High-field solution state DNP using cross-correlations

TL;DR

This paper analytically evaluates multiple cross-correlated relaxation processes in liquid state DNP at high magnetic fields, revealing a new mechanism that enhances nuclear polarization beyond traditional dipolar cross-relaxation.

Contribution

It introduces a comprehensive analytical framework for all rotationally driven relaxation processes and uncovers a novel cross-correlation DNP mechanism involving inter-electron exchange coupling.

Findings

Identified a new cross-correlation DNP mechanism (CCDNP) for 2e1n systems.

Demonstrated stronger nuclear polarization achievable at high fields with specific spin Hamiltonian parameters.

Provided analytical and numerical tools for optimizing DNP in liquid state NMR.

Abstract

At the magnetic fields of common NMR instruments, electron Zeeman frequencies are too high for efficient electron-nuclear dipolar cross-relaxation to occur in solution. The rate of that process fades with the electron Zeeman frequency as omega^{-2} - in the absence of isotropic hyperfine couplings, liquid state dynamic nuclear polarisation (DNP) in high-field magnets is therefore impractical. However, contact coupling and dipolar cross-relaxation are not the only mechanisms that can move electron magnetisation to nuclei in liquids: multiple cross-correlated (CC) relaxation processes also exist, involving various combinations of interaction tensor anisotropies. The rates of some of those processes have more favourable high-field behaviour than dipolar cross-relaxation, but due to the difficulty of their numerical - and particularly analytical - treatment, they remain largely uncharted. In this communication, we report analytical evaluation of every rotationally driven relaxation process in liquid state for 1e1n and 2e1n spin systems, as well as numerical optimisations of the steady-state DNP with respect to spin Hamiltonian parameters. A previously unreported cross-correlation DNP (CCDNP) mechanism was identified for the 2e1n system, involving multiple relaxation interference effects and inter-electron exchange coupling. Using simulations, we found realistic spin Hamiltonian parameters that yield stronger nuclear polarisation at high magnetic fields than dipolar cross-relaxation.