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A 93 Nb Solid‐State NMR and Density Functional Theory Study of Four‐ and Six‐Coordinate Niobate Systems
Author(s) -
Hanna John V.,
Pike Kevin J.,
Charpentier Thibault,
Kemp Thomas F.,
Smith Mark E.,
Lucier Bryan E. G.,
Schurko Robert W.,
Cahill Lindsay S.
Publication year - 2010
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200901581
Subject(s) - castep , electric field gradient , density functional theory , chemistry , anisotropy , solid state nuclear magnetic resonance , ab initio quantum chemistry methods , chemical shift , quadrupole , ab initio , nuclear magnetic resonance , analytical chemistry (journal) , molecular physics , computational chemistry , physics , atomic physics , molecule , optics , organic chemistry , chromatography
A variable B 0 field static (broadline) NMR study of a large suite of niobate materials has enabled the elucidation of high‐precision measurement of 93 Nb NMR interaction parameters such as the isotropic chemical shift ( δ iso ), quadrupole coupling constant and asymmetry parameter ( C Q and η Q ), chemical shift span/anisotropy and skew/asymmetry ( Ω /Δ δ and κ / η δ ) and Euler angles ( α , β , γ ) describing the relative orientation of the quadrupolar and chemical shift tensorial frames. These measurements have been augmented with ab initio DFT calculations by using WIEN2k and NMR‐CASTEP codes, which corroborate these reported values. Unlike previous assertions made about the inability to detect CSA (chemical shift anisotropy) contributions from Nb V in most oxo environments, this study emphasises that a thorough variable B 0 approach coupled with the VOCS (variable offset cumulative spectroscopy) technique for the acquisition of undistorted broad (−1/2↔+1/2) central transition resonances facilitates the unambiguous observation of both quadrupolar and CSA contributions within these 93 Nb broadline data. These measurements reveal that the 93 Nb electric field gradient tensor is a particularly sensitive measure of the immediate and extended environments of the Nb V positions, with C Q values in the 0 to >80 MHz range being measured; similarly, the δ iso (covering an approximately 250 ppm range) and Ω values (covering a 0 to approximately 800 ppm range) characteristic of these niobate systems are also sensitive to structural disposition. However, their systematic rationalisation in terms of the NbO bond angles and distances defining the immediate Nb V oxo environment is complicated by longer‐range influences that usually involve other heavy elements comprising the structure. It has also been established in this study that the best computational method(s) of analysis for the 93 Nb NMR interaction parameters generated here are the all‐electron WIEN2k and the gauge included projector augmented wave (GIPAW) NMR‐CASTEP DFT approaches, which account for the short‐ and long‐range symmetries, periodicities and interaction‐potential characteristics for all elements (and particularly the heavy elements) in comparison with Gaussian 03 methods, which focus on terminated portions of the total structure.