Radiation chemical formation of radical cations of halogenoalkanes and their electron spin resonance spectra and structure

Abstract

Exposure of dilute solutions of chloro-, bromo- and iodo-alkanes in trichlorofluoromethane to ⁶⁰Co γ-rays at 77 K gave the corresponding cations. However, for RCI⁺ and RBr⁺ the hole is shared with one chlorine atom of a solvent molecule via the formation of a σ bond, as in [RCI[graphic ommitted]CICFCI₂]⁺. For alkyl chlorides the hyperfine coupling constants of the two chlorine nuclei are nearly equal. The (RBr)⁺ adduct shows e.s.r. spectra consisting of dominant quartet splittings from bromine and additional quartet splittings from chlorine. For the iodo-cations, no clear evidence for such bonding is obtained, and there are very large shifts in the g-values resulting from marked spin-orbit coupling. For more concentrated solutions, the e.s.r. spectra show septet hyperfine patterns arising from coupling to two equivalent halogen nuclei, which indicates the formation of dimer radical cations, (RX)₂⁺, again having the unpaired electron in a σ* orbital. In the particular case of t-butyl chloride, neither the parent cation, nor its complex with a solvent molecule were detected. Instead, a multiline spectrum was obtained which we assign to (Me₂CCH₂)⁺ cations weakly complexed with HCl. Theoretical calculations on the parent cations, their dimers, and adducts with CFCl₃ support these assignments.