Radical cations of ethylene oxide and other oxiranes prepared by exposure to ionizing radiation. An electron spin resonance study

Abstract

Exposure of dilute solutions of tetramethyl oxirane in trichlorofluoromethane to ⁶⁰C γ-rays at 77 K gave a species identified as the ring-opened radical cation, [graphic omitted]. Its e.s.r. spectrum comprised 13 lines with A(¹H)= 15.1 G. This species had an intense violet colour, and there was no evidence for the cyclic cation. Similarly, the cations of cyclopentene oxide and cyclohexene oxide gave red–violet colours and complex e.s.r. spectra which were interpreted in terms of the oxirane ring opening to give 6- and 7-membered cyclic cations only.

Both the 2,3-cis-dimethyl and the monomethyl derivatives on exposure at 4 K gave poorly defined e.s.r. spectra and no coloration. On warming to ca. 30 K the spectra became better defined and at 77 K intense red colours appeared together with new e.s.r. features which could be analysed in terms of the open-ring structures.

Oxirane itself gave a spectrum with splitting due to four equivalent protons, together with small extra splittings probably due to coupling to a chlorine nucleus. The proton coupling of ca. 16 G is close to that for the two α-protons of the open-chain radical (ĊHMeOĊHMe)⁺, which strongly supports an open structure. However, there was no visible absorption band under all conditions in CFCl₃. In marked contrast, a very similar e.s.r. spectrum of five lines was obtained from solutions in SF₆ at 77 K, but these solutions had an intense orange–red coloration. These results are tentatively interpreted in terms of a cyclic cation for oxirange in CFCl₃, but an ‘open’ cation in SF₆, both having very similar proton hyperfine coupling constants. Similarly, the acquisition of colour and change in the e.s.r. spectrum of the monomethyl and dimethyl derivatives is explained in terms of ring-opening in the 40–70 K region for solutions in CFCl₃.