An aluminum-27 NMR study of chemical exchange and NMR line broadening in molten butylpyridinium chloride + AlCl 3. II

Abstract

27Al NMR spectra of butylpyridinium chloride + AlCl 3 melts were obtained between 39 and 80 mol% AlCl 3 at various temperatures. The lineshapes observed at the concentrations except for those of less than 50 mol% AlCl 3 and except for the concentration of 67 mol% AlCl 3 were not purely Lorentzian. The non-Lorentzian shapes, which consisted of two overlapping resonances, were successfully simulated on the basis of the Bloch equations of transverse relaxation modified by chemical-exchange effects. The intensity of each resonance absorption of nuclei on site m was given as the imaginary part of a general solution of the equations above and was represented in terms of the transverse relaxation rates R 2,m ∗ in the absence of chemical exchange, the fractions of nuclei f m , the chemical-exchange lifetimes τ m , and the nuclear resonance frequencies ν m of the individual components A and B. A and B correspond to AlCl 4 − and Al 2Cl 7 −, Al 2Cl 7 − and Al 3Cl 10 − , and Al 3Cl 10 − and Al 2Cl 6 in the melts of 55 ± 2, 71 ± 1, and 80 mol% AlCl 3, respectively. Since τ mΔω 2π ranged from 1 to 1.5 with Δω 2π (=ν m − ν n) ≅ 50 ∼ 150 Hz, the very slow exchange rate in the melts is equal to ca. 20 to 100 s −1.