Nature of the chlorine-centred paramagnetic species in irradiated crystals of cytosine hydrochloride doped with thiocytosine.

It has been demonstrated that the paramagnetic species formed in crystals of cytosine hydrochloride doped with thiocytosine are most likely the radicals formed by association of thiocytosine and chlorine. The unpaired electron is positively identified on the Cl atom and on at least one of the pyrimidine atoms, and the data are consistent with the formation of the 'three-electron' S-Cl sigma sigma * bonds. The spin density derived from the measured 35Cl and 14N coupling tensors compare reasonably well with the MO calculations for the 'in-plane' conformations of three hypothetical radicals formed by initial ionization of either thiocytosine or Cl- followed by their reaction. Two distinct and related centres, A and B, probably represent two different conformations of chemically the same radical in the crystal lattice of cytosine-HCL.