The relative motions of ions in solution II. An NMR relaxation study of attractive ions in water at low ionic strength

Abstract

In this paper we describe a nuclear magnetic resonance study of the relative motion of the attractive ion pairs (CH₃)₄N⁺/=ON(SO₃)^-2₂ and (CH₃)₄N⁺/ in dilute D₂O solutions of varying ionic strength. Our major purpose is to test previous theoretical calculations for different model electrolytes. We have measured the interparticle dipole-dipole spin-lattice relaxation rate at 400 MHz of the (CH₃)₄N⁺ protons due to their interaction with the electronic spins of the paramagnetic nitroxide ions. At 400 MHz this relaxation rate is proportional to the reduced spectral density ρ₂(ω_pΤ) which depends upon the relative ion distribution and motion, and can be directly compared with theoretical results. The present experiments verify the large charge and ionic strength effects predicted by the theory and allow us to distinguish between molecular and continuum solvent models. For the molecular solvent picture good agreement is obtained between the experimental and theoretical results. This is true for both ion pairs and over a range of ionic strengths. The primitive or continuum solvent model is found to be very poor. This is particularly evident for the (CH₃)₄N⁺/·ON(SO₃)^-2₂ pair where the continuum theory underestimates ρ₂(ω_pΤ) by roughly a factor of 4 at low ionic strength.