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.

High-Order Ca(II)–Chloro Complexes in Mixed CaCl2–LiCl Aqueous Solution: Insights from Density Functional Theory and Molecular Dynamics Simulations

“…Here, x is the number of Cl -, W is the abbreviation of water molecule, n denotes the total number of water molecules, mL indicates the total coordination number of Ca 2+ . The B3LYP/aVDZ calculations show that [CaCl x ] 2−x hydrates prefer six-coordinated structures, which agrees with the results of Rudolph et al17 . As observed from…”

“…1-4 Inorganic CaCl 2 salt plays vital roles in many biochemical processes 1,2 and is a major component of the geothermal brines [5][6][7] and energy storage materials. [8][9][10] This great importance has prompted numerous studies on CaCl 2 aqueous solutions, both experimentally [11][12][13][14][15][16][17][18][19][20] and theoretically, [15][16][17][18][19][20][21][22][23] in recent decades. In the published works, ion-associated characteristics in CaCl 2 aqueous solutions, in particular contact ion pair (CIP) which was firstly proposed by Phutela and Pitzer 11 to explain an anomaly in the thermodynamic behavior in near 5.0 mol·kg -1 CaCl 2 aqueous solutions, were generally suspected.…”

Ion association characteristics in MgCl2and CaCl2aqueous solutions: a density functional theory and molecular dynamics investigation

“…Geometries CIP structures in work of Phutela and Pitzer [16], whereas the existence of CIP or SSIP/s structures was frequently doubted in early studies, especially in some CPMD results and experiment work [17][18][19][20][21][22]. Since the constraint of Ca 2 + on the second hydration shell cannot be neglected, as pointed out by the work of Bai et al [8], the stabilities of CIP and SSIP conformers in aqueous solution should be investigated when the perturbation of bulk water or outer hydration shells on these ion associated species has been properly considered.…”

“…Phutela and Pitzer suggested the formation of contact ion pair (CIP) in CaCl 2 aqueous solution, compared to the MgCl 2 aqueous solution [16]. However, in early works, the formation of CIP conformer in CaCl 2 aqueous solution was generally suspected both in some MD simulations and experimental investigations [17][18][19][20][21][22]. Recently, combined with X-ray absorption fine structure spectral and an X-ray diffraction spectral analyses, the work of Pham and Fulton demonstrated the presence of CIP and solvent-shared ion pair (SSIP) 2 Q.…”

A Raman spectroscopic investigation of speciation in La2(SO4)3(aq)

“…5 The O-atoms of the three waters in the equatorial plane (capping position) are separated from the cation by a bond distance of 2.64Å, while six water molecules at the vertices of the trigonal prism have a La-O bond distance of 2.515Å. 5,6 A recent Raman spectroscopic investigation revealed that in concentrated La(ClO 4 ) 3 (aq) ion pairs are formed while in dilute solutions the undisturbed [La(H 2 O) 9 ] 3+ species exist.…”