ESR Evudence for Localized Forms of C - C Ring-Opened Oxirane Radical Cations

Article abstract of DOI:10.1021/j100263a006

Localized forms of the C...C ring-opened oxirane and methyl-substituted oxirane radical cations have been detected by ESR spectroscopy following γ-irradiation of solid solutions of the parent compounds in the CFCl2CF2Cl matrix.For oxirane, 1,2-dimethyloxirane, and tetramethyloxirane, he localized forms possess the RCH2., RCHMe., and the RCMe2. radical centers, respectively.In the unsymmetrically substituted methyloxiranes, the radical center is localized at the oxirane carbon with the least number of methyl groups.The temperature needed to bring about the formation of of the localized species increases with methyl substitution at the localized radical center, the oxirane and methoxirane species being formed during irradiation at 77 K while the tetramethyloxirane species is produced subsequently from the delocalized planar form of the of ring-opened cation at 105-110 K.In contrast, only the delocalized forms of the oxirane cations have been observed in CFCl3, CF3CCl3, and CCl4, even at much higher temperatures (145-215 K) close to the softening points of these matrices.The possible structures of the localized forms of the oxirane radical cations are discussed, the two most reasonable models being either an orthogonal structure with noninteracting radical and carbocation centers or a similar species in which the carbocation center is complexed with the solvent or combined with a chloride ion.Despite the lack of ESR and other evidence for some type of solvent haloen participation in the localized structure, the latter model cannot be ruled out at present.