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    Direct Contact versus Solvent-Shared Ion Pairs in Saturated NiCl2 Aqueous Solution: A DFT, CPMD, and EXAFS Investigation
    Update time: 2013-03-10
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    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 to 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.


    Original paper link:http://pubs.acs.org/doi/abs/10.1021/jp405168r

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