# Isotropic Hyperfine Interactions Drive Cross-Effect Dynamic Nuclear Polarization

## TL;DR

This paper introduces a new method for dynamic nuclear polarization using isotropic interactions, which improves efficiency and reduces dependence on sample orientation.

## Contribution

The novel approach uses isotropic hyperfine interactions to enable cross-effect dynamic nuclear polarization independent of sample orientation.

## Key findings

- Isotropic hyperfine interactions can fulfill the cross-effect condition in dynamic nuclear polarization.
- Experiments and simulations confirm the feasibility of using Mn(II) dopants with 55Mn nuclear spin for this approach.
- This method allows polarization enhancements independent of magic-angle spinning frequency.

## Abstract

Dynamic nuclear polarization
(DNP) is a powerful route
for overcoming
the inherent sensitivity limitation of solid-state nuclear magnetic
resonance (ssNMR) spectroscopy by transferring high electron spin
polarization to surrounding nuclear spins. Cross-effect (CE) DNP is
the most efficient mechanism in solids. CE requires several conditions
to be met, primarily the presence of two coupled electron spins with
resonance frequencies separated by the nuclear Larmor frequency. This
condition is typically achieved through the presence of large anisotropic
spin interactions, which shift the transition frequencies of the two
coupled electron spins with respect to each other. Here we present
an alternative approach, where the CE condition is met via isotropic
interactions. This is advantageous as it makes CE independent of the
sample orientation, thus making the enhancements independent of the
MAS frequency and enabling the use of fast relaxing polarizing agents.
We demonstrate the feasibility of the approach in experiments and
simulations for Mn­(II) dopants as polarizing agents, making use of
the isotropic hyperfine interactions with its 55Mn nuclear
spin to achieve the required frequency difference.