Zinc(II) hydration in aqueous solution: A Raman spectroscopic investigation and an ab initio molecular orbital study of zinc(II) water clusters

Raman spectra of aqueous Zn(II)-perchtorate solutions were measured over br ed concentration (0.50-3.54 mol-L-1) and temperature (25-120 degrees C) ran ges. The weak polarized band at 390 cm(-1) and two depolarized modes at 270 and 214 cm(-1) have been assigned to nu(1)(a(1g)), nu(2)(e(g))r and nu(5)( f(2g)) of the zinc-hexaaqua ion. The infrared-active mode at 365 cm(-1) has been assigned to nu(3)(f(1u)). The vibrational analysis of the species [Zn (OH2)(6)(2+)] was done on the basis of O-h symmetry (OH2 as point mass). Th e polarized mode nu(1)(a(1g))-ZnO6 has been followed over the full temperat ure range and band parameters (band maximum, full width at half height, and intensity) have been examined. The position of the nu(1)(a(1g))-ZnO6 mode shifts only about 4 cm(-1) to lower frequencies and broadens by about 32 cm (-1) for a 95 degrees C temperature increase. The Raman spectroscopic data suggest that the hexaaqua-Zn(II) ion is thermodynamically stable in perchlo rate 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)(6)(2)] were carried out at the Hartree-Fock and second-order Moller-Plesset lev els 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. Th e unsealed Vibrational frequencies of the [Zn(OH2)(6)(2+)] are reported. Th e unsealed vibrational frequencies of the ZnO6 unit are lower than the expe rimental frequencies(ca. 15%), but scaling the frequencies reproduces the m easured frequencies. The theoretical binding enthalpy for [Zn(OH2)(6)(2+)] was calculated and accounts for ca. 66% of the experimental single-ion hydr ation enthalpy for Zn(II). Ab initio geometry optimizations and frequency c alculations are also reported for a [Zn(OH2)(18)(2+)] (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 solu tion. The nu(1)-ZnO6 (unscaled) mode occurs at 388 cm(-1) almost in perfect correspondence to the experimental value. The theoretical binding enthalpy for [Zn(OH2)(18)(2+)] 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 hydra tion shell because of the strong polarizing effect of the Zn(II) ion. The i mportance of the second hydration sphere is discussed.