Electron paramagnetic resonance studies of molecular orientation and nuclear exchange in single crystals of morphine N-oxide.
Weerts, Gary Lee
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Low temperature (-150°C) EPR spectra of the nitroxide generated photolytically in a single crystal of N-nitromorpholine are complicated by the presence of two magnetically distinct orientations of the N-0 bond within the crystal. As the temperature is raised, the spectra are further complicated by motional effects. Since at -150° the molecule is essentially frozen in the crystal lattice with respect to the time of the EPR interaction, and since the nitrogen coupling tensor is markedly anisotropic, the observed EPR spectrum varies widely as the orientation of the applied field changes. The anisotropy of the nitrogen coupling tensor made it possible to unequivocally determine the orientation of the crystal with respect to the applied magnetic field. A series of experimental spectra was then obtained for several combinations of crystal orientation and temperature.In addition, computer programs were developed which simulate low, intermediate, and high temperature EPR spectra. By using a combination of experimental spectra and these programs, it was possible to determine the angle between the two orientations of the N-0 bond in the crystal and also to determine the energy barrier to exchange.
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