An aluminium-27 nuclear magnetic resonance study of ligand exchange. Kinetic and equilibrium properties

Abstract

²⁷ Al Nuclear magnetic resonance measurements have been carried out between –5 and 85 °C for a methanolic aqueous Al₂(SO₄)₃ solution: the ²⁷Al longitudinal magnetization recovery (l.m.r.) was measured from the free-induction decays following the second pulse of a 180°–τ–90° pulse sequence. The ²⁷Al n.m.r. spectra consisted of two resonance signals due to a first-sphere complex [Al(H₂O)₅SO₄]⁺(= A) and to a hexa-aquo cation [Al(H₂O)₆]³⁺(= B), respectively. The equilibrium and kinetic properties of the exchange of H₂O and SO^2–₄ ligands were determined by reproducing the observed l.m.r. and lineshape with the aid of the theory developed by one of the authors (K.I.). The observed rate of l.m.r. under the influence of ligand exchange was not equal to the energy relaxation rate R^*_1,A or R^*_1,B. The lifetimes τ_A and τ_B ranged from 10^–3 to 5 s; the temperature dependence of both fractional populations f_A or f_B was determined. These data gave a high value of ΔH^‡_BA and a positive ΔS^‡_BA and showed a dissociative reaction for the forward path; a relatively small value of ΔH^‡_AB and a large negative ΔS^‡_AB showed an associative reaction for the backward path in the chemical exchange reaction Al^III(in B)⇌ Al^III(in A). The reaction was endothermic, but the product, [Al(H₂O)₅SO₄]⁺, increased with increasing temperature because of the high entropy contribution.