Radical-induced Hetero-Nuclear Mixing and Low-field $^{13}$C Relaxation in Solid Pyruvic Acid

TL;DR

This study investigates how radicals influence $^{13}$C relaxation and hetero-nuclear mixing in solid pyruvic acid, revealing a linear relaxation dependence on magnetic field and extending models to carbon nuclei.

Contribution

It extends existing models of radical-induced hetero-nuclear mixing from protons to carbon, and identifies additional effects like linebroadening and diffusion impacting relaxation.

Findings

Carbon $T_1$ relaxation is linearly dependent on magnetic field between 5 mT and 2 T.

Radical-enhanced hetero-nuclear mixing occurs via the Non-Zeeman reservoir from 20 mT to beyond 1 T.

Experimental polarization transfer confirms the model predictions and observed effects.

Abstract

Radicals serve as source in dynamic nuclear polarization, but may also act as polarization sink. If the coupling between the electron spins and different nuclear reservoirs is stronger than any of the reservoirs' couplings to the lattice, radicals can mediate hetero-nuclear mixing. Here, we report radical-enhanced $^{13}$C relaxation in pyruvic acid doped with trityl. We find a linear dependence of the carbon $T_1$ on field between 5 mT and 2 T. We extend a model, employed previously for protons, to carbon, and predict efficient proton-carbon mixing via the radical Non-Zeeman reservoir, for fields from 20 mT to beyond 1 T. Discrepancies between the observed carbon relaxation and the model are attributed to enhanced direct hetero-nuclear mixing due to trityl-induced linebroadening, and a field-dependent carbon diffusion from the radical vicinity to the bulk. Measurements of hetero-nuclear polarization transfer up to 600 mT confirm the predicted mixing as well as both effects inferred from the relaxation analysis.