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Characterizing the Turbostratic Disorder in COF-5 with NMR Crystallography
preprintsubmitted on 05.10.2020, 23:39 and posted on 06.10.2020, 10:06 by Giovanna Pope, Demetrius Vazquez, Fernando Uribe-Romo, James K. Harper
Since its initial synthesis in 2005, COF-5 has been known to have intrinsic disorder in the placement of the 2D layers relative to one another (i.e. turbostratic disorder). Prior studies of have demonstrated that the eclipsed layering found in the space group originally assigned to COF-5 (P6/mmm) is inconsistent with energy considerations. Herein it is demonstrated that eclipsed layers are also inconsistent with 13C solid-state NMR data. Crystal structure predictions are made in five alternative space groups and good agreement is obtained in P21/m, Cmcm, and C2/m. We posit that all three space groups are present within the stacked 2D layers and show that this conclusion is consistent with evidence from 13C solid-state NMR linewidths and chemical shifts, powder x-ray diffraction data and energy considerations. An alternative explanation involving a mixture of multiple pure phases is rejected because the observed NMR spectra don’t exhibit the characteristic features of such mixed phase materials.