Pairwise Trapping of Dissimilar Radical Species and Radical Conversion in a Single Crystal of Monofluoroacetamide γ Irradiated at 77°K as Studied by Electron Spin Resonance

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Abstract
This is the first report of the pairwise trapping of dissimilar radical species in irradiated crystals. Besides the isolated radical of ·CH2CONH2, a radical pair between ·CH2CONH2 and ·CHFCONH2 was found in a single crystal of CH2FCONH2, γ irradiated at 77°K. The paired radicals were formed with one specific orientation and separation which were the same as with the molecules in the undamaged crystal. The observed principal values of the D tensor due to the magnetic dipole interaction of the two unpaired electrons gave a mean distance between the paired radicals of 7.03 Å. The alignment of the paired radicals was also determined by the principal directions of the D tensor. Both the separation and the alignment of the paired radicals thus obtained are in very good agreement with those calculated from the structure of the mother crystal. Furthermore, on warming the crystals the isolated radical ·CH2CONH2 was quantitatively converted into the radical ·CHFCONH2 which is the same species as found in room-temperature irradiation. The pairwise trapping of dissimilar radicals and the radical conversion found must give important information in considering the primary process of the radiation-induced reaction.