Primary reactions in crystalline halogenated acetic acids studied by ESR spectroscopy and pulse radiolysis

Abstract

The radical anion BrCH2COOOH− was identified by ESR spectroscopy in single crystals of monobromoacetic acid γ-irradiated at 77K. The primary radical gave approximately axially symmetric hyperfine couplings to the bromine with A| = 283 G, A⊥ = 101 G and hyperfine couplings to the protons (≈15 G). The first order decay of the radical anion was studied by pulse radiolysis. This radical has an optical absorption at 440 nm and decomposes by breaking the CBr bond. The activation energy for this process is 12.3 kcal/mole. The radical anion ICH2COOH− was identified by ESR in powder samples of monoiodoacetic acid at 77 K. The hyperfine coupling to the iodine is A| = 360 G and A⊥ = 163 G. The decay of this radical was followed by ESR. The spin density distribution in halogenated anions containing Cl, Br and I is discussed in terms of the electronegativity of these atoms. The secondary radical ·CH2COOH formed in the anionic decay was detected in both acids. The radical ·CH2Br probably formed in the cationic decay was observed. A mechanism consistent with the observed species is proposed.