Raman and nuclear magnetic resonance studies on the concentration dependence of orientational relaxation times of the nitrate ion in dilute aqueous solution

The perpendicular orientational relaxation time (τ⊥) of the nitrate ion was determined as a function of concentration by the Raman line shape analysis (ν1 stretch) in aqueous LiNO3 and KNO3 solutions in the range of 0.2–1 M and by the nuclear magnetic resonance (NMR) T1 measurement (14N) in MNO3 (M=Li, Na, K, and Cs) solutions in the range of 0.02–1 M; the parallel orientational relaxation time (τ∥) was also determined by the NMR measurement for the solutions enriched in 17O. Both τ⊥ and τ∥ increase linearly with increasing concentration up to 1 M; the limiting values of τ⊥ and τ∥ were 1.25 and 1.9 ps, respectively. The slope increases with the surface charge density of the countercation except for Li+. These trends are predicted in very dilute solutions by the continuum theory developed in the previous paper. The anomalous behavior of Li+ was explained in terms of its strong hydration.