X-irradiated cyanoacetylurea CNCH₂CONHCONH₂ and x-irradiated dicyandiaraide NCNC(NH₂)₂ single crystals were studied,by Electron Spin Resonance. Two radical species were formed when cyanoacetylurea, CNCH₂CONHCONH₂, was irradiated with x-rays. One was a π-electron radical, CNCHCONHCONH₂, and the other a σ-electron radical, CNCH₂CONHCONH . The σ electron radical was found to have a large, isotropic proton coupling tensor and an anisotropic nitrogen coupling tensor very similar to those of

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... ONH . The π-electron radical had similar proton and nitrogen coupling tensors as CNCHCOOH which was formed in γ-irradiated cyanoacetic acid. In x-irradiated dicyandiamide crystals, the main species formed was shown to be NCNC(NH₂)NH, having also a large, isotropic proton coupling tensor, and hyperfine interactions with two nitrogen coupling nuclei were also observed and measured. ESR studies were carried out both at room and at liquid nitrogen temperatures. The effect of temperature on the spectra is discussed. INDO-SCF-LCAO-MO calculations were carried out for a model compound HCONH , with the amide proton assuming different inplane and out of plane positions. The spin density was found to vary over a wide range and could be used to interpret the large proton coupling of the dicyandiamide radical. MO calculations were also performed on the radical NCCHCOO ̄. The calculated results correlate fairly satisfactorily with the observed ones. Comparison of the direction of the unpaired electron p-orbital symmetry axis with bond directions and with normals to the fragment planes showed that while malonamide radical, H₂NCOCH₂CONH was a genuine σ-electron radical, dicyandiamide radical, NCNC(NH₂)NH was most likely a π-electron radical.