  J-driven Dynamic Nuclear Polarization (JDNP) was recently proposed for
enhancing the sensitivity of solution-state nuclear magnetic resonance (NMR),
while bypassing the limitations faced by conventional (Overhauser) DNP at
magnetic fields of interest in analytical applications. Like Overhauser DNP,
JDNP also requires saturating the electronic polarization using high-frequency
microwaves, known to have poor penetration and associated heating effects in
most liquids. The present microwave-free JDNP (MF-JDNP) proposal seeks to
enhance the sensitivity of the solution state NMR by shuttling the sample
between higher and lower magnetic fields, with one of these fields providing an
electron Larmor frequency that matches the inter-electron exchange coupling
Jex. If spins cross this so-called JDNP condition sufficiently fast, we predict
that a sizable nuclear polarization will be created without microwave
irradiation. This MF-JDNP proposal requires radicals whose singlet/triplet
self-relaxation rates are dominated by dipolar hyperfine relaxation, and
shuttling times that can compete with these electron relaxation processes. This
communication discusses the theory behind the MF-JDNP, as well as proposals for
radicals and conditions that could enable this new approach to NMR sensitivity
enhancement.
