Heteronuclear DNP of 1H and 19F nuclei using BDPA as a polarizing agent

Abstract

This work explores the dynamic nuclear polarization (DNP) of ¹H and ¹⁹F nuclei in a sample of 25/75 (% v/v) fluorobenzene/toluene containing the radical 1,3-bisphenylene-2-phenylallyl radical (BDPA) as a polarizing agent. Previously, heteronuclear effects in DNP were studied by analysing the shapes of DNP spectra, or by observing cross-relaxation between nuclei of different types. In this work, we report a rather specific DNP spectrum, where ¹H and ¹⁹F nuclei obtain polarizations of opposite signs upon microwave (MW) irradiation. In order to explain this observation, we introduce a novel mechanism called heteronuclear thermal mixing (hn-TM). Within this mechanism the spectra of opposite signs can then be explained due to the presence of four-spin systems, involving a pair of dipolar coupled electron spins and hyperfine coupled nuclear spins of ¹H and ¹⁹F, such that a condition relating their Larmor frequencies |ω_1e − ω_2e| ≈ ω_H − ω_F is satisfied. Under this condition, a strong mixing of electron and nuclear states takes place, enabling simultaneous four-spin flip-flops. Irradiation of electron spin transitions with MW followed by such four-spin flip-flops produces non-equilibrium populations of |α_Hβ_F〉 and |β_Hα_F〉 states, thus leading to the enhancements of opposite signs for ¹H and ¹⁹F. Signal enhancements, build-up times and DNP-spectra as a function of MW power and polarizing agent concentration, all provide additional support for assigning the observed DNP mechanism as hn-TM and distinguishing it from other possible mechanisms. We also develop a quantum mechanical model of hn-TM based on averaging of spin Hamiltonians. Simulations based on this model show very good qualitative agreement with experimental data. In addition, the system exhibits cross-relaxation between ¹H and ¹⁹F induced by the presence of BDPA, which was detected by measuring the ¹⁹F signal build-up upon saturation of ¹H nuclei with a train of radio-frequency pulses. We demonstrate that such cross-relaxation most likely originates due to the same electron and nuclear states mixing in the four-spin systems.