Radical cations of organic sulphides and disulphides formed by radiolysis: an electron spin resonance study

Abstract

Exposure of dilute solutions of various organic sulphides in fluorotrichloromethane at 77 K to ⁶⁰Co γ-rays gave species identified by their e.s.r. spectra as the parent radical cations. The average β-proton coupling (R₂CHSR)⁺ of ca. 20 G is about half that for the corresponding ether cations, indicating greatly reduced σ-π delocalisation. However, the spread of g-values (ca. 2.032, 2.015, 2.00) is much greater than that for the ether cations. These cations readily react with other disulphide molecules to form (R₂S[graphic ommitted]S₂)⁺ dimer cations. The spectrum for tetrahydrothiophene (tetramethylene sulphide) exhibits a large coupling to two axial protons (ca. 42 G) and two equatorial protons (ca 19 G), the coupling constants again being about half those for tetrahydrofuran cations. However, conformational inversion did not occur below the softening point of the solids (ca. 160 K) in marked contrast with the ether cations. The three-and four-membered ring cations (ethylene and trimethylene sulphide cations) gave very similar spectra with g_X=g_Y, and four equivalent protons having an isotropic coupling of ca. 31G. Thus their structures are similar to the normal R₂S·⁺ cations, the equivalence of the g-values being interpreted in terms of effective C–Ŝ–C bond angles (θ) close to 90°. This, in turn, implies that θ >90° for the unconstrained cations. It is noteworthy that the ethylene oxide (oxirane) cation exhibits a smaller coupling to its four protons (16 G). This implies a drastic change in structure for the oxirane cation which is clearly not the case for the sulphur analogue. Disulphides of structure R-S-CH₂-S-R form cyclic σ* radicals with weak S-S bonding, in marked contrast with the oxygen analogues (acetal cations) which have π-structures conferring very high spin-density onto the CH₂ protons. Other molecules containing two RSR units form similar cyclic σ* radicals. Persulphides of structure RS-SR readily give the π-cations (RS˙S)⁺, characterised by g-values in the region of 2.035, 2.018, 2.002. The smaller range of g values for (PhS[graphic ommitted]SPh)⁺ cations is interpreted in terms of ca. 30% spin-delocalisation into both the benzene rings.