Issue 11, 2013

Hydration of the calcium(ii) ion in an aqueous solution of common anions (ClO4, Cl, Br, and NO3)

Abstract

Raman spectra of aqueous calcium salt solutions, Ca(ClO4)2, CaCl2, CaBr2, and Ca(NO3)2, were measured from the concentrated solution stage to more dilute solutions (6.08–0.1 mol L−1) at 23 °C in water and heavy water down to 40 cm−1. In aqueous Ca(ClO4)2 solutions a strongly polarized band at 283 cm−1 (full width at half height (fwhh) = 68 cm−1) was observed. The mode at 283 cm−1 was assigned to the Ca–O symmetric stretching vibration of the hexa-aqua Ca2+ ion, [Ca(OH2)6]2+, and the integrated band intensity showed a linear dependency with Ca(ClO4)2 concentration. In a Ca(ClO4)2 solution of heavy water a similar band was observed at 268 cm−1 (fwhh = 64 cm−1) of the deuterated species, [Ca(OD2)6]2+. In the OH stretching region of water a band of weakly H-bonded O–H oscillators was detected at 3550 cm−1 due to O–H⋯ClO4. In D2O solutions a similar band was found at 2590 cm−1 due to O–D⋯ClO4. The band at 283 cm−1, in addition to the restricted translation mode of water at ∼180 cm−1, was also observed in dilute to moderately concentrated CaCl2 and CaBr2 solutions. This fact is strong evidence that neither Cl nor Br penetrate the first hydration sphere of Ca2+ in solution with mol ratio H2O : CaCl2/CaBr2 ≥ 18 : 1 and the coordination number is unchanged. Furthermore, the influence of CaCl2 on the water bands of the librational band region (300–900 cm−1), the deformation band of water and the O–H stretching region has been described. In a hydrate melt and very concentrated solutions of CaCl2 with a mol ratio H2O : CaCl2 ≤ 9 : 1, however, contact ion pairs between Ca2+ and Cl are formed and the 283 cm−1 band vanishes. Preliminary DFT calculations on the contact ion pair, [Ca(OH2)5Cl]+, confirm its existence in such hydrate melts. In aqueous solutions of Ca(NO3)2, NO3 penetrates the first hydration sphere and spectroscopic evidence of weak nitrato-complex formation could be detected. This is the first comprehensive report on the symmetric stretching vibration of the hydrated Ca2+ ion, [Ca(OH2)6]2+, in aqueous solution. DFT calculations concerning geometry optimizations and frequency calculations at the B3LYP/6-311+G(d,p) level on the hexa-aqua Ca2+ ion in the gas phase and including a solvation-sphere were performed. The calculations on [Ca(OH2)6]2+ and [Ca(OD2)6]2+ with a solvation-sphere allowed the determination of the six CaO6 skeletal modes and supported the assignment of the symmetric stretching mode, ν1CaO6 of [Ca(OH2)6]2+ and [Ca(OD2)6]2+. Discrete cluster calculations on a cluster with six inner sphere and twelve outer sphere water molecules, [Ca(OH2)6(OH2)12]2+ at the same level of theory, led to a Ca–O internuclear distance at 2.383 Å and 4.475 Å for the inner sphere and the outer sphere respectively.

Graphical abstract: Hydration of the calcium(ii) ion in an aqueous solution of common anions (ClO4−, Cl−, Br−, and NO3−)

Supplementary files

Article information

Article type
Paper
Submitted
30 Jul 2012
Accepted
03 Jan 2013
First published
07 Jan 2013

Dalton Trans., 2013,42, 3919-3935

Hydration of the calcium(II) ion in an aqueous solution of common anions (ClO4, Cl, Br, and NO3)

W. W. Rudolph and G. Irmer, Dalton Trans., 2013, 42, 3919 DOI: 10.1039/C2DT31718D

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