NMR chemical shifts of urea loaded copper benzoate. A joint solid-state NMR and DFT study

We report solid-state C-13 NMR spectra of urea-loaded copper benzoate, Cu-2(C6H5CO2)(4) -2(urea), a simplified model for copper paddlewheel-based metal-organic frameworks (MOFs), along with first-principles density functional theory (DFT) computation of the paramagnetic NMR (pNMR) chemical shifts. 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, the observed S(C-13) values are reproduced reasonably well at the PBE0-1/3/IGLO-II/PBEO-D3/AE1 level. Using the proposed assignments of the signals, the mean absolute deviation between computed and observed C-13 chemical shifts is below 30 ppm over a range of more than 1100 ppm.