Raman spectral studies of ionic motion and ionic interaction in aqueous nitrate solutions

Abstract

The Raman spectra of the totally symmetric ν₁(A ₁′) mode of an NO₃ ^- ion have been recorded in aqueous LiNO₃, NaNO₃, KNO₃ and NH₄NO₃ solutions at various concentrations. The vibrational and rotational correlation functions have been calculated from the Fourier transform of the Raman profiles. Reorientation of the symmetry axis of an NO₃ ^- ion in the aqueous solutions may be represented by the inertial motion for short times (t ≲ 0·15 ps, jump angle 17°), and by the rotational diffusion for longer times (t ≳ 0·3 ps). An effective moment of inertia for short times has been found to be I _⊥ = 20 × 10^-39 g cm², which suggests, as compared with the free NO₃ ^- value I _⊥ ⁰ = 6 × 10^-39 g cm², some association with the surrounding water molecules or cations. The mean-square torque acting on an NO₃ ^- ion from the moment analysis has been found to increase linearly with nitrate concentration. The torque on an NO₃ ^- ion in concentrated nitrate solutions is predominantly from other ions, while the contribution from an NO₃ ^--water interaction is small. It is concluded that the vibrational band width of 6–13 cm^-1 in these nitrate solutions is to be attributed to the pure vibrational dephasing, due to an NO₃ ^--water interaction in dilute solutions, and due to NO₃ ^--water and increased NO₃ ^--cation interactions in concentrated solutions. The contribution from the induced dipole-induced dipole resonance vibrational energy transfer has been found to be negligible.