An aluminium-27 nuclear magnetic resonance study of chemical exchange between different polyatomic species in butylpyridinium chloride–AlCl3 melts

Abstract

²⁷ Al n.m.r. measurements have been made for molten n-butylpyridinium chloride–AlCl₃ mixtures at 61 mol% AlCl₃ and at various temperatures between 20 and 100 °C. The time evolution of the ²⁷Al longitudinal magnetization recovery (l.m.r.) was obtained from the free-induction decays measured using the inversion recovery method. The observed ²⁷Al l.m.r. and n.m.r. spectrum consisted of two contributions which were attributed to Al₂Cl^–₇(A) and AlCl^–₄(B). The experimental results of the 27Al l.m.r. and n.m.r. spectrum were reproduced with the aid of the general theory describing the effect of chemical exchange on l.m.r. and lineshape. Thus the kinetic and equilibrium properties of the chemical exchange process were determined from the temperature dependences of the fractional population, ƒ_α, and the lifetime τ_α(α= A or B), which ranged from 10^–3–10^–1s. The rate of the exchange reaction Al (in B)⇌ Al (in A) is not very fast, since it is small compared with R^*_{1, A}( > R^*_{1, B}) even at high temperatures for 61 mol% AlCl₃. The observed rate of l.m.r. at high temperatures under the influence of chemical exchange between the different polyatomic species B and A was not equal to the spin–lattice relaxation rates R^*_{1, A} and R^*_{1, B}.