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.