Vinyl radical cations formed by radiolysis: an electron spin resonance study

Abstract

Exposure of dilute solutions of a range of vinyl compounds, H₂CCHX or H₂CCMeX in trichlorofluoro-methane to ⁶⁰Co γ-rays at 77 K generated the corresponding cations, most of which were well characterised by their e.s.r. spectra. Results for X = Me are discussed in terms of possible out-of-plane twisting for the cation. The 2-vinylpyridine cation favours a π SOMO similar to that of the styrene cation, whereas the 4-vinyl-derivative favours a SOMO which is largely the n(σ)-orbital on nitrogen. The cations of allyl chloride and bromide exhibit large hyperfine coupling to the halogen showing that these atoms favour the extreme out-of-plane site, with considerable delocalisation of the unpaired electron on to the halogen. Results for methyl acrylate and methyl methacrylate cations are difficult to interpret in terms of the expected π-structures. However, for X = CHO, the SOMO is clearly the n(O) orbital in the radical plane rather than the π-orbital. The cyano cations favour a π SOMO with significant spin density on nitrogen. For X = Br, large hyperfine coupling to bromine indicates ca. 39% delocalisation. For the vinyl ethers and acetate the SOMO is quite strongly localised on the H₂Ċ-unit with α-proton couplings of ca. 19 G. Results for X = SiMe₃ are of interest in that they provide clear evidence for out-of-plane twisting, as was recently established by Sakurai and co-workers. Also, on annealing to ca. 120 K these cations lost methyl radicals, which were detected by their characteristic e.s.r. spectrum.