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

摘要

Highly disordered and X-ray amorphous aluminum isopropoxide fluorides can be seen as intermediates formed during the synthesis of high-surface AlF3. Their ~(27)Al MAS NMR spectra were recorded at different magnetic fields of up to 21.1 T. The ~(27)Al 3QMAS experimental data derived at B_0 = 14.1 T enabled the simulation of the ~(27)Al MAS NMR spectrum. In addition to 6-fold coordinated species, the presence of four- and 5-fold coordinated AlF4(O~iPr)_(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, AlF_xO_(4-x) and AlF_xO_(4-x) units present in the solids could be assigned based on a comprehensive graphical interpretation (~(27)Al 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 ~(27)Al MAS NMR spectra of related Al/F/O systems.