Seminar

NMR in the Presence of Paramagnetic Centers: From Shifts and Relaxation to DNP

Daniel Jardon-Alvarez

The magnetic moment of electron spins is orders of magnitude larger than that from nuclear spins, therefore, the presence of even small amounts of paramagnetic centers can significantly alter the NMR properties of otherwise diamagnetic samples. Effects arising from the presence of paramagnetic centers are enhanced relaxation rates, line broadening, and frequency shifts. One of the most important parameters that determine how the paramagnetic interactions will manifest in the NMR spectrum is the electron relaxation time, T1e. In this talk I will first show how the ratio of nuclear T1/T2 relaxation times can be used to directly obtain T1e when paramagnetic relaxation enhancement is the dominating relaxation mechanism. Application of this analysis enabled us to understand the drastic changes in the NMR response as a function of paramagnetic dopant content in crystalline LiMg(1 x)MnxPO4 over a wide range of concentrations. Next, I will show how paramagnetic metal ions when introduced as a minor dopant into an otherwise diamagnetic host lattice can be used as polarizing agents to increase the NMR sensitivity of bulk nuclei through dynamic nuclear polarization (DNP). This approach can lead to signal enhancements of various orders of magnitude, enabling, for instance, acquisition of 17O spectra at natural abundance. Finally, focusing on heavily paramagnetic samples, I will present a novel experimental approach for spectral assignment, consisting in combining EPR and magnetic susceptibility measurements to predict paramagnetic shifts in the NMR spectrum.