Relevance of electron spin dissipative processes to dynamic nuclear polarization via thermal mixing

TL;DR

This paper demonstrates that electron spin relaxation processes significantly influence dynamic nuclear polarization (DNP) performance and introduces a modified thermal mixing model that accounts for these effects, explaining experimental observations and the impact of gadolinium doping.

Contribution

The authors develop a new thermal mixing model including electron spin relaxation, explaining DNP behavior and the effects of gadolinium doping on polarization enhancement.

Findings

Nuclear steady state polarization is independent of internuclear distance.

Electron spin relaxation dominates DNP performance.

Gadolinium doping enhances polarization by shortening electron spin-lattice relaxation time.

Abstract

The available theoretical approaches aiming at describing Dynamic Nuclear spin Polarization (DNP) in solutions containing molecules of biomedical interest and paramagnetic centers are not able to model the behaviour observed upon varying the concentration of trityl radicals or the polarization enhancement caused by moderate addition of gadolinium complexes. In this manuscript, we first show experimentally that the nuclear steady state polarization reached in solutions of pyruvic acid with 15 mM trityl radicals is substantially independent from the average internuclear distance. This evidences a leading role of electron (over nuclear) spin relaxation processes in determining the ultimate performances of DNP. Accordingly, we have devised a variant of the Thermal Mixing model for inhomogenously broadened electron resonance lines which includes a relaxation term describing the exchange of magnetic anisotropy energy of the electron spin system with the lattice. Thanks to this additional term, the dependence of the nuclear polarization on the electron concentration can be properly accounted for. Moreover, the model predicts a strong increase of the final polarization on shortening the electron spin-lattice relaxation time, providing a possible explanation for the effect of gadolinium doping.