CHARACTERIZATION OF THE GAO4AL12(OH)24(H2O)127+ POLYOXOCATION BY MAS NMR AND INFRARED SPECTROSCOPIES AND POWDER X-RAY-DIFFRACTION

Powder X-ray diffraction studies of the selenate salts of the GaO4Al12(OH)24(H2O)12(7+) polyoxocation (GaAl12) have confirmed unambiguously that it is structurally analogous to the AlO4Al12(OH)24(H2O)12(7+) species (Al13). Lines in the solid-state MAS NMR spectra of GaAl12, in which a gallium rather than an aluminum atom sits in the central tetrahedral position, are much sharper than those for the Al13 species. At least in part, this is because the GaAl12 structure is less distorted than the Al13 ion. Line widths in the solution phase NMR spectra of the Al13 ion (and perhaps the GaAl12 ion) are also affected by exchange broadening. Changes in the infrared spectra between ca. 900 and 400 cm-1 which result from replacing the central tetrahedral aluminum in Al13 with gallium, thereby forming GaAl12, and also the changes which occur upon deuteration have been studied, and an assignment of the bands in this region has been proposed.