Direct Contact versus Solvent-Shared Ion Pairs in Saturated NiCl

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Article

Direct Contact Versus Solvent-Shared Ion Pairs in Saturated NiCl Aqueous Solution: A DFT, CPMD and EXAFS Investigation 2

Feifei Xia, Dewen Zeng, Hai-Bo Yi, and Chunhui Fang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp405168r • Publication Date (Web): 02 Aug 2013 Downloaded from http://pubs.acs.org on August 7, 2013

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The Journal of Physical Chemistry

Direct Contact Versus Solvent-Shared Ion Pairs in Saturated NiCl2 Aqueous Solution: A DFT, CPMD and EXAFS Investigation

Fei-Fei Xia,a Dewen Zeng,*a,b Hai-Bo Yi,*c and Chunhui Fanga a

b

Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, Qinghai 810008, China

College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, China c

College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan 410082, China

*Corresponding authors, Email: [email protected]; [email protected]

1

ACS Paragon Plus Environment


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Abstract: In this work, a systematic investigation of the competition coordination of H2O and Cl– with Ni2+ in saturated NiCl2 aqueous solution at room temperature was conducted using density functional theory (DFT), Car-Parrinello molecular dynamics (CPMD) simulations, and extended x-ray absorption fine structure (EXAFS) spectra. The calculated results reveal that the six-coordinated structure is favorable for [NiClx(H2O)n]2−x (x = 0−2; n = 1−12) clusters in the aqueous phase. The hydration energy calculation shows that the six-coordinated solvent-shared ion pair (SSIP) ([Ni(H2O)6(H2O)n−6Cl]+) is more stable than its contact ion pair (CIP) ([NiCl(H2O)5(H2O)n−5]+) isomer as n ≥ 9 in the aqueous phase, and the six-coordinated solvent-shared ion pair with a dissociated double Cl− (SSIP/d) ([Ni(H2O)6(H2O)n−6Cl2]0) is more preferable than its CIP ([NiCl2(H2O)4(H2O)n−4]0) and solvent-shared ion pair with single dissociated Cl− (SSIP/s) ([NiCl(H2O)5(H2O)n−5Cl]0) isomers as n ≥ 11. The six-coordinated SSIP/d ([Ni(H2O)6(H2O)n−6Cl2]0) conformers are the dominant structures in a saturated NiCl2(aq) solution (NiCl2 concentration: ~5.05 mol·kg−1, corresponding n ≈ 11). The CPMD simulations exhibited that there are six water molecules with Ni−O distance at ~205.0 pm on average around each Ni2+ in the first hydration sphere, even in the saturated NiCl2 aqueous solution (~5.05 mol·kg−1) at room temperature, and no obvious Ni−Cl interaction was found. The EXAFS spectra revealed that the first solvation shell of Ni2+ is an octahedral structure with six water molecules tightly bound in the NiCl2(aq) solution with a concentration ranging from 1.00 to 5.05 mol·kg−1, and there is no obvious evidence of Ni−Cl contact ion pairs. A comprehensive conclusion from the DFT, CPMD and EXAFS studies is that there is no obvious direct contact between Ni2+ and Cl–, even in saturated NiCl2 aqueous solution at room temperature. Keywords: ion hydration, hydrated cluster, density functional theory, molecular dynamics, extended x-ray absorption fine structure

2


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1. Introduction Insight into the local coordination structure around Ni2+ in NiCl2 aqueous solution is necessary for designing new methods to separate impurity elements, such as Co2+ and Cu2+, from NiCl2 aqueous solutions in hydrometallurgical processes.1−3 One key point lies in whether the Cl− enters the inner coordination sphere of the central Ni2+ in the NiCl2 aqueous solution. In the past few decades, the local structures of NiCl2 aqueous solutions have been extensively studied using various experimental techniques, e.g., extended x-ray absorption fine structure4−7 (EXAFS), neutron diffraction8−12 (ND), x-ray diffraction13−16 (XRD), x-ray absorption17 (XRA), ultra-violet spectroscopy18,19 (UV), and nuclear magnetic relaxation20 (NMR); however, the results are not consistent with each other. For instance, the authors in some studies4−13,17 detected no direct contact between Ni2+ and Cl− in NiCl2 aqueous solution, others14−16,21 reported the definite existence of direct Ni−Cl contact ion pairs (CIP) in NiCl2 aqueous solution. Liu et al.19 derived the stability constant of [NiCl]+aq species in NiCl2 aqueous solution from their UV detections but gave no account of the inner or outer sphere coordination between Ni2+ and Cl− in the [NiCl]+aq species. To obtain a comprehensive understanding of the structure of the NiCl2 aqueous solution, especially in its saturated concentration, we conducted a series studies using density functional theory (DFT),22,23 Car-Parrinello molecular dynamics (CPMD), and EXAFS methods.

2. Calculational and Experimental Methods 2.1 DFT Calculation DFT22,23 with Becke’s three-parameter exchange potential and Lee-Yang-Parr correlation functional (B3LYP)24 was used to determine the local minimum energy structures of the [NiClx(H2O)n]2−x (x = 0−2; n = 1−12) clusters, as used by Kim et al.25,26 for sodium and cesium halide aqueous solutions and by us27−29 for the CuCl2 aqueous solution. Dunning’s correlation consistent basis sets30 were also used, i.e., aug-cc-pVDZ for the non-metallic elements O, H, and Cl. For Ni, the relativistic effective core potentials (RECP) developed by the Stuttgart group were used 3



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in conjunction with the basis set to describe the metal valence electrons, and two sets of f and one set of g polarization functions were added.31 The valence space was described by the corresponding (6s5p3d) basis sets. The combination of these basis sets is abbreviated as aVDZ. The basis-set superposition errors (BSSE) of energies were corrected by the standard Boys-Bernardi counterpoise scheme.32 Recently, Goddard III et al.33 and Clark et al.34 employed mixed cluster/continuum models to investigate the solvation of Cu2+ and Pb2+, respectively, and their calculated results agree well with experimental observations. Thus, to investigate the long range electrostatic effect of the polarized continuum, we employed the polarized continuum model (PCM)33−36 in the calculation of the hydration energy for various hydrated [NiClx(H2O)n]2−x (x = 0−2; n = 1−12) clusters in the aqueous phase. It is well known that the dielectric constant ε of an aqueous solution decreases with the increasing salt concentration, to understand the effect of the dielectric constant on the calculated energy parameters of each conformers, we carried out the calculation in two cases, i.e., ε = 40.0 and ε = 78.4. The former is the hypothetical value of the continuum solvent H2O, equivalent to some real electrolyte solution with decreasing ε, the latter is for pure water at 298.15 K. All geometry optimization and frequency calculations were performed at the B3LYP/aVDZ level using the Gaussian 03 software package.37 An approximation to a complete solvation shell around [NiClx]2−x (x = 0−2) was calculated as in our previous articles25,26 (more details are given in the supporting information of this paper). The stabilities of the [NiClx(H2O)n]2−x (x = 0−2, n = 1-12) clusters can be compared using the hydration energy (∆E):

∆E = E[NiCl

2− x x (H 2 O)n ]

− ENi2+ − xECl− − E(H2O)n ( x = 0 − 2)

(1)

which corresponds to the process:

Ni 2+ + xCl- + (H 2 O)n ® [NiCl x (H 2 O) n ]2- x ( x = 0 - 2) The Gibbs energy of the above solvation reaction, ∆Gsolv, was obtained using PCM within the context of: 4


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∆Gsolv = ∑ Gproduct −∑ Greact

(2)

The Gibbs energy of each species is expressed as: 0 Gsolv = Egas + Gcorr + Gscrf + Gss

(3)

0 where Gcorr = PV − TS and Egas is the hydration energy of the standard state (298.15 K, 1 atm) in

the gas phase, and: Gscrf = Gelectrostatic + Gnonelectrostatic = Gelectrostatic + Gcavition + Gdispersion + Grepulsion

(4)

In the case of water:

Gss = RT ln( Pw / P 0 ) / n

(5)

Pw is the pressure of liquid water at room temperature, P0 is the saturated vapor pressure of liquid

water, R is the universal gas constant, and T is the room temperature in degrees kelvin.

2.2 CPMD Simulation The ab initio calculations were performed based on DFT22,23 implemented in the CPMD code.38 The electron exchange and correlation were taken into account by the generalized gradient approximation using the Becke–Lee–Yang–Parr (BLYP) functional,24c,39 which provides a good description of the structural properties of hydrated clusters. The interaction between the core and valence electrons is described by Troullier–Martins norm-conserving pseudopotential40 for the O, H, and Cl atoms with a plane-wave cutoff of 70 Ry. Furthermore, a nonlinear core correction (NLCC)41 was applied with a cutoff radius r0 = 1.6 Bohr for the Ni atom. Temperature control during the simulations was accomplished using a Nosé–Hoover algorithm to keep the systems in the so-called canonical ensemble (nVT-ensemble).42,43 The electrons were given a fictitious mass of 400 a.u.,44 and the hydrogen atoms in the simulation system were assigned the mass of deuterium to allow for a larger time step. A time step of 4.0 a.u. (0.097 fs) was employed to ensure suitable control of the conserved quantities. The CPMD simulations were run for 19.4 ps (200000 steps), with the first 100000 steps used for equilibration and the final 100000 steps used to collect the statistical data (in 5



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intervals of 10 steps).

2.3 EXAFS Spectra for NiCl2 Aqueous Solutions Five concentrations (1.00 m, 2.00 m, 3.00 m, 4.00 m, and 5.05 m (m means mol/kg water in this paper)) of NiCl2 aqueous solutions were studied by EXAFS spectra. Solid compounds, NiCl2·6H2O (AR, 98.0%) and NiSO4·6H2O (AR, 98.5%), were used as a comparison to the EXAFS spectra of the NiCl2 aqueous solutions. The EXAFS spectra at the Ni K-edge (8333.0 eV) were recorded in the transmission mode at the Beijing Synchrotron Radiation Facility (BSRF). The spectra were collected at room temperature with a Si(111) double-crystal monochromator, and a 50.0% harmonic rejection was achieved by slightly detuning the two crystals from parallel alignment. The storage ring ran at an energy of 1.5~2.2 GeV with 80 mA positron currents. The solution was kept in a cell with Kapton film windows. The EXAFS data analysis45−47 was performed using the IFEFFIT software,48−51 which is based on the theoretical calculation of the x-ray absorption fine spectra signal and a subsequent refinement of the structure parameters. The EXAFS oscillations, χ(k), are given by: 2R

χ (k ) = ∑ i

Fi (k ) S02 N i −2 k 2σ i2 − λ ( ki ) e e sin[2kRi + δ i (k )] kRi2

(6)

The sum in equation 6 covers all possible single scattering paths and all of the significant multiple scattering

paths.

The

wavenumber

of

the

ejected

photoelectron

is

expressed

by

k = 2me ( E − E0 ) / h 2 , where E0 is the absorption edge energy. Ri, Ni, and σ i2 are the path length, the number of equivalent paths and the Debye–Waller factor for a group of equivalent scattering paths, respectively. For single scattering paths, R and N are equal to the distance and the coordination number of the scattering atoms, respectively. σ i2 represents the mean-square variation in Ri due to both static and thermal disorder. Fi(k), δi(k) and λ(k) are the amplitude, phase and mean-free-path factors, respectively, which are derived from theoretical standards calculated by the software IFEFFIT.48,49 S02 is the core-hole or amplitude-reduction factor, which is usually 6


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treated empirically. The Ni χ(k) data were generally weighted by k2 and windowed between 2.0 < k < 11.0 Å using a Hanning window with dk = 1.0 Å. The fitting was applied to both the real and imaginary parts of χ(R) in the region of 1.3 < R < 3.0 Å.

3. Results and Discussion 3.1 Structures and Energetics To investigate the hydration nature of NiCl2 aqueous solutions, a series of low-lying conformers of the [NiClx(H2O)n]2−x (x = 0−2; n = 1−12) clusters were studied at the B3LYP/aVDZ level based on their bond and energy parameters. More low-lying conformers of the [NiClx(H2O)n]2−x (x = 0−2; n = 1−12) clusters and their bond and energy parameters are presented in supporting information. 3.1.1 [Ni(H2O)n]2+ Clusters The B3LYP/aVDZ optimized low-lying energy structures of the [Ni(H2O)n]2+ (n = 6-12) clusters in the gas phase are shown in Figure 1, and the bond and energy parameters for each conformer at the B3LYP/aVDZ level are listed in Table 1.

W6-4L

W6-5L

W6-6L

W7-4L

W7-5L

W7-6L

W8-4L

W8-5L

W8-6L

W9-4L

W9 -5L

W9-6L

7



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W10-5L

W10-6L

W11-5L

W11-6L

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W12-5L

W12-6L

Figure 1. Typical optimized structures of the [Ni(H2O)n]2+ clusters for n = 6–12 at the B3LYP/aVDZ level. More optimized structures are collected in the supporting information. W and L are the abbreviations for water and ligand, respectively. The hydrogen bonds are expressed as dashed lines.

TABLE 1: B3LYP/aVDZ Bond and Energy Parameters of the [Ni(H2O)n]2+ Clusters for n = 6–12 in the Gas and Aqueous Phases bond geometries parametersa RNi−O W6-4L 197.2 W6-5L 203.7 W6-6L 207.8 W7-4L 196.4 W7-5L 203.5 W7-6L 208.1 W8-4L 195.6 W8-5L 203.5 W8-6L 208.3 W9-4L 195.4 W9-5L 202.7 W9-6L 208.1 W10-5L 202.2 W10-6L 207.5 W11-5L 202.1 W11-6L 207.8 W12-5L 202.2 W12-6L 207.8

∆E -306.2 -309.8 -312.5 -314.9 -325.2 -325.5 -320.8 -330.2 -329.5 -328.8 -342.1 -340.6 -352.5 -352.5 -357.6 -360.7 -357.9 -362.3

gas phase ∆E0 -303.1 -307.4 -310.7 -311.2 -324.0 -324.6 -319.7 -329.9 -330.0 -328.1 -341.4 -340.5 -352.5 -352.6 -357.0 -359.6 -359.3 -363.1

∆G -291.4 -295.6 -298.8 -302.1 -315.2 -315.4 -312.8 -323.0 -323.6 -321.8 -334.1 -332.9 -347.0 -344.7 -350.6 -350.9 -355.3 -356.7

energy parametersb aqueous phase (ε = 40.0) ∆Esolv ∆Esolv0 ∆Gsolv -478.8 -475.7 -464.0 -487.7 -485.3 -473.5 -494.5 -492.7 -480.9 -480.1 -476.4 -467.2 -494.2 -492.7 -483.8 -499.7 -499.1 -489.9 -479.1 -477.9 -471.0 -493.6 -493.3 -496.4 -498.5 -499.1 -491.8 -478.8 -478.1 -471.8 -495.3 -494.6 -487.3 -499.7 -499.6 -491.9 -496.2 -496.2 -490.7 -501.1 -501.2 -493.3 -496.7 -496.4 -490.3 -504.0 -502.7 -494.0 -500.9 -502.3 -498.3 -507.7 -508.5 -502.1

aqueous phase (ε = 78.4) ∆Esolv ∆Esolv0 ∆Gsolv -481.3 -478.2 -466.5 -490.3 -487.9 -476.1 -497.1 -495.4 -483.5 -481.9 -478.2 -469.1 -496.7 -495.2 -486.3 -502.2 -501.6 -492.5 -481.3 -480.2 -473.3 -495.9 -495.6 -488.8 -500.9 -501.4 -495.0 -481.0 -480.3 -474.0 -497.5 -496.8 -489.5 -502.0 -501.9 -494.2 -498.4 -498.4 -492.9 -503.3 -503.4 -495.5 -500.6 -500.3 -494.2 -506.1 -504.9 -496.3 -503.0 -504.4 -500.4 -509.8 -510.6 -504.2

a

RNi−O is the averaged Ni–O distance in pm for [Ni(H2O)n]2+ clusters. b∆E is the hydration energy, ∆E0 is the

zero-point corrected electronic energy, ∆G is free energy in the gas phase, and ∆Esolv, ∆Esolv0 and ∆Gsolv are the hydration energy, zero-point corrected electronic energy and free energy in the aqueous phase, respectively. ε is the dielectric constant of aqueous solution; Energies are in kcal/mol (298.15 K and 1 atm); ∆Esolv, ∆Esolv0 and ∆Gsolv were obtained using the PCM-B3LYP/aVDZ method.

Based on the parameters ∆E and ∆Esolv0 in Table 1, the four-coordinated conformer is less stable than the five- and six-coordinated conformers in both the gas phase and aqueous phase (ε =78.4) for the [Ni(H2O)n]2+ (n ≥ 6) clusters, whereas the six-coordinated conformer is the most stable structure in the aqueous phase (ε = 78.4). The six-coordinated conformers are approximately 5.0−7.0 and 17.0−23.0 kcal/mol more stable than the five- and four-coordinated isomers in the aqueous phase (ε = 78.4), respectively. Meanwhile, it can be seen from Figure 2 that the five- and 8


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six-coordinated conformers are iso-energetic in the gas phase for n = 6−10, while six-coordinated conformers are preferable as n ≥ 6 in the aqueous phases (ε = 78.4) and four-coordinated conformers are unfavorable in both gas phase and aqueous phase (ε = 78.4). Thus, Ni2+ prefers to form the six-coordinated conformer rather than the four- and five-coordinated conformers in a solution with nH 2 O : nNi 2+ ≥ 6. aqueous phase

4L 5L 6L

∆Esolv(kcal/mol)

-480 -488 -496 -504 -512 6

7

8

9 10 n/H2O

11

12

Figure 2. The stability of the different coordinated conformers for the [Ni(H2O)n]2+ cluster in the aqueous phase (ε = 78.4) with the increasing number of water molecule n.

The hydration energy of the six-coordinated [Ni(H2O)6]2+ clusters of approximately 310.0 kcal/mol indicates that Ni2+ is strongly restricted by water molecules in the first hydration shell. However, the averaged Ni−O distance of the six-coordinated conformers varies slightly as the involving water molecule increases to 6, which suggests that the outer hydration shell water molecule has little impact on the inner hydration shell. 210 203 RNi-O/pm

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


196 189 2

4

6 8 n/H2O

10

12

Figure 3. The variation of Ni−O bond distance for the [Ni(H2O)n]2+ cluster with the increasing number of water molecule n.

3.1.2 [NiCl(H2O)n]+ Clusters The B3LYP/aVDZ optimized low-lying energy structures of the [NiCl(H2O)n]+ (n = 6-12) 9



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clusters in the gas phase are shown in Figure 4, and the bond and energy parameters for each conformer at the B3LYP/aVDZ level are listed in Table 2.

W6Cl-6L-CIP

W6Cl-6L-SSIP

W7Cl-6L-CIP

W7Cl-6L-SSIP

W9Cl-6L-CIP

W9Cl-6L-SSIP

W10Cl-6L-CIP

W10Cl-6L-SSIP

W8Cl-6L-CIP

W11Cl-6L-SSIP

W8Cl-6L-SSIP

W12Cl-6L-SSIP

Figure 4. Typical optimized structures of the [NiCl(H2O)n]+ clusters with n = 6–12 at the B3LYP/aVDZ level. More optimized structures are collected in the supporting information. W and L are the abbreviations for water and ligand, respectively. The hydrogen bonds are expressed as dashed lines.

TABLE 2: B3LYP/aVDZ Bond and Energy Parameters of the [NiCl(H2O)n]+ Clusters for n = 6–12 in the Gas and Aqueous Phases bond parametersa geometries RNi−Cl RNi−O W6Cl-6L-CIP 232.9 210.7 W6Cl-6L-SSIP 358.7 208.4 W7Cl-6L-CIP 235.3 211.1 W7Cl-6L-SSIP 359.2 208.5 W8Cl-6L-CIP 240.7 209.7 W8Cl-6L-SSIP 405.9 208.5 W9Cl-6L-CIP 247.1 209.3 W9Cl-6L-SSIP 409.3 208.7 W10Cl-6L-CIP 248.5 209.0 W10Cl-6L-SSIP 419.6 208.3 W11Cl-6L-CIP 251.1 208.1 W11Cl-6L-SSIP 421.1 208.7 W12Cl-6L-CIP 250.2 208.3 W12Cl-6L-SSIP 431.8 208.9

∆E -522.6 -509.4 -528.3 -516.4 -527.9 -516.4 -530.6 -523.4 -537.9 -533.1 -541.9 -537.6 -539.4 -533.0

gas phase ∆E0 ∆G -519.1 -498.9 -506.9 -486.0 -525.7 -507.4 -514.7 -496.6 -526.3 -509.7 -515.5 -498.4 -528.6 -511.9 -521.4 -503.1 -535.4 -517.8 -530.7 -513.0 -540.0 -522.8 -534.4 -515.6 -539.0 -523.6 -532.1 -514.9

energy parametersb aqueous phase (ε = 40.0) ∆Esolv ∆Esolv0 ∆Gsolv -573.0 -569.4 -549.2 -571.5 -568.9 -548.0 -577.7 -575.1 -556.8 -576.8 -575.0 -557.0 -574.8 -573.1 -556.5 -574.4 -573.5 -556.4 -573.1 -571.1 -554.4 -575.0 -573.0 -556.7 -575.5 -573.0 -556.2 -580.0 -577.6 -559.8 -577.2 -575.3 -558.1 -580.8 -578.6 -559.7 -581.9 -581.5 -566.0 -585.2 -584.3 -567.0

aqueous phase (ε = 78.4) ∆Esolv ∆Esolv0 ∆Gsolv -574.0 -570.4 -550.2 -572.7 -570.2 -549.2 -578.7 -576.1 -557.8 -578.0 -576.3 -558.2 -575.7 -574.0 -557.4 -575.0 -574.6 -557.5 -573.9 -572.0 -555.3 -576.3 -575.0 -557.7 -576.3 -573.8 -556.2 -581.0 -578.7 -560.9 -578.1 -576.2 -559.0 -582.8 -579.6 -560.8 -582.8 -582.4 -567.0 -586.0 -585.0 -567.8

a

RNi−Cl and RNi−O are the Ni−Cl and Ni–O distances in pm for [NiCl(H2O)n]+ clusters, respectively. b∆E is the

hydration energy, ∆E0 is the zero-point corrected electronic energy, ∆G is free energy in the gas phase. ∆Esolv, ∆Esolv0 and ∆Gsolv are the hydration energy, zero-point corrected electronic energy and free energy in the aqueous phase, respectively. ε is the dielectric constant of aqueous solution; Energies are expressed in kcal/mol (298.15 K and 1 atm); ∆Esolv, ∆Esolv0 and ∆Gsolv were obtained using the PCM-B3LYP/aVDZ method. 10


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Similar to the [Ni(H2O)n]2+ clusters, the six-coordinated CIP ([NiCl(H2O)5(H2O)n−5]+) and SSIP ([Ni(H2O)6(H2O)n−6Cl]+) conformers are generally more stable than their four- and five-coordinated isomers in both the gas phase and aqueous phase (ε = 78.4) (more data in supporting information). From Table 2, we can see that the six-coordinated CIP conformer ([NiCl(H2O)5(H2O)n−5]+) is more stable than the SSIP isomer ([Ni(H2O)6(H2O)n−6Cl]+) in the gas phase for n = 6−12 and less stable in the aqueous phase (ε = 78.4) for n ≥ 9. Meanwhile, Figure 5 also shows that the six-coordinated SSIP ([Ni(H2O)6(H2O)n−6Cl]+) conformer is more stable than its six-coordinated CIP isomer ([NiCl(H2O)5(H2O)n−5]+) in the aqueous phase (ε = 78.4) as n ≥ 9. -572

aqueous phase

∆ Esolv(kcal/mol)

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


6L CIP 6L SSIP

-576 -580 -584 6

7

8

9 10 n/H2O

11

12

Figure 5. The stability of the six-coordinated CIP and SSIP conformers for the [NiCl(H2O)n]+ cluster in the aqueous phase with the increasing number of water molecule n.

3.1.3 [NiCl2(H2O)n]0 Clusters The B3LYP/aVDZ optimized low-lying energy structures of the [NiCl2(H2O)n]0 (n = 6-12) clusters in the gas phase are shown in Figure 6, and the bond and energy parameters for each conformer at the B3LYP/aVDZ level are listed in Table 3.

W6Cl2-6L-CIP

W9Cl2-6L-SSIP/s

W6Cl2-6L-SSIP/s

W9Cl2-6L-SSIP/d

W7Cl2-6L-CIP

W7Cl2-6L-SSIP/s

W8Cl2-6L-CIP

W8Cl2-6L-SSIP/s

W10Cl2-6L-SSIP/s W10Cl2-6L-SSIP/d W11Cl2-6L-SSIP/d W12Cl2-6L-SSIP/d 11



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 6. Typical optimized structures of [NiCl2(H2O)n]0 (n = 6–12) at the B3LYP/aVDZ level. More optimized structures are collected in the supporting information. W and L are the abbreviations for water and ligand, respectively. The hydrogen bonds are expressed as dashed lines.

TABLE 3: B3LYP/aVDZ Bond and Energy Parameters of the [NiCl2(H2O)n]0 Clusters for n = 6–12 in the Gas and Aqueous Phases geometries W6Cl2-6L-CIP W6Cl2-6L-SSIP/s W7Cl2-6L-CIP W7Cl2-6L-SSIP/s W8Cl2-6L-CIP W8Cl2-6L-SSIP/s W9Cl2-6L-SSIP/s W9Cl2-6L-SSIP/d W10Cl2-6L-SSIP/s W10Cl2-6L-SSIP/d W11Cl2-6L-SSIP/s W11Cl2-6L-SSIP/d W12Cl2-6L-SSIP/s W12Cl2-6L-SSIP/d

bond parametersa RNi−O RNi−Cl RNi−Cld 211.2 240.7 211.1 234.2 371.8 210.2 242.4 210.1 234.9 438.5 209.4 245.4 210.2 240.4 433.1 210.1 242.9 437.2 209.4 396.8 209.6 248.6 436.4 208.7 433.4 211.5 245.8 441.0 209.5 433.5 211.6 242.1 449.8 210.1 436.9

∆E -644.4 -642.6 -646.4 -646.3 -640.4 -641.4 -643.1 -637.4 -647.2 -644.2 -645.5 -647.9 -640.1 -640.9

gas phase ∆E0 ∆G -640.2 -612.3 -638.2 -609.9 -644.2 -620.0 -643.1 -617.0 -639.8 -617.5 -639.2 -614.5 -640.9 -616.4 -634.9 -609.2 -645.1 -620.8 -642.0 -616.9 -642.7 -617.3 -644.4 -617.3 -639.5 -615.8 -639.7 -614.8

energy parametersb aqueous phase (ε = 40.0) aqueous phase (ε = 78.4) ∆Esolv ∆Esolv0 ∆Gsolv ∆Esolv ∆Esolv0 ∆Gsolv -646.6 -642.4 -614.5 -647.0 -642.8 -615.0 -647.0 -642.5 -614.2 -647.4 -643.0 -614.7 -650.4 -648.3 -624.0 -650.9 -648.7 -624.5 -652.1 -648.9 -622.9 -652.6 -649.4 -623.4 -646.9 -646.3 -624.0 -647.4 -646.8 -624.5 -648.7 -647.2 -623.6 -650.7 -648.5 -623.8 -648.7 -646.5 -622.0 -649.2 -646.9 -622.5 -648.3 -645.8 -620.2 -648.9 -646.4 -620.7 -649.7 -647.6 -623.3 -650.2 -648.1 -623.8 -650.9 -649.0 -623.5 -651.1 -649.0 -623.8 -646.4 -643.6 -618.1 -646.9 -644.1 -618.7 -651.4 -647.8 -620.8 -651.9 -648.4 -621.3 -650.8 -650.1 -626.5 -651.4 -650.7 -627.1 -653.1 -651.8 -627.0 -653.7 -652.4 -627.6

a

RNi−Cl and RNi−O are the Ni−Cl and Ni–O distances in pm for [NiCl2(H2O)n]0 clusters, and RNi−Cld is the distance

between Ni2+ and the dissociated Cl−. b∆E is the hydration energy, ∆E0 is the zero-point corrected electronic energy, ∆G is free energy in the gas phase. ∆Esolv, ∆Esolv0, ∆Gsolv are the hydration energy, zero-point corrected electronic energy and the free energy in the aqueous phase, respectively. ε is the dielectric constant of aqueous solution; Energies are expressed in kcal/mol (298.15 K and 1 atm); ∆Esolv, ∆Esolv0 and ∆Gsolv were obtained using the PCM-B3LYP/aVDZ method.

We systematically calculated the hydration energy of the four-, five- and six-coordinated conformers of the [NiCl2(H2O)n]0 clusters and found that the six-coordinated conformers were preferable for both the CIP and SSIP structures in the gas phase and aqueous phase (ε = 78.4). The structure configurations and thermodynamic properties of the six-coordinated conformers are presented in Figure 6 and Table 3 and those of the four- and five-coordinated conformers are listed in the supporting information of this paper. The energy parameters in Table 3 present that the six-coordinated CIP ([NiCl2(H2O)4(H2O)n−4]0) and SSIP/s ([NiCl(H2O)5(H2O)n−5Cl]0) conformers clusters are iso-energetic in the aqueous phase (ε = 78.4) as n = 6−8. As the water molecules increase to 9, the six-coordinated SSIP/s 12


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([NiCl(H2O)5(H2O)n−5Cl]0) conformer becomes more favorable than its six-coordinated CIP isomer ([NiCl2(H2O)4(H2O)n−4]0) in the aqueous phase (ε = 78.4), which implies that one Cl− tends to dissociate under these conditions. Meanwhile, the six-coordinated SSIP/s ([NiCl(H2O)5(H2O)n−5Cl]0) and SSIP/d ([Ni(H2O)6(H2O)n−6Cl2]0) conformers are also iso-energetic as n = 9 and 10 in the aqueous phase (ε = 78.4). For n ≥ 11, the six-coordinated SSIP/d ([Ni(H2O)6(H2O)n−6Cl2]0) conformer (double dissociated Cl−) is more stable than the SSIP/s ([NiCl(H2O)5(H2O)n−5Cl]0) and CIP ([NiCl2(H2O)4(H2O)n−4]0) conformers in both the gas phase and aqueous phase (ε = 78.4), that is, the second Cl− also tends to dissociate. Figure 7 also shows that the six-coordinated SSIP/d ([Ni(H2O)6(H2O)n−6Cl2]0) conformers are preferable in the aqueous phase (ε = 78.4) as n ≥ 11. aqueous phase ∆Esolv(kcal/mol)

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


-640

6L CIP 6L SSIP/s 6L SSIP/d

-644 -648 -652 6

7

8

9 10 n/H2O

11

12

Figure 7. The stability of the six-coordinated CIP, SSIP/s and SSIP/d conformers for the [NiCl2(H2O)n]0 cluster in the aqueous phase (ε = 78.4) with the increasing number of water molecule n.

Besides, we calculated the energy parameters of each conformers in a hypothetical aqueous solution with the dielectric constant ε = 40.0 and presented them in Tables 1−3. It can be easily drawn that the variation of the dielectric constant for the aqueous phase in a certain extent doesn’t change qualitatively the calculated results. These results suggest that Ni2+ exists mainly as the SSIP/d conformer in a saturated NiCl2 aqueous solution at 298.15 K, which can be approximately treated as the [Ni(H2O)6(H2O)n−6Cl2]0 cluster with n ≈ 11, corresponding to a solubility of 5.05 m.52,53

3.2 CPMD Simulations CPMD simulations were performed using the BLYP functional, and the wave functions of the valance electrons were represented by the plane wave basis sets. Considering the calculation accuracy and efficiency, we simulated two systems, 1 molecule of NiCl2 plus 44 molecules of H2O 13



and 4 molecules of NiCl2 plus 44 molecules of H2O, and present the simulated results in Figure 8. 8

18

30

9

60

15

6

6

40

3

20

12

20

6

10

0 0

1

2

3 4 rNi-O/Å

5

6

0 7

0 0

1

2

3 4 rNi-H/Å

5

6

gNiCl(r)

20

gNiH(r)

80

gNiO(r)

12

0 7

10

4

5

2

0 0

1

2

3 4 rNi-Cl/Å

5

6

nNiCl(r)

40

nNiH(r)

24

nNiO(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

Page 14 of 26

0 7

Figure 8. Radial distribution functions (RDFs, g(r), full lines) and running coordination number (n(r), dashed lines) of Ni2+-H2O and Ni2+-H2O interactions for two different simulation systems (red line stands the system consists 1NiCl2+44H2O, equal to 1.26 m NiCl2 aqueous solution; blue line stands the system consists 4NiCl2+44H2O, equal to 5.05 m NiCl2 aqueous solution).

As shown in Figures 8, the first Ni−O peak calculated by the CPMD simulation is located at 205.0 pm for both the 1.26 m and the 5.05 m NiCl2 aqueous solutions. Correspondingly, the running coordination number, n(r), for the first Ni−O peak is 6. The Ni−O distance is 207−209 pm, and the preferable coordination number is six according to the B3LYP/aVDZ calculation in this work for the 5.05 m NiCl2 solution (see Table 3); both results support the same conclusion. For the 5.05 m NiCl2 solution, the first Ni−Cl peak determined by the CPMD simulation is located at ~425 pm, which is consistent with the Ni−Cl distance (433 pm) for the [NiCl2(H2O)11]0 clusters in the SSIP/d structure, but much longer than the Ni−Cl distance (254 pm) in the CIP structure. Both the CPMD simulation and the DFT calculation suggest that there is no obvious Ni−Cl direct interaction even in saturated NiCl2 solution (5.05 m) at room temperature.

3.3 EXAFS Analysis of NiCl2 Aqueous Solutions An abundance of EXAFS detections have been reported for the NiCl2 aqueous solution at different concentrations, e.g., 2.78 m and 3.74 m,4 0.5 m and 4.0 m,5 4 m,6 and 2 m.7 All of these studies have found no evidence for the direct contact of Ni−Cl in NiCl2 aqueous solutions at concentrations below the solubility limit of NiCl2 at 298 K. Whether direct contact of Ni−Cl arises in the saturated NiCl2 solution (5.05 m) is unknown. We conducted EXAFS spectra detection on the saturated NiCl2 aqueous solution (5.05 m) and several solutions at lower concentrations. 14


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Additionally, nickel chloride hexahydrate (NiCl2·6H2O(s), with −[NiCl2(H2O)4]0 moieties54) and nickel sulfate hexahydrate (NiSO4·6H2O(s), with −[Ni(H2O)6]2+ moieties55) were also analyzed to provide qualitative fingerprinting information regarding the local coordination environment of Ni. The detected EXAFS spectra (Ni K-edge absorption spectra) are presented in Figure 9a and were transformed to the k-space spectra (Figure 9b) and the R-space spectra (Figure 9c) by Fourier transform. The Ni K-edge absorption spectra of NiCl2·6H2O(s) and NiSO4·6H2O(s) are similar except for small differences in the intensity and shifting of the first band from 8340 to 8370 eV (Figure 9a). These differences could be attributed to the replacement of two water molecules in NiSO4·6H2O(s) by two chloride ions in NiCl2·6H2O(s). The experimental EXAFS k2-weight ‫׀‬χ(R)‫ ׀‬spectra (Figure 9c) of the different concentrations of NiCl2 solutions are almost identical with those of NiSO4·6H2O(s). This indicates that few water molecules can be substituted by Cl− even in the saturated NiCl2 aqueous solution (5.05 m), because [Ni(H2O)6]2+ (an octahedral structure) is the unique moiety of Ni2+ in NiSO4·6H2O(s). 2.0

1.6

Normalized absorption

2.4 NiCl2· 6H2O

5.05 m NiCl2

1.0

4.00 m NiCl2 3.00 m NiCl2

0.5

2.00 m NiCl2

0.0 8300

1.00 m NiCl2

8400

8500 8600 Energy/eV

8700

–3


0.0


-0.8 -1.6

NiCl2·6H2O

NiSO4· 6H2O

0.8

|χ(R)| (Å )

NiSO4·6H2O

–2

1.5

2

NiCl2·6H2O

k χ(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


1.00 m NiCl2

0

(a) Ni K-edge absorption spectra

2

4

6 –1 8 10 12 k (Å ) (b) EXAFS k-space spectra

NiSO4·6H2O

1.8


1.2


0.6

1.00 m NiCl2

0.0 0

1 2 3 R/Å (c) EXAFS R-space spectra

4

Figure 9. Experimental EXAFS results of nickel complexes (NiCl2·6H2O(s) and NiSO4·6H2O(s)) and different concentrations of NiCl2 solutions (1.00−5.05 m) at 25oC and 1 atm: (a) Ni K-edge absorption spectra; (b) EXAFS k-space spectra; (c) EXAFS R-space spectra.

TABLE 4: EXAFS Fitting Results of the Reference Solid Compounds (NiCl2·6H2O(s) and NiSO4·6H2O(s)) and NiCl2 Aqueous Solutions at Different Concentrations samples 1.00 m 2.00 m 3.00 m 4.00 m 5.05 m

structural parametersa CNO RNi−O/pm 5.99 205.6 5.94 205.8 5.86 205.9 5.95 206.4 5.93 207.5

σ2/Å2 x 10−3

∆E0/eV

k-range

R-range

R-factor

7.03 7.96 7.87 8.79 9.29

−5.18 −6.10 −5.24 −4.78 −4.46

2~11 2~11 2~11 2~11 2~11

1.3~3 1.3~3 1.3~3 1.3~3 1.3~3

0.00174 0.00135 0.00124 0.00079 0.00228

15



NiSO4·6H2O 6.50 204.3 8.14 −1.33 2~11 1.3~3 0.00331 NiCl2·6H2O 3.48 203.2 9.51 −3.80 2~11 1.3~3 0.00214 a These EXAFS fitting results are only the first solvation shell of Ni2+ for both the solid compounds and NiCl2 aqueous solutions.

5 4 NiSO4·6H2O –3

|χ(R)| (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 16 of 26

3

1.00 m NiCl2 2.00 m NiCl2 3.00 m NiCl2

2

4.00 m NiCl2

1

5.05 m NiCl2 NiCl2·6H2O

0 0

1

2 3 R(Å)

4

5

Figure 10. Ni EXAFS k2-weight ‫׀‬χ(R)‫ ׀‬plots for the solid compounds (NiCl2·6H2O(s) and NiSO4·6H2O(s)) and different concentrations of NiCl2 aqueous solutions. The solid lines are the experimental data, and the dashed lines are the fitting results. The Fourier transform spectra were calculated in the k-range 2.0−11.0 Å with no phase shift correction applied.

For fitting the experimental EXAFS spectra data, only the first coordination shell of Ni2+ was considered in this work. The fitted k2-weighted |χ(R)| plots (with no phase shift correction applied) are presented in Figure 10, and the obtained EXAFS parameters based on the k2-weighted fits in the R-space are given in Table 4. The main peak and the following weak peak are present for all of the NiCl2 solutions and NiSO4·6H2O(s) and correspond to the bonds of Ni−O and Ni−H in the first coordination shell. The bond distances of Ni−O and Ni−H are listed in Table 4 with the phase shift correction. Compared with the peaks for the NiCl2 aqueous solutions and NiSO4·6H2O(s), the main peak for NiCl2·6H2O(s) is increased in width and the following weak peak disappears. This information can be used as a reference for judging whether any H2O in the solvation shell is substituted by Cl−.

TABLE 5: Comparison of the Structural Parameters Obtained by Different Methods for the Different Concentrations of NiCl2 Aqueous Solutions NiCl2 concentration

methods

CNO

RNi−O/pm

16


reference

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


2.00 m 2.78 m 3.74 m 4.00 m 5.05 m 5.05 m 5.05 m

EXAFS EXAFS EXAFS EXAFS EXAFS CPMD B3LYP

6.5±1.0 7.0±1.0 6.2±1.0 6.8±1.0 5.93 6.0 6.0

206.0 206.0 207.0 205.9 207.5 206.0 209.5

7 4 4 6 this work this work this work

We summarized the coordination number and the Ni−O distance in the first solvation shell of Ni2+ determined by different methods in this work and in the literature4,6,7 and present the data in Table 5. The Ni−O bond distances obtained by the B3LYP/aVDZ, CPMD, and EXAFS methods in this work agree with each other and with the values reported in the literature.4,5,7 In addition, the coordination number of Ni2+ is about 6.0 for all of these different methods. Therefore, the SSIP/d conformer with double dissociated Cl− is the dominant species even in saturated NiCl2 aqueous solutions, and there is no obvious Ni−Cl direct contact.

4. Conclusions In the present paper, [NiClx(H2O)n]2−x (x = 0−2; n = 1−12) clusters were exhaustively and systematically investigated using the B3LYP/aVDZ method. In addition, the saturated NiCl2 aqueous solution at 298 K was studied by the CPMD and EXAFS methods. The following conclusions can be drawn: According to our DFT calculation, the six-coordinated structures of the [NiClx(H2O)n]2−x (x = 0−2) clusters are more stable than the five-coordinated structure in the aqueous phase. The hydration energy calculation shows that the six-coordinated SSIP ([Ni(H2O)6(H2O)n−6Cl]+) is more stable than its six-coordinated CIP ([NiCl(H2O)5(H2O)n−5]+) isomer as n ≥ 9, and the SSIP/d ([Ni(H2O)6(H2O)n−6Cl2]0) cluster is more stable than the CIP ([NiCl2(H2O)4(H2O)n−4]0) and the SSIP/s ([NiCl(H2O)5(H2O)n−5Cl]0) isomers as n ≥ 11. Therefore, the six-coordinated SSIP/d ([Ni(H2O)6(H2O)n−6Cl2]0) conformers are the dominant structures in a saturated NiCl2(aq) solution (5.05 m) at the room temperature. The CPMD simulation results indicated that there are six water molecules with an Ni−O distance at 205.0 pm on average around each Ni2+ in the first hydration sphere, and there was no obvious Ni−Cl direct interaction observed during our CPMD simulation. 17



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

Moreover, the EXAFS detection of the aqueous NiCl2 solution showed that the first solvation shell of Ni2+ is an octahedral structure with six tightly bound water molecules in all of the NiCl2 aqueous solutions (1.00−5.00 m), and there was no obvious evidence of Ni−Cl contact ion pairs even in the saturated aqueous NiCl2 solution. All of these calculated and experimental results show that the six-coordinated SSIP/d ([Ni(H2O)6(H2O)n−6Cl2]0, n ≥ 11) conformer is the predominant structure even in saturated NiCl2 aqueous solution at 298 K. The six-coordinated SSIP/s ([NiCl(H2O)5(H2O)6Cl]0) conformer also might exist in the saturated NiCl2 aqueous solution with such a small partition according to the DFT calculation that its existence cannot be detected by the methods of CPMD simulation and EXAFS detection.

18


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Acknowledgment This work was supported financially by the National Natural Science Foundation of China under contracts 21073056, 51134007 and 11079047. Prof. Tiandou Hu, Dr. Jing Zhang and Dr. Lirong Zheng are sincerely acknowledged for their support in the EXAFS experiments performed in the Beijing Synchrotron Radiation Facility (BSRF).

Supporting Information Available: More optimized local minimum energy structures and energy parameters of the [NiClx(H2O)n]2−x (x = 0−2) cluster for n = 1–12 at the B3LYP/aVDZ level are available free of charge via the Internet at http://pubs.acs.org.

19



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References and Notes (1) Habib, A.; Wahiduzzaman, M.; Rashid, H. O.; Islam, A.; Ferdoushi, F. K.; Alam, A. M. S. Selective Extraction of Co(II) in the Presence of Mn(II), Ni(II) and Cu(II) Using Salting-out Phase Separation Method. Pak. J. Anal. Environ. Chem. 2008, 9, 6−10. (2) Sadyrbaeva, T. Zh. Electrodialytic Separation of Cobalt(II) and Nickel(II) Using Liquid Membranes Based on Tri-n-octylamine and Trialkylbenzylammonium Chloride. Russ. J. Appl. Chem. 2013, 86, 186−191. (3) Ghomri, F.; Lahsini, A.; Laajeb, A.; Addaou, A. The Removal of Heavy Metal Ions (Copper, Zinc, Nickel and Cobalt) by Natural Bentonite. Larhyss Journal. 2013, 12, 37−54. (4) Sandstrom, D. R. Ni2+ Coordination in Aqueous NiCl2 Solutions: Study of the Extended X-ray Absorption Fine Structure. J. Chem. Phys. 1979, 71, 2381−2386. (5) Lagarde, P.; Fontaine, A.; Raoux, D.; Sadoc, A.; Migliardo, P. EXAFS Studies of Strong Electrolytic Solutions. J. Chem. Phys. 1980, 72, 3061−3069. (6) Licheri, G.; Paschina, G.; Piccaluga, G.; Pinna, G.; Vlaic, G. EXAFS Study of Ni2+ Coordination in Concentrated Aqueous Solution. Chem. Phys. Lett. 1981, 83, 384−387. (7) Licheri, G.; Paschina, G.; Piccaluga, G.; Pinna, G. EXAFS Study of Ni–Cl Bonding in Ni(II) Aqueous Solutions at Increasing Cl−/Ni2+ Ratios. J. Chem. Phys. 1983, 79, 2168−2171. (8) Neilson, G. W.; Enderby, J. E. The Hysration of Ni2+ in Aqueous Solution. J. Phys. C. Solid State Phys. 1978, 11, L625−L628. (9) Neilson, G. W.; Enderby, J. E. The Structure of an Aqueous Solution of Nickel Chloride. Proc. R. Soc. Lond. A 1983, 390, 353−371. (10) Powell, D. H.; Neilson, G. W. The Concentration Dependence of the Ni2+ Hydration Geometry in Aqueous Solution. J. Phys. Condens.Matter 1990, 2, 3871−3878. (11) Howell, I.; Neilson, G. W. Ni2+ Coordination in Concentrated Aqueous Solutions. J. Mol. Liq.

1997, 73,74, 337−348. (12) Badyal, Y. S.; Simonson. J. M.; Annis, B. K.; Londono, J. D. The Hydration Structure of Ni2+ 20


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Diffraction. J. Chem. Phys. 1969, 50, 4690−4696. (55) Rousseau, B.; Maes, S. T.; Lenstra, A. T. H. Systematic Intensity Errors and Model Imperfection as the Consequence of Spectral Truncation. Acta Cryst A. 2000, 56, 300−307.

25



TOC 2.4 NiCl2·6H2O NiSO4·6H2O

1.8

5.05 m NiCl2

–3

|χ(R)| (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

4.00 m NiCl2

1.2


0.6

1.00 m NiCl2

0.0 0

1

2 3 R/Å EXAFS R-space spectra

26


4

Page 26 of 26

Direct Contact versus Solvent-Shared Ion Pairs in Saturated NiCl

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