Predicting the NMR Spectra of Paramagnetic Molecules by DFT: Application to Organic Free Radicals and Transition-Metal Complexes

Nuclear shieldings, including the Fermi-contact and pseudocontact terms, have been calculated with DFT methods in a variety of open-shell molecules (nitroxides, aryloxyl and various transition-metal complexes), thereby predicting 1H and 13C chemical shifts. In general, when experimental data are reliable a good agreement with experimental values is observed, thus demonstrating the predictive power of DFT also in this context. However, the general accuracy is lower than for closed-shell species. A few inconsistencies in literature values are reconciled by re-assigning some shifts. Structural, magnetic and dynamic parameters have also been input to the Solomon-Bloembergen equation in order to predict signal lineshapes, in particular those of signals that are difficult to locate or undetectable. Guidelines are provided to predict the order of magnitude of relaxation rates. It is shown that DFT-predicted paramagnetic shifts can greatly aid in obtaining and understanding NMR spectra of paramagnetic molecules, which generally require different experimental strategies and exhibit problems in detection and assignment.