Electron Spin Resonance of a Gamma‐Irradiated Single Crystal of Barbituric Acid Dihydrate

Free radicals produced in single crystals of barbituric acid dihydrate by ionizing radiation have been identified by ESR spectroscopy and the parameters of the Hamiltonian describing the free radicals have been determined. The stable radical observed at room temperature is formed by loss of a hydrogen atom from C(5) of the original molecule, thus leaving an unpaired electron mainly in a 2pπ orbital on the carbon atom. Within experimental error, the free radicals have the same orientation in the crystal lattice as did the original molecules. Principal values of the g tensor characterizing the resonance were found to be gu=2.0044, gv=2.0056, gw=2.0022. Hyperfine interaction of the unpaired electron with the single hydrogen atom bonded to C(5) of the free radical splits the absorption into a doublet. Principal values of the hyperfine tensor were found to be Ax=11.2 G, Ay=30.0 G, Az=20.4 G. The principal axis systems for the g tensor and the hyperfine coupling tensor coincide. Two sets of satellite lines corr...