Molecular reorientations and local structure in nitrate aqueous solutions

We present an experimental study of anionic reorientational dynamics in Cu(NO3)2 and Ca(NO3)2 aqueous solutions. The anion reorientational autocorrelation time τor has been obtained by Fourier band shape analysis of the Raman 1050 cm−1 N–O stretching vibration line and by a multiple Lorentzian fitting of the depolarized Rayleigh wing spectrum. Its evolution as a function of solute concentration and of the nature of the counterion present (Cu2+, Ca2+) has been analyzed. Deviations from the Stokes–Einstein proportionality relation between Raman reorientational correlation times and the values of shear viscosity of the system have been used to obtain the microscopic structure contribution to the viscosity itself. From the depolarized Rayleigh wing spectra we have been able to investigate the contributions of both water and the nitrate ion to the reorientational relaxation in the solutions and to assess the contribution of collective effects.