ESR Study of Electron Capture by Guest Molecules in Irradiated Single Crystals of Impurity-Doped Organic Compounds: Fumaric Acid-Doped Succinic Acid

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Abstract
The highly selective capture of an ejected electron by a small amount of guest molecules has been found by the ESR study of a single crystal of fumaric acid‐doped succinic acidirradiated at 77°K. The four‐line spectrum due to the molecular anion found in an irradiatedsingle crystal of pure succinic acid was replaced by the three‐line spectrum when about 0.005 mole fraction of fumaric acid was doped in a single crystal of succinic acid. The analyses of the hyperfine and g tensors of this spectrum lead to the conclusion that the radical responsible for this spectrum is the molecular anion of fumaric acid, which is believed to be a precursor of the stable radical formed by the addition of the acidic proton to fumaric acid in the room temperature irradiation. Our results suggest that the hydrogen addition radical is formed by the acidic proton transfer from the host molecule to the fumaric anion. The mechanisms for the formation of the stable radical and for the stereospecific addition of the acidic proton to the C=C bond are proposed based on the orientation of the guest molecule in the host crystal, which was determined by comparing the principal directions of the hyperfine and g tensors with the x‐ray crystallographic data.