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17O NMR chemical shifts in oxometalates: from the simplest monometallic species to mixed-metal polyoxometalates

Lookup NU author(s): Dr Magda Pascual-Borras, Dr John Errington

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This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0).


Abstract

We report a theoretical analysis on O-17 NMR chemical shifts for a family of prototypical polyoxometalate anions. The huge diversity of structures and compositions in this family of oxometalates provides a unique resource for evaluating the influence of the metal type and connectivity over the resonance of O-17 nuclei. For a set of 75 signals, we show that DFT calculations performed with the GGA-type OPBE functional, including spin-orbit and scaling corrections, provide a mean absolute error < 30 ppm, a small value considering that the range of delta(O-17) values in these systems is similar to 1200 ppm. For terminal M=O oxygens, the chemical shifts primarily depend on the energy gap between pi(*)(M-O) and sigma(M-O) orbitals. When M is in its highest oxidation state, the energy of pi(*)(M-O) increases as we replace M going to the left and down in the periodic table. Consequently, we must expect large energy gaps and upfield shifts for O atoms linked to more electropositive ions. Although there is not a direct relationship between delta(O-17) and the negative charge of the oxygen, it is not entirely wrong to correlate atomic charge and chemical shift because the ionicity of the M-O bond, the orbital energy gap and the charge density of oxygen are related. The O-17 NMR chemical shifts move upfield with an increasing number of bound metal ions because of the larger energy gap in the involved orbitals. Finally, we explored the effect of protonation on delta(O-17) in oxometalates and demonstrated that O-17 NMR can be a powerful tool to identify the site(s) of protonation at low pH.


Publication metadata

Author(s): Pascual-Borras M, Lopez X, Rodriguez-Fortea A, Errington RJ, Poblet JM

Publication type: Article

Publication status: Published

Journal: Chemical Science

Year: 2014

Volume: 5

Issue: 5

Pages: 2031-2042

Print publication date: 01/05/2014

Online publication date: 17/03/2014

Acceptance date: 10/02/2014

Date deposited: 15/12/2014

ISSN (print): 2041-6520

ISSN (electronic): 2041-6539

Publisher: Royal Society of Chemistry

URL: http://dx.doi.org/10.1039/c4sc00083h

DOI: 10.1039/c4sc00083h


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