ENDOR study of the displacements of the HCO2 - groups in the formation of secondary radicals in an X-irradiated single crystal of ?-calcium formate

The results of a room temperature ENDOR investigation of the secondary radicals formed after X-irradiation in a single crystal of ?-calcium formate are reported and discussed. 39 different proton hyperfine tensors have been determined, belonging to magnetically or structurally different radicals. The hyperfine tensors are typical of ?-protons in ?-radicals, with a spin density ?c = 0·75 on the central carbon atoms. The radicals are identified therefore -O-?H-O-CO2 - in twelve slightly different conformations. The mechanisms giving rise to the reactions of primary ?-radicals CO2 - with the neighbouring undamaged molecules through cooperative rotations around the Ca-O bonds are discussed. It is shown that the experimentally determined C-H directions are very well reproduced by rotations of the formate groups around a Ca-O bond.