Structural Modeling of O/F Correlated Disorder in TaOF 3 and NbOF 3- x (OH) x by Coupling Solid-State NMR and DFT Calculations.

The structure of MOF 3 (M = Nb, Ta) compounds was precisely modeled by combining powder X-ray diffraction, solid-state NMR spectroscopy, and semiempirical dispersion-corrected DFT calculations. It consists of stacked ∞ (MOF 3 ) layers along the c⃗ direction formed by heteroleptic corner-connected MX 6 (X = O, F) octahedra. 19 F NMR resonance assignments and occupancy rates of the anionic crystallographic sites have been revised. The bridging site is shared equally by the anions, and the terminal site is occupied by F only. An O/F correlated disorder is expected since cis -MO 2 F 4 octahedra are favored, resulting in one-dimensional -F-M-O-M- strings along the <100> and <010> directions. Ten different 2 × 2 × 1 supercells per compound, fulfilling these characteristics, were built. Using DFT calculations and the GIPAW approach, the supercells were relaxed and the 19 F isotropic chemical shift values were determined. The agreement between the experimental and calculated 19 F spectra is excellent for TaOF 3 . The 1 H and 19 F experimental NMR spectra revealed that some of the bridging F atoms are substituted by OH groups, especially in NbOF 3 . New supercells involving OH groups were generated. Remarkably, the best agreement is obtained for the supercells with the composition closest to that estimated from the 19 F NMR spectra, i.e., NbOF 2.85 (OH) 0.15 .