Exploring NMR parameters of paramagnetic Cu complexes using density functional theory

Abstract

In this thesis, Density Functional Theory (DFT) methods have been validated to compute Nuclear Magnetic Resonance (NMR) chemical shifts for paramagnetic materials to complement solid-state NMR experiments carried out in the Ashbrook group. The materials studied include Cu phenolic oximes (which contain one paramagnetic centre), urea loaded Cu benzoate (containing two paramagnetic centres), and Metal-Organic Frameworks (MOFs) from the HKUST-1 and STAM families loaded with a variety of guest molecules (which have multiple paramagnetic centres). For Cu phenolic oximes, a combination of experiment and computation has revealed significant substituent effects on the ¹³C and ¹H chemical shifts. For urea loaded Cu benzoate, the observed δ (¹³C) values are reproduced reasonably well at the PBE0-⅓/II//PBE0-D3/AE1 level assuming a Boltzmann distribution between a diamagnetic open-shell singlet ground state (in a broken-symmetry Kohn-Sham DFT description) and an excited triplet state. Using the proposed assignments of the signals, the mean absolute deviation between the computed and observed ¹³C chemical shifts is below 30 ppm over a range of more than 1100 ppm. For HKUST-1 loaded with a variety of guests, a trimmed dimer (where three benzoate moieties of the dimer have been replaced with three acetate moieties) is shown to reproduce ¹³C chemical shifts for the carboxylate and benzoic carbons in the MOFs reasonably well at low computational cost. This reasonable accuracy can only be achieved after empirical scaling of the singlet-triplet energy gap (by factors close to 2). Molecular models with increasing numbers of dimer units (two or three joined by benzene linkers) have been validated against experimental ¹³C pNMR shifts for activated and hydrated MOFs in HKUST-1, STAM-1 and STAM-17. Using an appropriate selection of electronic states, calculations with lower scaling factors of the energy gaps between the spin states can fully reproduce the unusual temperature dependence of ¹³C shifts and substituent effects on these shifts in the STAM MOFs at the CAM-B3LYP/II//GFN2-xTB level.