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

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 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 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. Abinitio 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 T_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 (ca. 15%), but scaling the frequencies reproduces the measured frequencies. The theoretical binding enthalpy for [Zn(OH₂)₆²⁺] was calculated and accounts for ca. 64% of the experimental single ion hydration enthalpy for Zn(II). Abinitio 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 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 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.