EPR spectroscopy by dynamic nuclear polarization in low magnetic field

Dynamic nuclear polarization (DNP) is presented as an application for EPR spectroscopy in low magnetic field (below 40 G), An updated theory of DNP allows the calculation of the maximum DNP factor for all EPR transitions of N-14 or N-15 nitroxides. The DNP factor is considered as a function of the magnetic field and as a function of the direction of the applied RF field, The equations make it possible to calculate the theoretical DNP spectra of nitroxide which is essential for the interpretation of the DNP spectra of nitroxides in a very low magnetic field, The EPR spectra obtained by field-cycling dynamic nuclear polarization with an RF field for the EPR irradiation at 64, 72, and 74 MHz for the TXO(N-14) and 62 and 66 MHz for the PCD(N-15) nitroxides are presented, The RF field for EPR saturation was applied perpendicular to the static magnetic field in order to induce pi transitions, except for the 64 and 74 MHz spectra of the TXO(N-14) nitroxide, in which the RF field was applied parallel to the static magnetic field in order to induce sigma transitions. (C) 1996 Academic Press, Inc.