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Origin of the temperature dependence of 13C pNMR shifts for copper paddlewheel MOFs

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Date
03/02/2022
Author
Ke, Zhipeng
Dawson, Daniel
Ashbrook, Sharon E.
Buehl, Michael
Funder
EPSRC
The Royal Society
Grant ID
EP/M022501/1
WM150021
Keywords
QD Chemistry
DAS
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Abstract
An efficient protocol for the calculation of 13C pNMR shifts in metal-organic frameworks based on Cu(II) paddlewheel dimers is proposed, which involves simplified structural models, optimised using GFN2-xTB for the high-spin state, and CAM-B3LYP-computed NMR and EPR parameters. Models for hydrated and activated HKUST-1 and hydrated STAM MOFs with one, two and three Cu dimers have been used. The electronic ground states are low-spin and diamagnetic, with pNMR shifts arising from thermal population of intermediate- and high-spin excited states. Treating individual spin configurations in a broken symmetry (BS) approach, and selecting two or more of these to describe individual excited states, the magnetic shieldings of these paramagnetic states are evaluated using the approach by Hrobárik and Kaupp. The total shielding is then evaluated from a Boltzmann distribution between the energy levels of the chosen configurations. The computed pNMR shifts are very sensitive to temperature and, therefore, to the relative energies of the BS spin states. In order to reproduce the temperature dependence of the pNMR shifts seen in experiment, some scaling of the calculated energy gaps is required. A single scaling factor was applied to all levels in any one system, by fitting to experimental results at several temperatures simultaneously. The resulting scaling factors decreases with an increasing number of dimer units in the model (e.g., from ~1.7 for mono-dimer models to 1.2 for tri-dimer models). The approach of this scaling factor towards unity indicates that models with three dimers are approaching a size where they can be considered as reasonable models for the 13C shifts of infinite MOFs. The observed unusual temperature dependencies in the latter is indicated to arise both from the "normal" temperature dependence of the pNMR shifts of the paramagnetic states and the populations of these states in the thermal equilibrium.
Citation
Ke , Z , Dawson , D , Ashbrook , S E & Buehl , M 2022 , ' Origin of the temperature dependence of 13 C pNMR shifts for copper paddlewheel MOFs ' , Chemical Science , vol. Advance Article . https://doi.org/10.1039/D1SC07138F
Publication
Chemical Science
Status
Peer reviewed
DOI
https://doi.org/10.1039/D1SC07138F
ISSN
2041-6520
Type
Journal article
Rights
Copyright © 2022 The Author(s). Open Access. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Description
This work was supported by the EPSRC through the Collaborative Computational Project on NMR Crystallography (CCP-NC), via EP/M022501/1. S.E.A. would also like to thank the Royal Society and Wolfson Foundation for a merit award. Z.K. would like to thank the Chinese Scholarship Council (CSC) for a scholarship.
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  • University of St Andrews Research
URI
http://hdl.handle.net/10023/24890

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