Inorganic Chemistry, Vol.39, No.10, 2135-2145, 2000
Characterization of divalent metal metavanadates by V-51 magic-angle spinning NMR spectroscopy of the central and satellite transitions
V-51 quadrupole coupling and chemical shielding tensors have been determined from V-51 magic-angle spinning (MAS) NMR spectra at a magnetic field of 14.1 T for nine divalent metal metavanadates: Mg(VO3)(2), Ca(VO3)(2), Ca(VO3)(2). 4H(2)O, alpha-Sr(VO3)(2), Ba(VO3)(2), Zn(VO3)(2), alpha- and beta-Cd(VO3)(2), and Pb(VO3)(2). The manifold of spinning sidebands (ssbs) from the central and satellite transitions, observed in the V-51 MAS NMR spectra,have been analyzed using least-squares fitting and numerical error analysis. This has led to a precise determination of the eight NMR parameters characterizing the magnitudes and relative orientations of the quadrupole coupling and chemical shielding tensors. The optimized data show strong similarities between the NMR parameters for the isostructural groups of divalent metal metavanadates. This demonstrates that different types of metavanadates can easily be distinguished by their anisotropic NMR parameters. The brannerite type of divalent metal metavanadates exhibits very strong V-51 quadrupole couplings (i.e., C-Q = 6.46-7.50 MHz), which reflect the highly distorted octahedral environments for the V5+ ion in these phases. Linear correlations between the principal tensor elements for the V-51 quadrupole coupling tensors and electric field gradient tensor elements, estimated fi om point-monopole calculations, are reported for the divalent metal metavanadates. These correlations are used in the assignment of the NMR parameters for the different crystallographic V-51 sites of Ca(VO3)(2). 4H(2)O, Pd(VO3)(2), and Ba(VO3)(2). For alpha-Sr(VO3)(2), With an unknown crystal structure, the V-51 NMR data strongly suggest that this metavanadate is isostructural with Ba(VO3)(2), for which the crystal structure has been reported. Finally, the chemical shielding parameters for orthovanadates and mono- and divalent metal metavanadates are compared.
Keywords:SOLID-STATE NMR;NUCLEAR-MAGNETIC-RESONANCE;RAY PHOTOELECTRON-SPECTROSCOPY;ANISOTROPIC SHIELDING TENSORS;CRYSTAL-STRUCTURE;RAMAN-SPECTROSCOPY;RELATIVE ORIENTATION;OXIDE CATALYSTS;VANADIA;QUADRUPOLE