Characterization of Strongly Hyperfine-split Protons by DNP

TL;DR

This study investigates the mechanisms of polarization transfer in dynamic nuclear polarization, identifying the spatial extent of proton spin diffusion barriers and electron-electron spin diffusion processes using advanced NMR techniques.

Contribution

It combines reverse DNP and band-selective inversion pulses to characterize hyperfine-split protons and quantify spin diffusion barriers and rates in trityl samples.

Findings

Proton spin diffusion is quenched for hyperfine couplings >250 kHz.

The spin diffusion barrier radius is estimated between 5.4 and 6.8 Å.

Electron-electron spin diffusion influences the proton NMR spectrum.

Abstract

Dynamic nuclear polarization experiments use microwave irradiation to transfer the larger electron polarization to nuclear spins of interest, and thus enhance the NMR transitions above thermal equilibrium. How the polarization transfer from the electron spin to the nuclear spins in such experiments proceeds and which nuclear spins close to an unpaired electron get polarized and contribute through spin diffusion to the observable bulk nuclear magnetization is not fully understood. We address these questions by combining reverse DNP and band-selective inversion pulses on nuclear spins. We report the electron-detected NMR spectrum of proton spins involved in the direct DNP process in Ox063 trityl samples with protonated and deuterated solvents and variable radical concentrations. We also determine the spin-diffusion barrier surrounding trityl and find that proton spin diffusion is quenched for hyperfine coulings exceding ~250 kHz. This corresponds to a radius of the spin diffusion barrier in the range from 5.4 to 6.8 {\AA}. Burning a hole into the NMR spectrum of proton spins involved in the direct DNP step reveals an electron-electron spin diffusion process imprinted on the proton spectrum. We explain this diffusion process using a three-spin system consisting of two electron spins and one proton and quantify the electron spin diffusion rate constant.