Formation of ˙CF3 and ˙CCl3 radicals by unimolecular decomposition of (CF3COR)˙+ and (CCl3COR)˙+ radical cations

Abstract

Exposure of very dilute solutions of CF₃COCH₃ in CFCl₃ to ⁶⁰Co γ-rays at 77 K gave ^˙CF₃ radicals, which were clearly identified by their e.s.r. spectra. There was no indication of the presence of the parent cations, CF₃(CH₃)CO⁺, which were expected to form under these conditions, and it is concluded that these decompose at 77 K to give ^˙CF₃+CH₃CO⁺. Methyl radicals were not detected. These results are compared with recent gas-phase studies which suggest that ^˙CH₃ should have been formed with far higher probability than ^˙CF₃. Whilst acetone cations also fail to give ^˙CH₃ radicals, cyclobutanone cations undergo ring-opening at 77 K to give H₂ĊCCH₂CH₂CO⁺ radical cations. Under these circumstances, (CCl₃)₂CO gave only ^˙CCl₃ radicals at 77 K, also well characterised by their e.s.r. spectra. Again there was no sign of spectra for the parent cations, so decomposition into ^˙CCl₃+CCl₃CO⁺ is postulated. In contrast, CCl₃CHO gave a novel radical at 77 K, characterised by a 40 G doublet, together with a large positive g-shift. This species, identified as the parent radical cation, decomposed in the 120 K region to give in ^˙CCl₃ radical.