Electron spin resonance spectra and structure of the radical cations of dibromoalkanes and monobromoalkanes

Abstract

The radical cations of dibromoalkanes containing two bromines separated by an alkyl chain, have been generated by exposing dilute solutions of the dibromoalkanes in freon to X-rays at 77 K. Two types of e.s.r. spectra were observed for these cations. The spectra obtained for Br(CH₂)_nBr, n= 1–7, show septet features characteristic of two equivalent Br nuclei. The coupling constant of the septet increases and reaches a plateau, with increased n in these dibromoalkanes. This indicates that a positive hole in these cations is shared, not by the two Br nuclei connected through adelocalization over the chain, but by the two Br nuclei directly bonded to each other, in accordance with conclusions recently drawn from e.s.r. studies of dichloroalkane cations.

In contrast, the e.s.r. spectra obtained for Br(CH₂)_nBr, n > 7, are more complicated. When similar experiments were carried out with H(CH₂)_nBr, n > 7, analogous spectra were obtained, indicating that the spectra observed for these dibromoalkanes can be interpreted in terms of hyperfine interactions to only one of the two Br nuclei and to two nuclei with nuclear spins of ½. This result, considered together with the fact that similar cationic centres were detected for H(CH₂)_nBr, n > 3, and with the spectral change observed for Br(CH₂)_nBr and H(CH₂)_nBr cations in different matrices (CCl₂FCClF₂, CCl₃F, and CCl₄) indicates that cyclization also occurs for these cations, the positive hole being occupied in a three-centre bond comprising a Br nucleus, a C nucleus (four atoms away from the Br nucleus), and an H nucleus bonded to the C nucleus.