Projects per year
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.
Original language | English |
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Number of pages | 12 |
Journal | Chemical Science |
Volume | Advance Article |
Early online date | 3 Feb 2022 |
DOIs | |
Publication status | E-pub ahead of print - 3 Feb 2022 |
Fingerprint
Dive into the research topics of 'Origin of the temperature dependence of 13C pNMR shifts for copper paddlewheel MOFs'. Together they form a unique fingerprint.Projects
- 2 Finished
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RS Wolfson Merit Award: Exploiting NMR spectroscopy: Local structure and disorder in solids
Ashbrook, S. E. (PI)
1/09/15 → 31/08/20
Project: Standard
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NMR Crystallography: Collaborative Computational Project in NMR Crystallography - Collaborative Agreement
Ashbrook, S. E. (PI)
1/04/15 → 31/03/20
Project: Standard
Datasets
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Origin of the Temperature Dependence of 13C pNMR Shifts for Copper Paddlewheel MOFs (dataset)
Ke, Z. (Contributor), Dawson, D. M. (Contributor), Ashbrook, S. E. (Contributor) & Buehl, M. (Creator), University of St Andrews, 7 Feb 2022
DOI: 10.17630/233aeedf-1dc4-49e4-b5b8-2c136b2e2365
Dataset
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