Zinc(II) Hydration in Aqueous Solution: A Raman Spectroscopic Investigation and An ab initio Molecular Orbital Study of Zinc(II) Water Clusters

Abstract

Raman spectra of aqueous Zn(II)–perchlorate solutions were measured over broad concentration (0.50–3.54 mol-L−1) and temperature (25–120°C) ranges. The weak polarized band at 390 cm−1 and two depolarized modes at 270 and 214 cm−1 have been assigned to ν1(a 1g), ν2(e g), and ν5(f 2g) of the zinc–hexaaqua ion. The infrared-active mode at 365 cm−1 has been assigned to ν3(f 1u). The vibrational analysis of the species [Zn(OH2) 2 + ] was done on the basis of O h symmetry (OH2 as point mass). The polarized mode ν1(a 1g)-ZnO6 has been followed over the full temperature range and band parameters (band maximum, full width at half height, and intensity) have been examined. The position of the ν1(a 1g)-ZnO6 mode shifts only about 4 cm−1 to lower frequencies and broadens by about 32 cm−1 for a 95°C temperature increase. The Raman spectroscopic data suggest that the hexaaqua–Zn(II) ion is thermodynamically stable in perchlorate solution over the temperature and concentration range measured. These findings are in contrast to ZnSO4 solutions, recently measured by one of us, where sulfate replaces a water molecule of the first hydration sphere. Ab initio geometry optimizations and frequency calculations of [Zn(OH2) 2 + ] were carried out at the Hartree–Fock and second-order Moller–Plesset levels of theory, using various basis sets up to 6-31 + G*. The global minimum structure of the hexaaqua–Zn(II) species corresponds with symmetry T h. The unscaled vibrational frequencies of the [Zn(OH2) 2 + ] are reported. The unscaled vibrational frequencies of the ZnO6, unit are lower than the experimental frequencies (ca. 15%), but scaling the frequencies reproduces the measured frequencies. The theoretical binding enthalpy for [Zn(OH2) 2 + ] was calculated and accounts for ca. 66% of the experimental single-ion hydration enthalpy for Zn(II).Ab initio geometry optimizations and frequency calculations are also reported for a [Zn(OH2) 2 18 ] (Zn[6 + 12]) cluster with 6 water molecules in the first sphere and 12 in the second sphere. The global minimum corresponds with T symmetry. Calculated frequencies of the zinc [6 + 12] cluster correspond well with the observed frequencies in solution. The ν1-ZnO6 (unscaled) mode occurs at 388 cm−1 almost in perfect correspondence to the experimental value. The theoretical binding enthalpy for [Zn(OH2) 2 18 ] was calculated and is very close to the experimental single ion-hydration enthalpy for Zn(II). The water molecules of the first sphere form strong hydrogen bonds with water molecules in the second hydration shell because of the strong polarizing effect of the Zn(II) ion. The importance of the second hydration sphere is discussed.