SOLVENT ISOTOPE EFFECT ON ORIENTATIONAL CORRELATION TIMES OF THE NITRATE ION IN DILUTE AQUEOUS-SOLUTION

The perpendicular orientational correlation time (τ⊥) for NO3- in D2O was determined as a function of concentration (c) by analyzing the Raman line shape (v1 stretch, A1′) in LiNO3 and KNO3 solutions at 0.2-1 M (mol dm-3) and by measuring the 14N nuclear magnetic resonance (NMR) T1, values in MNO3 (M = Li, Na, K, and Cs) solutions at 0.2-1 M. The parallel orientational correlation time (τ∥) was also determined by the 17O NMR T1 measurement for the solutions enriched in 17O. Both τ⊥ and τ∥ increase linearly with increasing concentration as τ = τ°(1 + αc). The limiting values τ⊥° and τ∥° in D2O are 1.40 ± 0.01 and 2.17 ± 0.02 ps, respectively, being larger than the corresponding values of 1.25 ± 0.01 and 1.92 ± 0.02 ps in H2O; the ratios τ°(D2O)/τ°(H2O) are somewhat smaller than the solvent viscosity ratio η(D2O)/η(H2O). The quadrupole coupling constant of the 14N nucleus differs little from H2O to D2O within experimental errors. The α value, which depends on the countercation, shows a small solvent isotope effect in contrast to the prediction of the hydrodynamic interaction model. © 1990 American Chemical Society.