title: Solid-state NMR/NQR and first-principles study of two niobium halide cluster compounds.
abstract: Two hexanuclear niobium halide cluster compounds with a [Nb6X12](2+) (X=Cl, Br) diamagnetic cluster core, have been studied by a combination of experimental solid-state NMR/NQR techniques and PAW/GIPAW calculations. For niobium sites the NMR parameters were determined by using variable Bo field static broadband NMR measurements and additional NQR measurements. It was found that they possess large positive chemical shifts, contrary to majority of niobium compounds studied so far by solid-state NMR, but in accordance with chemical shifts of (95)Mo nuclei in structurally related compounds containing [Mo6Br8](4+) cluster cores. Experimentally determined δiso((93)Nb) values are in the range from 2,400 to 3,000 ppm. A detailed analysis of geometrical relations between computed electric field gradient (EFG) and chemical shift (CS) tensors with respect to structural features of cluster units was carried out. These tensors on niobium sites are almost axially symmetric with parallel orientation of the largest EFG and the smallest CS principal axes (Vzz and δ33) coinciding with the molecular four-fold axis of the [Nb6X12](2+) unit. Bridging halogen sites are characterized by large asymmetry of EFG and CS tensors, the largest EFG principal axis (Vzz) is perpendicular to the X-Nb bonds, while intermediate EFG principal axis (Vyy) and the largest CS principal axis (δ11) are oriented in the radial direction with respect to the center of the cluster unit. For more symmetrical bromide compound the PAW predictions for EFG parameters are in better correspondence with the NMR/NQR measurements than in the less symmetrical chlorine compound. Theoretically predicted NMR parameters of bridging halogen sites were checked by (79/81)Br NQR and (35)Cl solid-state NMR measurements.
authors.affiliation: Laboratory for Solid-State and Complex Compounds Chemistry, Division of Materials Chemistry, Ruđer Bošković Institute, Bijenička cesta 54, 10000-HR, Zagreb, Croatia. Electronic address: Berislav.Peric@irb.hr.
authors.forename: Berislav
authors.initials: B
authors.lastname: Perić
authors.affiliation: Institut des Sciences Chimiques de Rennes, UMR 6226, CNRS-Ecole Nationale Supérieure de Chimie de Rennes, CS 50837, 35708 Rennes Cedex 7, France.
authors.forename: Régis
authors.initials: R
authors.lastname: Gautier
authors.affiliation: Department of Physics and Astronomy, University College London, Gower Street, London, WC1E6BT, UK.
authors.forename: Chris J
authors.initials: CJ
authors.lastname: Pickard
authors.affiliation: Department of Physics, Faculty of Science, University of Zagreb, Bijenička cesta 32, HR-10000 Zagreb, Croatia.
authors.forename: Marko
authors.initials: M
authors.lastname: Bosiočić
authors.affiliation: Department of Physics, Faculty of Science, University of Zagreb, Bijenička cesta 32, HR-10000 Zagreb, Croatia.
authors.forename: Mihael S
authors.initials: MS
authors.lastname: Grbić
authors.affiliation: Department of Physics, Faculty of Science, University of Zagreb, Bijenička cesta 32, HR-10000 Zagreb, Croatia.
authors.forename: Miroslav
authors.initials: M
authors.lastname: Požek
keywords: (35)Cl NMR; (81/79)Br NQR; (93)Nb NMR; (93)Nb NQR; DFT calculations; GIPAW; Niobium halide clusters; Solid-state NMR