Hydration and speciation studies of Mn2+ in aqueous solution with simple monovalent anions (ClO4-, NO3-, Cl-, Br-)

Raman spectroscopic measurements have been performed on aqueous solutions of Mn(ClO4)2, MnCl2, MnBr2, and Mn(NO3)2 in the terahertz region (40-600 cm(-1)) and to wavenumbers up to 4200 cm(-1) employing an absorption gap in these light pink coloured solutions. In Mn(ClO4)2 solutions of water and heavy water, the hexahydrate and its deuterate, [Mn(OH2)6](2+) and [Mn(OD2)6](2+), were characterized as showing a very weak, strongly polarized band at 354 cm(-1) and 338 cm(-1), respectively. These modes were assigned to ν1 MnO6 of [Mn(OH2)6](2+) and [Mn(OD2)6](2+). In Mn(NO3)2(aq), the undisturbed mode at 354 cm(-1), representative of manganese hexahydrate, was also detected in dilute solutions up to ~3 mol L(-1) and no sign of nitrato complex formation could be obtained. In fairly dilute MnCl2(aq) up to ~1.5 mol L(-1) the hexaaqua ions were identified but in concentrated solutions, chloro-complexes of Mn(2+) were detected in the form of [Mn(OH2)(6-n)Cl(n)]((+2-n)) with n equal to one or two. In MnBr2(aq) a comparable picture to the one in MnCl2(aq) could be obtained. Density functional theory (DFT) calculations on Mn(2+) water clusters were carried out to optimize the geometry and calculate the frequencies of the [Mn(OH2)6](2+) cluster. For this purpose an unrestricted B3LYP functional was used with a triple-ζ basis set 6-311+G(d,p). In order to include the hydration effects around the [Mn(OH2)6](2+), a continuum model approach was employed where the solvent is described by a structureless dielectric polarizable continuum using the Polarizable Continuum Model of Solvation (PCM). The gas phase cluster of [Mn(OH2)6](2+) led to lower MnO6 frequencies compared to the measured ones. A second, much larger cluster model, with 18 water molecules containing 6 waters in the first shell and an explicit second hydration shell, [Mn(OH2)6(OH2)12](2+), was modelled. Again, the cluster in vacuo and the cluster surrounded by a structureless polarizable continuum were considered. The larger cluster including the polarizable continuum gives the most realistic frequency value of ν1 MnO6 and the other MnO6 skeleton modes. The hydration enthalpy, ΔH(hyd(l)) at 298 K, of [Mn(OH2)6](2+)(aq) was calculated by applying a Born-Haber cycle and correcting for the heat of vaporization, ΔH(vap), of water and the solvation enthalpy, ΔH(solv), released by transferring the gas phase cluster into the solution. The theoretical hydration enthalpy is in fair agreement with the measured hydration enthalpy.