A conspicuous 27Al-NMR signal at 72 ppm during isomerization of Keggin Al13 ions

Abstract

A sharp signal at 72 ppm was recently observed in 27Al-NMR spectra during the isomerization of Keggin-Al13 ions in the presence of calcium and glycine [1]. It has been proposed that this signal corresponds to one of the Keggin isomers. Conversion of the most common ε-isomer of the Keggin series of molecules, having the stoichiometry [AlO4Al12(OH)24(OH2)12]7+, is enhanced by the addition of calcium and glycine [2,3]. Here we show that a 72 ppm signal can also be observed in the absence of calcium or glycine; the magnitude of which depends on aluminum concentration and temperature. Additionally, isomerization of the ε-Keggin-Al13 to the γ-Keggin-Al13 isomer was observed in the absence of calcium and glycine at elevated temperatures after several days. Calcium and glycine were previously shown to enhance rates of γ-Keggin-Al13 formation.2.