Solid-state NMR/NQR and first-principles study of two niobium halide cluster compounds |
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| Institution: | 1. Laboratory for Solid-State and Complex Compounds Chemistry, Division of Materials Chemistry, Ru?er Bo?kovi? Institute, Bijeni?ka cesta 54, 10000-HR, Zagreb, Croatia;2. Institut des Sciences Chimiques de Rennes, UMR 6226, CNRS-Ecole Nationale Supérieure de Chimie de Rennes, CS 50837, 35708 Rennes Cedex 7, France;3. Department of Physics and Astronomy, University College London, Gower Street, London, WC1E6BT, UK;4. Department of Physics, Faculty of Science, University of Zagreb, Bijeni?ka cesta 32, HR-10000 Zagreb, Croatia;1. Institut des Sciences Chimiques de Rennes, ISCR-UMR CNRS 6226, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France;2. Institut National des Sciences Appliquées, INSA-ISCR, 35043 Rennes Cedex, France;1. Department of Chemistry, University of Zanjan, P.O. Box 45195-313, Zanjan, Islamic Republic of Iran;2. Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Islamic Republic of Iran;3. Department of Chemie, Ludwig-Maximilians-Universität München Butenandtstr. 5-13, Haus D81377 München, Germany;1. Instituto de Matemática Aplicada San Luis (CONICET-UNSL), (5700) San Luis, Argentina;2. Universidad Nacional de La Pampa, (6300) Santa Rosa, La Pampa, Argentina;3. Dpto. de Matemática, Facultad de Ciencias Exactas, Físico-Químicas y Naturales, Universidad Nacional de Río Cuarto, (5800) Río Cuarto, Córdoba, Argentina;4. Université d’Orléans, Laboratoire MAPMO, CNRS, UMR 7349, Fédération Denis Poisson, FR 2964, Bâtiment de Mathématiques, BP 6759, 45067 Orléans Cedex 2, France;1. LGM, National Engineering School of Monastir, University of Monastir, Tunisia;2. LGM, Preparatory Institute for Engineering Studies, University of Monastir, Tunisia;3. IUSTI, UMR 7343, Faculty of Science, University of Aix-Marseille, France |
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| 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 95Mo nuclei in structurally related compounds containing Mo6Br8]4+ cluster cores. Experimentally determined δiso(93Nb) values are in the range from 2400 to 3000 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 79/81Br NQR and 35Cl solid-state NMR measurements. |
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| Keywords: | DFT calculations GIPAW Solid-state NMR Niobium halide clusters |
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