Role of the glassy dynamics and thermal mixing in the dynamic nuclear polarization and relaxation mechanisms of pyruvic acid

TL;DR

This study investigates how glassy dynamics and thermal mixing influence nuclear relaxation and polarization in pyruvic acid at very low temperatures, revealing a quadratic temperature dependence and collective electron involvement in DNP.

Contribution

It demonstrates the role of structural dynamics and thermal mixing in nuclear relaxation and DNP mechanisms in pyruvic acid, highlighting the collective electron involvement.

Findings

Quadratic temperature dependence of relaxation rates in pyruvic acid.

Correlation between $^{13}$C build-up rate and electron relaxation rate.

Evidence supporting thermal mixing involving all electrons in DNP.

Abstract

The temperature dependence of $^1$H and $^{13}$C nuclear spin-lattice relaxation rate $1/T_1$ has been studied in the 1.6 K - 4.2 K temperature range in pure pyruvic acid and in pyruvic acid containing trityl radicals at a concentration of 15 mM. The temperature dependence of $1/T_1$ is found to follow a quadratic power law for both nuclei in the two samples. Remarkably the same temperature dependence is displayed also by the electron spin-lattice relaxation rate $1/T_{1e}$ in the sample containing radicals. These results are explained by considering the effect of the structural dynamics on the relaxation rates in pyruvic acid. Dynamic nuclear polarization experiments show that below 4 K the $^{13}$C build up rate scales with $1/T_{\text{1e}}$, in analogy to $^{13}$C $1/T_1$ and consistently with a thermal mixing scenario where all the electrons are collectively involved in the dynamic nuclear polarization process and the nuclear spin reservoir is in good thermal contact with the electron spin system.