Beyond the Limits of X-ray Powder Diffraction: Description of the Nonperiodic Subnetworks in Aluminophosphate-Cloverite by NMR Crystallography

Abstract

NMR crystallography of the fluorinated aluminophosphate cloverite is presented, with an emphasis on the description of the nonperiodic part of the compound, i.e., the fluorine and organic subnetworks, which are very difficult to access by usual X-ray powder diffraction methods. Multinuclear high-resolution 1D Al-27 and P-31 NMR support the main cloverite-type topological features previously proposed for aluminum cloverite from powder X-ray diffraction. Spatial proximities are extracted from the 2D P-31-P-31 and Al-27-P-31 NMR spectra, allowing a full assignment of the P-31 and Al-27 resonances to the corresponding phosphorus and aluminum sites in the structure. To go further into the description of the main periodic framework, C-13, N-15, and H-1 1D and H-1-H-1 and H-1-P-31 2D NMR measurements are employed, allowing the characterization and selective locations of the two costructural-directing agents in the pores and channels of the framework. The nonperiodic fluorine subnetwork is described by means of F-19-X (X = Al-27 and P-31) 2D NMR experiments. Two kinds of fluorine atoms are distinguished: F- ions trapped in D4R units and F atoms covalently bonded to terminal Al or P atoms and which interrupt the AlPO network. Through the example of aluminum cloverite, we show that, despite the considerable complexity of such systems, an extremely detailed structural model can be obtained, including the simple rules that allow the description of the nonperiodic subnetworks that tailor the structure and properties of a compound, by coupling powder X-ray diffraction and high-resolution NMR data in a generalized crystallography approach.