ESR Studies on Radical Conversions in Irradiated Single Crystals of Succinic Acid

The ESR studies on the radical conversions from primary to room temperature stable radicals in irradiated single crystals of succinic acid have been carried out with aid of fumaric acid‐doped crystals. Since in the doped crystals the spectrum of succinic acid anion was replaced by that of furmaric acid anion, it was possible to identify the spectrum arising from ĊH₂CH₂COOH, which has been postulated for a long time as a precursor of the room temperature stable radical CH₃ĊHCOOH. It has been further found that a part of ĊH₂CH₂COOH converts into HOOCĊHCH₂COOH at $\text{− 100°}\mathrm{C}$ and then the radical isomerization from ĊH₂CH₂COOH to CH₃CHCOOH takes place at $\text{− 100 − ∼ − 75°}\mathrm{C}$ following the conformational change which is favorable to the intramolecular hydrogen transfer. The freshly formed CH₃CHCOOH radical has a different conformation from that of the room temperature stable species. The stable conformation is achieved by the 180° rotation around the $\mathrm{C}–\mathrm{C}$ bond between the CH and carboxyl carbon atoms in the unstable species. Besides these, the conversion process from the succinic acid anion to HOOCĊHCH₂COOH and HOOCCH₂CH₂C=O has been also studied. These results are based on the hyperfine and g tensors determined in the present work for CH₂CH₂COOH and unstable and stable CH₃ĊHCOOH as well as the stable anion.