  One of solution-state Nuclear Magnetic Resonance (NMR)'s main weaknesses, is
its relative insensitivity. J-driven Dynamic Nuclear Polarization (JDNP) was
recently proposed for enhancing the sensitivity of solution-state NMR, by
bypassing the limitations faced by conventional Overhauser DNP (ODNP), at the
high magnetic fields where most analytical research is performed. By relying on
biradicals with inter-electron exchange couplings on the order of the electron
Larmor frequency, JDNP was predicted to introduce a transient enhancement in
the nuclear polarization at high magnetic fields, and for a wide range of
rotational correlation times of medium-sized molecules in conventional
solvents. This communication revisits the JDNP proposal, including additional
effects and conditions that were not considered in the original treatment.
These include relaxation mechanisms arising from local vibrational modes that
often dominate electron relaxation in organic radicals, as well as the
possibility of using biradicals with inter-electron exchange couplings of the
order of the nuclear Larmor frequency as potential polarizing agents. The
presence of these new relaxation effects lead to variations in the JDNP
polarization mechanism originally proposed, and indicate that
triplet-to-singlet cross-relaxation processes may lead to a nuclear
polarization enhancement that persists even at steady states. The physics and
potential limitations of the ensuing theoretical derivations, are briefly
discussed.
