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

Abstract

Thermal averaging over DFT-computed paramagnetic spin states can reproduce observed temperature-dependent shifts in 13C pNMR spectra of MOFs with Cu(ii) paddlewheel dimers.

13C pNMR shifts of MOFs based on Cu(ii)-paddlewheel dimers – DFT predictions for spin–1/2 defects

“…Most results in the previous work for models containing a single dimer were based on a structure with one Ph and three Me groups (model M4 in ref. 10) to reduce computational cost. However, owing to the reduced symmetry of models containing protonated defects we decided to retain the four phenyl groups in this study.…”

“…Using the same methodology as in ref. 10, we have computed the 13 C and 1 H chemical shifts for these models in their doublet (spin-1/2) states. Because the actual linkers in the MOFs have carboxylate substituents in the meta position we only report data for C1, C2, C3 and C3 0 (cf.…”

“…To this end, experimental solid-state NMR spectroscopy paired with theoretical prediction can aid in unravelling the structure of such paramagnetic MOFs and aid in the optimisation of their properties. Ke et al 10,11 have established a DFT-based methodology to compute the paramagnetic NMR (pNMR) chemical shifts of Cu-based MOFs using suitable model complexes. The paddlewheel type of system is special due to the coupling of the spins within each dimer.…”

Deconvolution of metal apportionment in bulk metal-organic frameworks

“…Here, we used the NorthWest computational Chemistry (NWChem, version 6.5) software package to perform DFT calculations ( 27 , 28 ). Calculations of chemical shifts in the presence of paramagnetic centers via DFT are fraught with difficulties, yet the order of calculated chemical shifts as a function of distance from the paramagnetic center is expected to follow the order of observed peak shifts, although the magnitude of calculated numbers may be quite different from observed shifts ( 6 , 12 – 15 , 29 , 30 ). Specifically, we used a cluster model consisting of five metal ions (fig.…”

Computational NMR of the iron pyrazolylborate complexes [Tp2Fe]+ and Tp2Fe including solvation and spin-crossover effects

“…[26][27][28][29][30] Recent papers also report investigations of spin-crossover systems and particularly their applications. [31][32][33][34][35][36] Adjacent to the topic, related spin equilibrium systems have been studied computationally using DFT and Boltzmann statistics by Ke et al 37,38 with the high-spin and low-spin configurations arising from two distinct paramagnetic centers. In this work, we examine the significance of the spin-crossover effect in predicting the pNMR chemical shifts in an isolated Tp 2 Fe(II) molecule, and how different computational fitting methods for handling its equilibrium state succeed in reproducing the experimental results.…”