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Identification of AlFx(OR)y Species in Strongly Disordered Aluminum Isopropoxide Fluoride Solids: A Field-Dependent MAS NMR Study

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journal contribution
posted on 03.09.2009, 00:00 authored by R. König, G. Scholz, A. Pawlik, C. Jäger, B. van Rossum, E. Kemnitz
Highly disordered and X-ray amorphous aluminum isopropoxide fluorides can be seen as intermediates formed during the synthesis of high-surface AlF3. Their 27Al MAS NMR spectra were recorded at different magnetic fields of up to 21.1 T. The 27Al 3QMAS experimental data derived at B0 = 14.1 T enabled the simulation of the 27Al MAS NMR spectrum. In addition to 6-fold coordinated species, the presence of four- and 5-fold coordinated AlFx(OiPr)CN−x (coordination number, CN, of 4 or 5) was unambiguously established for the first time for this system. A comparison of the chemical shifts observed for fluorine and aluminum isopropoxide fluorides with different F contents allows for a simple correlation for the appropriate species. As a main outcome, hints of corresponding F-species connected to 4-fold and 5-fold coordinated Al species were corroborated. Moreover, for the first time, AlFxO4−x and AlFxO5−x units present in the solids could be assigned based on a comprehensive graphical interpretation (27Al chemical shift trend analysis) that included known chemical shifts of solids containing pure AlO4, AlO5, AlF4, and AlF5 species. The correlations obtained are useful for the interpretation of 27Al MAS NMR spectra of related Al/F/O systems.