Effect of dielectric friction on the perpendicular reorientation of the nitrate ion in water and organic solvents

Abstract

The perpendicular orientational correlation time τ2R has been determined for the nitrate ion in tetrabutylammonium nitrate (Bu4NNO3) solutions in such an aprotic solvent as acetonitrile (MeCN) and such protic solvents as water (H2O), methanol (MeOH), ethanol (EtOH), and 1-propanol (PrOH) as a function of concentration (c) between 0.01 and 1 M at 30 °C by measuring the 14N nuclear magnetic resonance spin-lattice relaxation times. To ensure the presence of the common limit of τ2R at infinite dilution, the countercation effect on τ2R has been investigated for the potassium ion in water and for the lithium ion in ethanol. In all cases, the rotational correlation time depends linearly on concentration (c); τ2R=τ2R0(1+αc). The reduced slope α depends on the solvent and the cation size; α increases with a decrease in the solvent polarity and the countercation size. The effect on α of the solvent polarity and the surface charge density of the countercation can be explained qualitatively well by our hydrodynamic interaction model previously presented; the anomalously large slope for the Bu4N+ ion in water indicates its promoting effect on the water structure. The limiting correlation times (τ2R0) are 1.08±0.01, 1.14±0.01, 3.38±0.03, 6.12±0.06, and 11.6±0.1 ps, respectively, in MeCN, H2O, MeOH, EtOH, and PrOH. The solvent effect on τ2R0 is much stronger than that expected from viscosity, except for water. The friction coefficient which exceeds the viscosity contribution indicates a key role of the dielectric friction (DF), as predicted by the Hubbard–Onsager DF theory for ionic rotation.