Naslov
Solid-State NMR/NQR and First-Principles Study of Two Niobium Halide Cluster Compounds

Autori
Perić, Berislav ; Gautier, Régis ; Pickard, Chris J. ; Bosiočić, Marko ; Grbić, Mihael S. ; Požek, Miroslav

Izvornik
Solid state nuclear magnetic resonance (0926-2040) 59/60 (2014); 20-30

Vrsta, podvrsta i kategorija rada
Radovi u časopisima, članak, znanstveni

Ključne riječi
DFT calculations ; GIPAW ; solid-state NMR ; niobium halide clusters ; 93Nb NMR ; 93NbNQR ; 35Cl NMR ; 81/79Br NQR

Sažetak
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 the 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 [Nb6X12]2+ unit. Bridging halogen atom sites are characterized by large asymmetry of the 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 centre of the [Nb6X12]2+ cluster. 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.

Izvorni jezik
Engleski

Znanstvena područja
Fizika, Kemija

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