The Temperature Dependence of Raman Band Parameters for Aquated Mg(II) and Zn(II)

Abstract

The weak polarized Raman bands, previously assigned to the v₁ mode of the octahedral species Mg(H₂O)₆²⁺ and Zn(H₂O)₆²⁺, have been studied over the temperature ranges −32 to 99 °C and −29 to 80 °C respectively. The signal-to-noise ratio has been enhanced by multi-scan averaging with a spectrometer interfaced to a PDP/11 computer. The digitized band profiles have been examined analytically to extract these parameters: position of maximum, position of centroid, full width at half height (f.w.h.h.), and intensity. The dependence on concentration and acidity has also been measured. Whereas the 390 cm⁻¹ line of aquated zinc(II) shifts only 3 cm⁻¹ to lower frequencies and broadens by about 16 cm⁻¹ for a 110 °C temperature increase, the 362 cm⁻¹ band of aquated magnesium(II) shifts down by 15 cm⁻¹ but broadens to the same extent for a 130 °C temperature increase. The results are interpreted in terms of the difference in the magnitude of forces between these cations and hydrated water molecules.