Electron spin resonance study of lactone radical cations formed in Σ-irradiated freon matrices

Exposure of dilute solutions of a number of lactones in trichlorofluoromethane at 77 K to Co-60 Σ-rays resulted in electron loss centers which underwent fragmentation, or complex formation with the solvent, but gave no evidence for uncomplexed primary cations. For propiolactone the authors suggest that ring opening occurs to give the .CH2CH2OCO+ radical cation. For β-butyrolactone fragmentation of the ring is suggested to occur. Although the cyclic four-membered rings are suggested to open or fragment their results show that the larger five-, six-, and seven-membered rings form complex cations with the freon solvent. The complexes have large chlorine couplings (A/sub max/(Cl-35) = 62-73 G) which are in accord with sigma*-bonding between an oxygen lone pair and a chlorine atom on a CFCl3 molecule. Photobleaching or annealing the complexes results in new species in which H transfer has occurred. For Σ-butyrolactone hydrogen atom migration from one of the methylene groups to the carbonyl oxygen of the primary cation is inferred. In the case of Σ-valerolactone the results suggest that it is the unique tertiary hydrogen atom that migrates. Similar reactions are proposed for the cations of delta-valerolactones and epsilon-caprolactone. Molecular orbital calculations at the MINDO/2 level with full geometry optimization for several lactone cation radicals and propiolactone cation's likely ring-opened product were performed and clarify the nature and identity of the radicals. The experimental results are discussed in relation to recent work on esters and other carbonyl compounds. 24 references, 5 figures, 1 table