Polarized-Dependent IR ATR Study for the Structural Characterization of Solid-State Phosphonates:  Case of Aluminum (4-Carboxyphenyl)methylphosphonate

The hybrid organic−inorganic phosphonate Al(μ-OH)(O3PCH2C6H4COOH)·H2O has been structurally characterized by comparison of its polarized ATR-FTIR spectra, MAS NMR, and TGA analysis with those of Al(μ-OH)(O3PCH2C6H4Br)·H2O and Al(μ-OH)(O3PCH2C6H5)·H2O whose crystallographic structures are known. The polarized ATR (attenuated total reflection) IR study revealed the presence and orientation of selected bonds with respect to the sample holder. Moreover, for the three compounds, the polarized study allowed the identification of a symmetric CPO3 vibration mode around 1050 cm-1; usually, regular FTIR study of solid-state metallic phosphonates does not permit such precise attribution in this region of the IR spectrum. Polarized ATR FTIR spectroscopy, along with solid-state MAS NMR spectroscopy, appeared to be a very efficient tool for obtaining structural information on such solid-state hybrid compounds exhibiting highly anisotropic crystallites shapes.