Chemical behavior of 5-vinyl-2-norbornene, 5-ethylidene-2-norbornene, and related compounds as a key to understanding specifics of radiation-chemical processes: 3. The structure of 5-vinyl-2-norbornene and 2-vinylnorbornane radical cations

Abstract

Cyclic and distonic forms of radical cations were recorded by ESR in x-ray irradiated frozen solutions (1.0 vol %) of 5-vinyl-2-norbornene (VNB) and 2-vinylnorbornane in Freon 113 (CFCl2CF2Cl). In both cases, mixtures of isomers in a ratio of endo-: exo-= 66: 34 were used. It was shown that, in the general case, the endo-VNB radical cation rearranges into the radical cation of cis-3a,4,7,7a-tetrahydroindene through a multistage mechanism involving the intermediate formation of the distonic radical cation, not via the earlier suggested one-stage concerted mechanism. The interconversions of endo-and exo-VNB in γ-irradiated solutions of VNB in CCl4 were studied chromatographically. A general scheme of VNB transformations with allowance for the intermediate formation of the cyclic and distonic forms of the VNB radical cations is proposed. It was found that the structures of the radical cation distonic form and VNB homopolymer are identical, a resemblance that indicates the occurrence of VNB polymerization via the radical cation mechanism during its γ-irradiation.