This is really exciting work and the first time a vanadyl complex is used as the polarizing agent for DNP. Typically, nitroxides are used as polarizing agents, and this work clearly demonstrates the potential of metal complexes for DNP.
Gizatullin, Bulat, Marat Gafurov, Alexey Vakhin, Alexander Rodionov, Georgy Mamin, Sergei Orlinskii, Carlos Mattea, and Siegfried Stapf. “Native Vanadyl Complexes in Crude Oil as Polarizing Agents for In Situ Proton Dynamic Nuclear Polarization.” Energy & Fuels 33, no. 11 (November 21, 2019): 10923–32.
The presence of paramagnetic species such as vanadyl complexes (VO2+) and free carbon radicals in petroleum disperse systems (PDSs) such as crude oil, bitumen, or kerogen causes signiﬁcant interest of studying the structure of PDS, high-molecular weight components, and their eﬀects on the physical and chemical properties of PDS products by magnetic resonance techniques. However, the lack of detailed studies keeps the exact structure, aggregation mechanism, and interaction with complex composites of the PDS still disputable. In this contribution, detailed electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) investigations, including advanced fast ﬁeld cycling dynamic nuclear polarization, of heavy crude oil focused on vanadyl complexes are presented. A perceptible room-temperature 1H dynamic nuclear polarization (DNP) solid eﬀect at the X-band (magnetic ﬁeld of 300−400 mT corresponding to the EPR frequency of 9.5 GHz and NMR frequency of 14.6 MHz), with enhancement ±5, is observed at moderate microwave irradiation power in crude oil with a high concentration of VO2+, while no Overhauser DNP contribution is found. Using NMR T2-encoding, DNP spectra and molecular dynamics, two components are distinguished, from which the one with slower dynamics exhibits higher DNP enhancement via VO2+ complexes. The observed diﬀerence is discussed in terms of electron−nuclear interaction and relative parts of hyperpolarized nuclear spins using an advanced model for DNP data simulation.