A 93Nb Solid-State NMR and Density Functional Theory Study of Four- and Six-Coordinate Niobate Systems

Hanna, John V. and Pike, Kevin J. and Charpentier, Thibault and Kemp, Thomas F. and Smith, Mark E. and Lucier, Bryan E. G. and Schurko, Robert W. and Cahill, Lindsay S. (2010) A 93Nb Solid-State NMR and Density Functional Theory Study of Four- and Six-Coordinate Niobate Systems. Chemistry - A European Journal, 16 (10). pp. 3222-3239. ISSN 0947-6539

Abstract

A variable B0 field static (broadline) NMR study of a large suite of niobate materials has enabled the elucidation of high-precision measurement of 93Nb NMR interaction parameters such as the isotropic chemical shift (δiso), quadrupole coupling constant and asymmetry parameter (CQ and ηQ), chemical shift span/anisotropy and skew/asymmetry (Ω/Δδ and κ/ηδ) 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 NbV in most oxo environments, this study emphasises that a thorough variable B0 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 93Nb broadline data. These measurements reveal that the 93Nb electric field gradient tensor is a particularly sensitive measure of the immediate and extended environments of the NbV positions, with CQ values in the 0 to >80 MHz range being measured; similarly, the δiso (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 NbV 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 93Nb 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.