Overhauser effects in insulating solids.

TitleOverhauser effects in insulating solids.
Publication TypeJournal Article
Year of Publication2014
AuthorsCan, TV, Caporini, MA, Mentink-Vigier, F, Corzilius, B, Walish, JJ, Rosay, M, Maas, WE, Baldus, M, Vega, S, Swager, TM, Griffin, RG
JournalJournal of Chemical Physics
KeywordsOverhauser effect insulating solid

We report magic angle spinning, dynamic nuclear polarization (DNP) expts. at magnetic fields of 9.4 T, 14.1 T, and 18.8 T using the narrow line polarizing agents 1,3-bisdiphenylene-2-phenylallyl (BDPA) dispersed in polystyrene, and sulfonated-BDPA (SA-BDPA) and trityl OX063 in glassy glycerol/water matrixes. The 1H DNP enhancement field profiles of the BDPA radicals exhibit a significant DNP Overhauser effect (OE) as well as a solid effect (SE) despite the fact that these samples are insulating solids. In contrast, trityl exhibits only a SE enhancement. Data suggest that the appearance of the OE is due to rather strong electron-nuclear hyperfine couplings present in BDPA and SA-BDPA, which are absent in trityl and perdeuterated BDPA (d21-BDPA). In addn., and in contrast to other DNP mechanisms such as the solid effect or cross effect, the exptl. data suggest that the OE in non-conducting solids scales favorably with magnetic field, increasing in magnitude in going from 5 T, to 9.4 T, to 14.1 T, and to 18.8 T. Simulations using a model two spin system consisting of an electron hyperfine coupled to a 1H reproduce the essential features of the field profiles and indicate that the OE in these samples originates from the zero and double quantum cross relaxation induced by fluctuating hyperfine interactions between the intramol. delocalized unpaired electrons and their neighboring nuclei, and that the size of these hyperfine couplings is crucial to the magnitude of the enhancements. Microwave power dependent studies show that the OE sats. at considerably lower power levels than the solid effect in the same samples. Our results provide new insights into the mechanism of the Overhauser effect, and also provide a new approach to perform DNP expts. in chem., biophys., and phys. systems at high magnetic fields. (c) 2014 American Institute of Physics. [on SciFinder(R)]