Dynamic nuclear polarization of nitrogen-15 in benzamide

A N-15 dynamic nuclear polarization (DNP) experiment is reported in which a N-15 DNP enhancement factor of approximately 2.6 x 10(2) is obtained on free radical doped samples of 99% N-15 labeled benzamide. The free radicals BDPA (1:1 complex of alpha,gamma-bisdiphenylene-beta-phenylallyl with benzene) and DPPH (2,2-Di(4-tert-octylphenyl)-1-picrylhydrazyl) are used as dopants and the spin relaxation effects of adding these dopants are studied by means of changes in proton and nitrogen T-1 values of the samples. The combination in solids of a very low natural abundance, 0.37%, a small gyromagnetic ratio, and a long spin-lattice relaxation time for N-15 nuclei create severe sensitivity problems that, in large part, are ameliorated by the signal enhancement observed in the N-15 DNP experiment on samples containing free electrons. (C) 1997 Elsevier Science B.V.