ESR Studies of the Thietane and Thiirane Radical Cations in Freon Matrices. Evidence for Ethylene Molecule Extrusion from the ? Thiirane Dimer Radical Cation .+>

Article abstract of DOI:10.1021/ja00256a036

ESR spectroscopy has been used to study the structures and reactions of the radical cations produced from the three- and four-membered sulfur-containing ring compounds, thiirane and thietane, the radical cations being generated by γ irradiation of dilute solutions of the parent compounds in Freon matrices at 77 K.With use of CFCl3 as the matrix, the monomer radical cations have been identified and characterized as having (2)B₁ ring-closed structures with the unpaired electron localized on the sulfur atom.The hyperfine coupling to the four equivalent β-hydrogens in the thietane cation (31.1 G) is normal, but the corresponding value for the thiirane cation (16.1 G) is lower by almost a factor of 2, suggesting that hyperconjugation to methylene groups is much reduced in three-membered rings.In the more mobile CFCl2CF2Cl and CF3CCl3 matrices, dimer radical cations of thiirane and thietane are produced by the combination of a monomer radical cation with a neutral molecule at low temperatures (<100 K).ESR studies show that these dimer species are centrosymmetric, the binding between the molecules resulting from the formation of a 3-electron ?²?^*1 S-S bond.Both of the dimer cations are unstable above 105 K in the CFCl2CF2Cl matrix, the thietane dimer radical cation decomposing to give the 2-thietanyl radical as a result of hydrogen atom or proton transfer, whereas the thiirane dimer radical cation undergoes a novel reaction involving the extrusion of an ethylene molecule.Evidence for ethylene formation in the latter reaction comes indirectly from the ESR observation of secondary radicals with the structure RCH2CH2 on annealing γ-irradiated solutions of thiirane in CFCl2CF2Cl.Similar radicals are also generated in γ-irradiated thiirane solutions in CF3CCl3 at 145-150 K, the ESR spectrum being indistinguishable from that of identically treated ethylene solutions.The ethylene extrusion is depicted as a concerted reaction in which the driving force is supplied by the transfer of the unpaired electron from the ?^* (S-S) orbital of the dimer cation to a vacant p orbital on the terminal sulfur atom of the remaining C2H4S2⁺ moiety.