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Lookup NU author(s): Dr Magda Pascual-BorrasORCiD, Dr John ErringtonORCiD
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0).
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.
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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