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NMR crystallography to probe the breathing effect of the MIL-53(Al) metal-organic framework using solid-state NMR measurements of 13C-27Al distances.

The metal-organic framework MIL-53(Al) (aluminium terephthalate) exhibits a structural transition between two porous structures with large pore (lp) or narrow pore (np) configurations. This transition, called the breathing effect, is observed upon changes in temperature or external pressure, as well as with the adsorption of guest molecules, such as H2O, within the pores. We show here how these different pore openings can be detected by observing the dephasing of 13C magnetization under 13C-27Al dipolar couplings using Rotational-Echo Saturation-Pulse Double-Resonance (RESPDOR) solid-state NMR experiments with Simultaneous Frequency and Amplitude Modulation (SFAM) recoupling. These double-resonance NMR experiments between 13C and 27Al nuclei, which have close Larmor frequencies, are feasible thanks to the use of a frequency splitter. The experimental SFAM-RESPDOR signal fractions agree well with those simulated from the MIL-53(Al)-lp and -np crystal structures obtained from powder X-ray diffraction analysis. Hence, these 13C-27Al solid-state NMR experiments validate these structures and confirm their rigidity. A similar agreement is reported for the framework ligands in the as-synthesized (as) MIL-53(Al), in which the pores contain free ligands. Furthermore, in this case, 13C-{27Al} SFAM-RESPDOR experiments allow an estimation of the average distance between the free ligands and the 27Al nuclei of the framework.