Nuclear magnetic resonance study of ion hydration. 3. Anisotropic rotational motion of water molecules bound to the aluminum(III) ion

Abstract

The H spin-lattice relaxation times, T1, of the separate free and coordinated water signals of aqueous Al(ClO4)3 solutions have been independently measured at temperatures between +10 and -65C and with varying salt concentrations. All inversion-recovery measurements of T1 showed a single exponential form in the above temperature range. Under low temperature conditions, only the intramolecular proton-proton interaction undergoing rotational motion contributes to T1. The experimental results showed that the difference between the two signals' T1 values of the free and coordinated waters arises as the temperature falls below ca -20C. It is assumed that the coordinated water molecules anisotropically rotate as ellipsoidal molecules at low temperatures, whereas the bulk water molecules isotropically rotate as spherical molecules throughout the temperature range. The theory of spin-lattice relaxation due to the anisotropic rotational diffusion by Woessner is applied to the analysis of the motion of the coordinated water molecules and the following conclusions are reached: (1) at low temperatures and low aluminum concentrations, the coordinated water molecules undergo anisotropic rotational motion as an axially symmetric ellipsoidal body whose axis of symmetry is the C2 axis of the H2O molecule; (2) the motion of the hydration water molecule is slowed for low Al concentrations.more » 27 references, 4 figures.« less