Identification of a Carbon-Centered Radical in ?-Irradiated Pimelic Acid Single Crystal: An EPR Study

Abstract

This study presents the first comprehensive single-crystal Electron Paramagnetic Resonance (EPR) investigation of gamma-irradiated pimelic acid. The EPR spectra were recorded at 125 K by rotating the crystal around the a*, b, and c crystallographic axes, and the angular variations of the spectra were analyzed to determine the magnetic parameters of the radiation-induced radical. The experimental spectra, together with spectral simulations, confirm the formation of a single carbon-centered radical (HOOC-(CH)-H-center dot-(CH2)4-COOH), generated by hydrogen abstraction from a methylene group. The principal values of the g-tensor (gaa = 2.00269, gbb = 2.00195, gcc = 2.00128) and hyperfine coupling tensors (Aaa= 2.294, Abb=2.032, Acc=1.738 mT for the alpha-proton and; Aaa = 1.099, Abb = 0.86, Acc= 0.705 mT for the beta-proton) were successfully determined, revealing significant anisotropy in the magnetic interactions. Temperature-dependent measurements demonstrated that the radical remains stable up to 420 K. The excellent agreement between the experimental and simulated spectra confirms the reliability of the determined spin-Hamiltonian parameters. These findings provide the first complete characterization of the radiation-induced radical in pimelic acid and contribute to the systematic understanding of radical formation in homologous aliphatic dicarboxylic acids.