Hyperfine Coupling Constants in Cu-Based Crystalline Compounds: Solid-State NMR Spectroscopy and First-Principles Calculations with Isolated-Cluster and Extended Periodic-Lattice Models

Abstract

NMR spectroscopy is a powerful tool for structural analysis of solids, especially if it is complemented by computations of NMR observables, such as chemical shifts and quadrupole coupling constants. In paramagnetic solids, chemical shifts can be greatly affected by hyperfine couplings among the unpaired electrons and atomic nuclei. In this study 13 C MAS NMR spectra of three representative crystalline solids (a simple coordination compound of copper and alanine and two complex copper-based metal−organic framework materials) were measured, and the observed paramagnetic shifts were correlated with the hyperfine coupling constants, calculated along two different density functional theory-based approaches. The first approach employed an isolated-cluster model, and the second one used an extended-lattice model and periodic boundary conditions. For both approaches the calculated isotropic hyperfine coupling constants correlated very well with the measured paramagnetic shifts and thus allowed the assignment of the 13 C NMR signals, spread over the region of 1000 ppm. This assignment to individual carbon crystallographic sites was in complete agreement with the experimentally derived assignment.

Crystal and Substituent Effects on Paramagnetic NMR Shifts in Transition-Metal Complexes

“…To investigate the hyperfine effects of paramagnetic neighbors on the NMR shifts of the central molecule of 2a in the solid state, 36 , 37 we resorted to the smallest cluster, the three molecules shown in Figure 5 b. Because the magnetic coupling constant and zero-field splitting are negligible, the electronic ground state of this clust(3 ↑↓ ) is eight-times degenerate (one quartet and two doublets).…”

Low-Dimensional Magnetism in Multivariate Copper/Zinc MOF-74 Materials Formed via Different Mechanochemical Methods

“… 13 C MAS NMR spectra of mixed-metal CuZn-MOF-74, ZnCu-MOF-74, and Zn-Cu-MOF-74-alloyed compared to the spectra of neat Cu-MOF-74 and Zn-MOF-74. Assignment of the carbon NMR signals of Zn-MOF-74 is based on the solution spectrum of H 2 dobdc, whereas the assignment of the carbon NMR signals of Cu-MOF-74 is based on 13 C spin-lattice relaxation measurements and density functional theory-based calculations of hyperfine coupling constants . Asterisks denote spinning sidebands.…”

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

“…However, in extended solids the magnetic moments of the individual paramagnetic sites may communicate and couple differently, complicating the modelling using a periodic approach. 29 Moreover, the paramagnetic MOFs of interest in this work, HKUST-1 and the STAM family, have the additional complication of interacting spins on the paramagnetic centres in the paddlewheel dimer (and the possibility of a thermal equilibrium of differently (ferro-or antiferromagnetically) coupled states), in addition to potential couplings between dimers. In recent work we have shown that the experimental 13 C d iso of a solid containing molecular Cu(II) paddlewheel dimers can be reproduced computationally assuming a thermal equilibrium between a diamagnetic ground state (with antiferromagnetic spin coupling on the Cu dimer) and an excited triplet state (with ferromagnetically coupled spins for a single molecule).…”