liquid/Water Mixtures. I. Orientational Structure. J. Chem. Phys. 2007, 127, 234503. [13] Hanke, C. G.; Lynden-Bell, R. M. A Simulation Study of WaterDialkylimidazolium Ionic Liquid Mixtures. J. Phys. Chem. B 2003, 107, 10873–10878. [14] Jiang, W.; Wang, Y.; Voth, G. A. Molecular Dynamics Simulation of Nanostructural Organization in Ionic Liquid/Water Mixtures. J. Phys. Chem. B 2007, 111, 4812–4818. [15] Dimitrakis, G.; Villar-Garcia, I. J.; Lester, E.; Licence, P.; Kingman, S. Dielectric Spectroscopy: a Technique for the Determination of Water Coordination within Ionic Liquids. Phys. Chem. Chem. Phys. 2008, 10, 2947–2951. [16] Wang, Y.; Li, H.; Han, S. A Theoretical Investigation of the Interactions between Water Molecules and Ionic Liquids. J. Phys. Chem. B 2006, 110, 24646–24651. [17] Zhong, X.; Fan, Z.; Liu, Z.; Cao, D. Local Structure Evolution and its Connection to Thermodynamic and Transport Properties of 1-Butyl-3methylimidazolium Tetrafluoroborate and Water Mixtures by Molecular Dynamics Simulations. J. Phys. Chem. B 2012, 116, 3249–3263. 25



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

[18] Migliorati, V.; Ballirano, P.; Gontrani, L.; Triolo, A.; Caminiti, R. Thermal and Structural Properties of Ethylammonium Chloride and Its Mixture with Water. J. Phys. Chem. B 2011, 115, 4887–4899. [19] Migliorati, V.; Ballirano, P.; Gontrani, L.; Russina, O.; Caminiti, R. Crystal Polymorphism of Propylammonium Chloride and Structural Properties of Its Mixture with Water. J. Phys. Chem. B 2011, 115, 11805–11815. [20] 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. [21] Cesare Marincola, F.; Piras, C.; Russina, O.; Gontrani, L.; Saba, G.; Lai, A. NMR Investigation of Imidazolium-Based Ionic Liquids and Their Aqueous Mixtures. ChemPhysChem 2012, 13, 1339–1346. [22] Singh, T.; Kumar, A. Aggregation Behavior of Ionic Liquids in Aqueous Solutions: Effect of Alkyl Chain Length, Cations, and Anions. J. Phys. Chem. B 2007, 111, 7843–7851. [23] 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. [24] 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. [25] Spezia, R.; Duvail, M.; Vitorge, P.; Cartailler, T.; Tortajada, J.; D’Angelo, P.; Gaigeot, M.-P. A Coupled Car-Parrinello Molecular Dynamics and EXAFS Data Analysis Investigation of Aqueous Co2+ . J. Phys. Chem. A 2006, 110, 13081–13088. [26] D’Angelo, P.; Chillemi, G.; Barone, V.; Mancini, G.; Sanna, N.; Persson, I. Experimental Evidence for a Variable First Coordination Shell of the Cadmium(II) Ion in Aqueous, Dimethyl Sulfoxide, and N,N ‘dimethylpropyleneurea Solution. J. Phys. Chem. B 2005, 109, 9178– 9185.

26


Page 26 of 30

Page 27 of 30

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


[27] 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. [28] Migliorati, V.; Zitolo, A.; Chillemi, G.; D’Angelo, P. Influence of the Second Coordination Shell on the XANES Spectra of the Zn2+ Ion in Water and Methanol. ChemPlusChem 2012, 77, 234–239. [29] 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. [30] 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. [31] Migliorati, V.; Mancini, G.; Chillemi, G.; Zitolo, A.; D’Angelo, P. Effect of the Zn2+ and Hg2+ Ions on the Structure of Liquid Water. J. Phys. Chem. A 2011, 115, 4798–4803. [32] D’Angelo, P.; Zitolo, A.; Migliorati, V.; Bodo, E.; Aquilanti, G.; Hazemann, J. L.; Testemale, D.; Mancini, G.; Caminiti, R. X-Ray Absorption Spectroscopy Investigation of 1-alkyl-3-methylimidazolium Bromide Salts. J. Chem. Phys. 2011, 135, 074505. [33] 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. Submitted to J. Phys. Chem. B 2013, [34] Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. The Missing Term in Effective Pair Potentials. J. Phys. Chem. 1987, 91, 6269–6271. [35] Jorgensen, W. L.; Ulmschneider, J. P.; Tirado-Rives, J. Free Energies of Hydration from a Generalized Born Model and an All-Atom Force Field. J. Phys. Chem. B 2004, 108, 16264–16270. [36] Pálinkás, G.; Riede, W. O.; Heinzinger, K. A Molecular Dynamics Study of Aqueous Solutions. VII. Improved Simulation and Comparison with

27



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

X-ray Investigations of a Sodium Chloride Solution. Z. Naturforsch. teil A 1977, 32A, 1137–1145. [37] Canongia Lopes, J. N.; Pádua, A. A. H. Molecular Force Field for Ionic Liquids III: Imidazolium, Pyridinium, and Phosphonium Cations; Chloride, Bromide, and Dicyanamide Anions. J. Phys. Chem. B 2006, 110, 19586–19592. [38] Smith, W.; Forester, T. DL POLY 2.0: A General-Purpose Parallel Molecular Dynamics Simulation Package. J. Mol. Graphics 1996, 14, 136–141. [39] Nosé, S. A Unified Formulation of the Constant Temperature Molecular Dynamics Methods. J. Chem. Phys. 1984, 81, 511–519. [40] Evans, D. J.; Holian, B. L. The Nosé-Hoover Thermostat. J. Chem. Phys. 1985, 83, 4069–4074. [41] 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. [42] Di Cicco, A.; Aquilanti, G.; Minicucci, M.; Principi, E.; Novello, N.; Cognigni, A.; Olivi, L. Novel XAFS Capabilities at ELETTRA Synchrotron Light Source. J. Phys. Conf. Series 2009, 190, 012043. [43] Filipponi, A.; Di Cicco, A.; Natoli, C. R. X-ray-Absorption Spectroscopy and n-body Distribution Functions in Condensed Matter. I. Theory. [44] D’Angelo, P.; Di Cicco, A.; Filipponi, A.; Pavel, N. V. Double-Electron Excitation Channels at the Br K Edge of HBr and Br2 . Phys. Rev. A 1993, 47, 2055–2063. [45] D’Angelo, P.; Pavel, N. V. Complete Spectrum of Multielectron Excitations at the Br− K Edge X-ray Absorption Spectra. Phys. Rev. B 2001, 64, 233112. [46] Burattini, E.; D’Angelo, P.; Giglio, E.; Pavel, N. V. Exafs Study of Probe Molecules in Micellar Solutions. J. Phys. Chem. 1991, 95, 7880–7886.

28


Page 28 of 30

Page 29 of 30

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


[47] 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. [48] Ohtaki, H.; Radnai, T. Structure and Dynamics of Hydrated Ions. Chem. Rev. 1993, 93, 1157–1204. [49] D’Angelo, P.; Di Nola, A.; Filipponi, A.; Pavel, N. V.; Roccatano, D. An Extended X-ray Absorption Fine Structure Study of Aqueous Solutions by Employing Molecular Dynamics Simulations. J. Chem. Phys. 1994, 100, 985. [50] Tanida, H.; Sakane, H.; Watanabe, I. Solvation Structures for Bromide Ion in Various Solvents by Extended X-ray Absorption Fine Structure. J. Chem. Soc., Dalton Trans. 1994, 15, 2321–2326. [51] Ferlat, G.; Miguel, A. S.; Jal, J. F.; Soetens, J. C.; Bopp, P. A.; Daniel, I.; Guillot, S.; Hazemann, J. L.; Argoud, R. Hydration of the Bromine Ion in a Supercritical 1:1 Aqueous Electrolyte. Phys. Rev. B 2001, 63, 134202. [52] Filipponi, A.; De Panfilis, S.; Oliva, C.; Ricci, M. A.; D’Angelo, P.; Bowron, D. T. Ion Hydration under Pressure. Phys. Rev. Lett. 2003, 91, 165505. [53] 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. [54] 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. [55] Spickermann, C.; Thar, J.; Lehmann, S. B. C.; Zahn, S.; Hunger, J.; Buchner, R.; Hunt, P. A.; Welton, T.; Kirchner, B. Why Are Ionic Liquid Ions Mainly Associated in Water? A Car-Parrinello Study of 1-ethyl-3-methyl-imidazolium Chloride Water Mixture. J. Chem. Phys. 2008, 129, 104505.

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