Raman spectroscopy study on the dynamic behavior of nitrate anion in zinc nitrate solution at high temperatures and pressure Original Research Article

We have demonstrated the rotational relaxation of the N–O symmetric stretching fundamental of the anion of zinc nitrate in aqueous solution between ambient temperature and a high temperature of 340°C at a high pressure of 30 MPa by the Raman line shape analysis. The perpendicular orientational relaxation time (τ⊥) significantly decreases with increasing temperature up to 340°C; the values of τ⊥ were 1.86 and 0.25 ps at 20 and 340°C, respectively. The Arrhenius plot gives two activation energies depending on temperature: 2.1 kcal mol−1 up to 300°C and 6.4 kcal mol−1 above 300°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, so we assume that the orientational motion of the anion above 300°C is energetic enough to break the remaining water–water hydrogen bonds at high temperatures. Two linear correlations of τ⊥ on ηs/T above and below around 300°C also suggest different local environments experienced by the NO−3 species and water molecules. The experimental perpendicular correlation time scaled by the theoretical correlation time of the free gas molecule, χ⊥, were 1.35, 1.11, and 0.86 at high temperatures of 300, 320, and 340°C, respectively, suggesting strong inertial effects on the rotational relaxation processes. These inertial effects at the high temperatures are further supported by that our data did not satisfy the Hubbard relation, which is derived on the basis of the diffusion model.