Institut für Organische Chemie, Freie Universität Berlin, D-1000 Berlin 33
We have reinvestigated a series of 1-halofluorenone oxime iminoxyls, previously studied by Gilbert and Norman , primarily with the aim of finding out whether ENDOR spectroscopy can be performed with these systems which show an interesting syn/anti isomerism. ENDOR experiments on these radicals actually proved to be quite difficult, not only because of problems with radical concentration and stability, but also because of very unfavourable relaxation properties. The latter problems are caused by large spin populations at the hetero nuclei (nitrogen, oxygen and halogen) and also manifest themselves in the EPR spectra (high g values, large line widths in the case of quadrupolar nuclei 79Br/81Br and 127I). After all, 14N and 1H ENDOR experiments could be performed successfully on the chloro compound. The large 14N hyperfine coupling (a_N = 93.92 MHz) gives rise to pronounced second-order splittings in the 14N ENDOR spectra (2s = 0.46 MHz). Relative signs of some proton couplings could be determined by means of the CRISP technique. Chlorine and small proton hyperfine couplings were extracted from the EPR spectrum by means of computer fitting procedures . The data given in ref. 1 have to be revised in part.
 B. C. Gilbert and R. O. C. Norman, J. Chem. Soc. (B) 1967, 981; ibid., 1966,
 B. Kirste, J. Magn. Reson. 73, 213 (1987).