A solid-state 93Nb and 19F NMR spectroscopy and X-ray diffraction study of potassium heptafluoroniobate(V):: Characterization of 93Nb, 19F coupling, and fluorine motion

A variety of NMR interactions have been characterized by solid-state NMR spectroscopy in potassium heptafluoroniobate, K2NbF7, which contains fluorine atoms arranged about a central niobium atom in a heptacoordinate, capped trigonal prism arrangement. Simulations of Nb-93 MAS NMR spectra acquit-ed at 11.7 T and at high spinning speeds (35 kHz) yielded the nuclear quadrupole coupling constant, C-Q(Nb-93), the asymmetry parameter, eta, and the isotropic chemical shift, delta (iso). From the analysis of Nb-93 NMR spectra of stationary samples of K2NbF7, the niobium chemical shielding anisotropy (span, Omega = 200 ppm) and the relative orientation of the electric field gradient (EFG) and chemical shielding (CS) tensors were determined. The Nb-93 MAS NMR spectra acquired at lower spinning speeds, where the spinning sidebands are not separated from the centerband, were also simulated by using an efficient time propagation algorithm based on Floquet theory. The C-Q(Nb-93) is seen to increase with decreasing temperature, varying from 29 to 40 MHz from 150 to 0 degreesC, respectively, with the following parameters determined at room temperature: C-Q = 38.5(2) MHz, eta = 0.35(2), and delta (iso) = -1600(5) ppm. Slightly distorted ten-peak multiplets are observed in the solid-state F-19 MAS NMR spectra, which arise from J-coupling and residual dipolar coupling between the F-19 and Nb-93 nuclei. Simulations of the F-19 MAS NMR spectra yield values of (1)J(Nb-93,F-19) = 204(2) Hz and delta (iso)(F-19) = 76.28(2) ppm. Variable-temperature F-19 MAS NMR experiments demonstrate that intramolecular fluorine motion becomes significant above -100 degreesC resulting in a reduction in the fluorine second moments and the Nb-93, F-19 dipolar couplings. An irreversible phase transition is observed at ca. 160 degreesC by F-19 and Nb-93 NMR, as well as by time-resolved synchrotron X-ray powder diffraction techniques.