H-1 dynamic nuclear polarization in supercritical ethylene at 1.4 T

H-1 dynamic nuclear polarization (DNP) has been measured in supercritical e thylene in the pressure range 60-300 bar in an external field of 1.4 T. A s ingle-cell sapphire tube was used as a high-pressure cell, and powdered 1,3 -bisdiphenylene-2-phenyl allyl (BDPA) free radicals were added and distribu ted at the wall of the cell. At all pressures the dominant DNP mechanism wa s a positive Overhauser enhancement, caused by proton-electron contact inte ractions at the fluid/solid radical interface. The observed enhancements va ried from 12 at 60 bar to 17 at 300 bar. Besides the Overhauser enhancement , small solid state and thermal mixing enhancements also were observed, ind icating that part of the ethylene was adsorbed at the radical surface for a prolonged time. The impacts of the experimental conditions on the Overhaus er enhancement factors are discussed, and enhancements of at least 40-60 ar e estimated when the EPR saturation factor and the leakage factor become ma ximal. These data indicate that DNP-enhanced NMR has the potential of exten ding the impact of NMR in research areas involving supercritical fluids. (C ) 2000 Academic Press.