Optimizing the signal enhancement in cryogenic ex situ DNP-NMR spectroscopy

Dynamic nuclear polarization (DNP) is often carried out at low temperatures to utilize the high spin polarization of the radical electrons at these temperatures. We have observed that in the absence of microwave irradiation and radical a transient negative polarization is observed for the methyl signals of acetone and DMSO. We suggest that this polarization arises from rotational tunneling levels in this system. For samples dissolved in these solvents, we see a diminished polarization with a microwave frequency of ω_e - ω_N compared to ω_e + ω_N, and we relate the two effects due to constructive/destructive interference with proton spin diffusion and spin symmetry diffusion mediating the intermolecular transfer of polarization. Perdeuteration of the solvents acts to nullify this effect. Awareness of this additional factor in the temperature jump DNP experiment allows improved experiment design and polarization of samples which have otherwise proven virtually impossible.