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

Gennaro, Antonio; Karabanov, Alexander; Potapov, Alexey; Kockenberger, Walter

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 |ω1e-ω2e |≈ ωH-ωF 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 |αHβF ⟩ and |βHαF ⟩ 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 1F NMR 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.

Citation

Gennaro, A., Karabanov, A., Potapov, A., & Kockenberger, W. (2020). Heteronuclear DNP of 1H and 19F nuclei using BDPA as a polarizing agent. Physical Chemistry Chemical Physics, 22(15), 7803-7816. https://doi.org/10.1039/d0cp00892c

• Journal Article Type: Article
• Acceptance Date: Mar 26, 2020
• Online Publication Date: Mar 31, 2020
• Publication Date: Apr 21, 2020
• Deposit Date: Apr 4, 2020
• Publicly Available Date: Apr 1, 2021
• Journal: Physical Chemistry Chemical Physics
• Print ISSN: 1463-9076
• Electronic ISSN: 1463-9084
• Publisher: Royal Society of Chemistry
• Peer Reviewed: Peer Reviewed
• Volume: 22
• Issue: 15
• Pages: 7803-7816
• DOI: https://doi.org/10.1039/d0cp00892c
• Keywords: Physical and Theoretical Chemistry; General Physics and Astronomy
• Public URL: https://nottingham-repository.worktribe.com/output/4254897
• Publisher URL: https://pubs.rsc.org/en/content/articlelanding/2020/CP/D0CP00892C#!divAbstract
• Additional Information: : This document is Similarity Check deposited; : Supplementary Information; : Single-blind; : Received 17 February 2020; Accepted 26 March 2020; Accepted Manuscript published 31 March 2020