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(ClO(4))(2), CaCl(2), CaBr(2), and Ca(NO(3))(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(ClO(4))(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 Ca(2+) ion, [Ca(OH(2))(6)](2+), and the integrated band intensity showed a linear dependency with Ca(ClO(4))(2) concentration. In a Ca(ClO(4))(2) solution of heavy water a similar band was observed at 268 cm(-1) (fwhh = 64 cm(-1)) of the deuterated species, [Ca(OD(2))(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···ClO(4)(-). In D(2)O solutions a similar band was found at 2590 cm(-1) due to O-D···ClO(4)(-). 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 CaCl(2) and CaBr(2) solutions. This fact is strong evidence that neither Cl(-) nor Br(-) penetrate the first hydration sphere of Ca(2+) in solution with mol ratio H(2)O : CaCl(2)/CaBr(2)≥ 18 : 1 and the coordination number is unchanged. Furthermore, the influence of CaCl(2) 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 CaCl(2) with a mol ratio H(2)O : CaCl(2)≤ 9 : 1, however, contact ion pairs between Ca(2+) and Cl(-) are formed and the 283 cm(-1) band vanishes. Preliminary DFT calculations on the contact ion pair, [Ca(OH(2))(5)Cl](+), confirm its existence in such hydrate melts. In aqueous solutions of Ca(NO(3))(2), NO(3)(-) 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 Ca(2+) ion, [Ca(OH(2))(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 Ca(2+) ion in the gas phase and including a solvation-sphere were performed. The calculations on [Ca(OH(2))(6)](2+) and [Ca(OD(2))(6)](2+) with a solvation-sphere allowed the determination of the six CaO(6) skeletal modes and supported the assignment of the symmetric stretching mode, ν(1)CaO(6) of [Ca(OH(2))(6)](2+) and [Ca(OD(2))(6)](2+). Discrete cluster calculations on a cluster with six inner sphere and twelve outer sphere water molecules, [Ca(OH(2))(6)(OH(2))(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.