Abstract
On heating to 180°c, γ-irradiated crystals of sodium formate undergo an irreversible change. The CO2 - radical trapped in the crystal lattice reacts with a neighbouring formate ion to form a new free radical in a specific lattice orientation. The electron spin resonance spectrum of this secondary radical exhibits hyperfine structure due to the interaction of the free electron with one proton and (in a 13C enriched crystal) two carbon nuclei. Measurements of these interactions are presented and from them it is inferred that the secondary radical has the structure of CO2 -. O. CH. O-. The location of the radical in the lattice is discussed.