A DNP INVESTIGATION OF PI-TRANSITION AND SIGMA-TRANSITION FOR NITROXIDE SPIN PROBES AT 1.53 MT

A weak field DNP study of 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinylo xy (4-hydroxy-TEMPO), 4-amino-2,2,6,6-tetramethyl-1-piperidinyloxy (4- amino-TEMPO), and 2,2-Di(4-tert-octylphenyl)-1-picrylhydrazyl (DPPH) f ree radicals in various solvents was performed at 1.53 mT using a doub le resonance spectrometer. pi transitions, in which the RF field has a component perpendicular to the static magnetic field B-0, and sigma t ransitions, in which the RF field has a component parallel to B-0, wer e recorded. In order to observe the a transitions a new probe head was designed. The sigma transitions were obtained at the frequencies expe cted from theoretical calculations. However, the results obtained for the observed enhancements deviated from the values expected theoretica lly. (C) 1998 Academic Press.