Raman spectral studies of aqueous zinc nitrate solution at high temperatures and at a high pressure of 30 MPa

We present the first direct measurement of dynamic behavior of ions in an aqueous solution at high temperatures and pressure using Raman spectroscopy. We have studied the N-O symmetric stretching mode at high temperatures up to 340 degrees C and at a high pressure of 30 MPa. The Raman spectra of 1.3 M aqueous zinc nitrate solution have been analyzed by curve fitting. The zinc ion forms two species. In one species Zn2+ is bound more strongly to the NO3-, and in the other Zn2+ is bound more strongly to H2O. The ratio of the former to the latter remains unaltered with temperatures below 300 degrees C, but above 300 degrees C the ratio increases significantly. The average number of water molecules bound to Zn2+ (n(H2O)) is estimated using the intensity of peak frequency of the symmetric stretching mode of the haxaaquazinc(II) cation. As the temperature increases, the n(H2O) gradually decreases, but above 300 degrees C it shows a large decrease, suggesting the displacement of water molecules from the first solvation shell around Zn2+ and the concomitant entry of NO3- into the shell. The perpendicular orientational relaxation time (tau(perpendicular to)) decreases significantly with temperature; the values of tau(perpendicular to) were 1.86 and 0.25 ps, respectively, at 20 and 340 degrees C. The Arrhenius plot gives two activation energies, 2.1 kcal mol(-1) below 300 degrees C and 6.4 kcal mol(-1) above 300 degrees C. The activation energy for the orientational motion, 6.4 kcal mol(-1), is larger than that for orientational motion of water, 4-5 kcal mol(-1), and we assume that orientational motion of the anion above 300 degrees C requires the breaking of water-water hydrogen bonds. Furthermore, the experimental values of the perpendicular diffusion constant (D-perpendicular to) at higher temperatures than 300 degrees C are in agreement with those of D-i calculated from the slightly damped free-rotor (SDFR) model, and the rotation around the C-3 axis of the anion is confirmed to proceed rapidly and approach that in the free dilute gas phase.