ENDOR studies on X-irradiated single crystals of acridine and phenazine

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Abstract
Stable radicals formed by room temperature X-irradiation of single crystals of acridine and phenazine have been identified and their structures determined by detailed ENDOR studies at 77 K. The E.S.R. spectra are of little assistance in the identification of these radicals. However, the high resolution of ENDOR has made it possible to study all the proton hyperfine interactions in detail. The radicals investigated in this study are formed as a result of the addition of atomic hydrogen to the nitrogen atom of the acridine and phenazine molecules. ENDOR lines of all the protons have been identified and analysed. Spin densities have been deduced from the proton hyperfine tensors. Although the unpaired electron is extensively delocalized throughout the molecule, there are large spin densities (0·475 and 0·584, respectively) on C9 in acridine and the second nitrogen in phenazine. Excellent agreement has been obtained between the observed spin densities and those computed by INDO molecular orbital calculations. The design and construction of a simple rectangular ENDOR cavity operating in the TE101 mode is described.