Hydration of cadmium(II) in aqueous perchlorate solution: A Raman and ab-initio approach

The weak, polarized Raman band assigned to the nu(1) CdO6 mode of the hexaq uo-Cd(II) ion (T-h symmetry) has been studied over the temperature range fr om 25 degrees C to 152 degrees C. The isotropic scattering geometry in R-fo rmat was employed in order to measure the true vibrational contribution of the band and account for the Boltzmann temperature factor B. The band profi le as a function of temperature has been examined analytically to extract t he parameters: position of band maximum, full width at half height (FWHH), integral intensity of the band and relative molar scattering coefficient, S -h over the temperature range measured. The dependence on concentration has also been measured. The 358 cm(-1) band of hexaquo-Cd(II) shifts only 3 cm (-1) to lower frequencies and broadens about 32 cm(-1) for a 127 degrees C temperature increase. Two depolarized modes at 235 cm(-1) and 185 cm(-1) co uld be assigned to nu(2)(e(g)) and nu(5)(f(2g)), respectively. The Raman sp ectroscopic data suggest that the hexaquo-Cd(II) ion is stable in perchlora te solution over the temperature range measured. These findings are in cont rast to CdSO4 solutions, recently measured by one of us, where sulfate repl aces a water molecule of the first hydration sphere. Ab initio geometry opt imizations of [Cd(OH2)(6)(2+)] were carried out at the Hartree-Fock and sec ond order Moller-Plesset levels of theory, using various basis sets up to 6 -31+G*. The vibrational frequencies of the [Cd(OH)(6)(2+)] cation were also calculated. Scaling the Hartree-Fock vibrational frequencies (HF/6-31G* an d HF/6-31+G* level) reproduced the measured frequencies of the CdO6 unit. T he theoretical binding energy for the hexaquo-Cd(II) ion was calculated and accounts for 66% of the experimental hydration enthalpy of Cd(II). The imp ortance of the second hydration sphere is also discussed.