Electron Spin Resonance of Gamma-Irradiated Single Crystal of Dihydrouracil

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Abstract
Free radicals in gamma-irradiated dihydrouracil have been identified as –C(5)H2–C(6)H–. Two sets of resonance lines due to two magnetically distinct molecules were observed. From the direction cosines of the α-proton coupling tensor it has been concluded that the N(6) atom, originally significantly out of the plane defined by the atoms N(1), C(2), N(3), and C(4), comes to that plane after the abstraction of a hydrogen atom from C(6). The nonequivalence of the β-proton couplings proves that the C(5) atom remains out of the plane even in the radical form. Calculated bond energies for the four C–H bonds indicate that the H(6′) hydrogen atom should be the most easily removed. That is in agreement with the observations.