Impact of non-polar solvents in dynamic nuclear polarization at high magnetic fields

Abstract

Dynamic nuclear polarization (DNP) in liquids can enhance NMR signals by up to two orders of magnitude at magnetic fields greater than 9.4 T. The DNP experiment relies on driving electron spin transitions through microwave irradiation of the sample, which requires the solvent/sample to be transparent to microwaves. The physical models describing spin polarization transfer neglect the role of the solvent, despite recent experimental results suggesting that its impact on DNP efficiency can be as much as a factor of three. In this study, we aim to clarify how and why the solvent may affect DNP experiments at high magnetic fields. We examined known systems (¹³C-CCl₄/TEMPO and PPh₃/BDPA) dispersed in CCl₄, heptane, and benzene. By measuring their EPR properties, simulating microwave propagation patterns, and quantitatively assessing the DNP enhancements at 14.1 T, we determined that the choice of non-polar solvent is not critical to the outcome of a DNP experiment. Furthermore, our experimental results and electromagnetic simulations enable us to assess the state-of-the-art capabilities of DNP instruments at high magnetic fields and propose directions for possible future improvements.