Zinc(II) hydration in aqueous solution. A Raman spectroscopic investigation and an ab-initio molecular orbital study

Abstract

Raman spectra of aqueous Zn(II) perchlorate solutions were measured over a broad concentration (0.50-3.54 mol L^-1) and temperature (25-120°C) range. 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 hexaaquazinc(II) ion, respectively. The infrared active mode at 365 cm^-1 has been assigned to ν₃(f_1u). The vibrational analysis of the species [Zn(OH₂)₆²⁺] was done on the basis of <i>O</i>_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 of half height and band intensity) have been examined. The position of the ν₁(a_1g) ZnO₆ mode shifts only about 4 cm^-1 to lower frequencies and broadens about 32 cm^-1 for a 95°C temperature increase. The Raman spectroscopic data suggest that the hexaaquazinc(II) ion is thermodynamically stable in perchlorate solution over the temperature and concentration range measured. <i>Ab</i><i>initio</i> geometry optimizations and frequency calculations of [Zn(OH₂)₆²⁺] were carried out at the Hartree-Fock and second order Møller-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 <i>T</i>_h. The unscaled vibrational frequencies of the [Zn(OH₂)₆²⁺] were reported. The unscaled vibrational frequencies of the ZnO₆ unit are lower than the experimental frequencies (<i>ca</i>. 15%), but scaling the frequencies reproduces the measured frequencies. The theoretical binding enthalpy for [Zn(OH₂)₆²⁺] was calculated and accounts for <i>ca</i>. 64% of the experimental single ion hydration enthalpy for Zn(II). <i>Ab</i><i>initio</i> geometry optimizations and frequency calculations are also reported for a [Zn(OH₂)₁₈²⁺] (Zn[6+12]) cluster with 6 water molecules in the first sphere and 12 water molecules in the second sphere. The global minimum corresponds with <i>T</i> symmetry. Calculated frequencies of the zinc [6+12] cluster correspond well with the observed frequencies in solution. The ν₁ ZnO₆ (unscaled) mode occurs at 389 cm^-1 in good agreement with the experimental value. The theoretical binding enthalpy for [Zn(OH₂)₁₈²⁺] 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 H-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.