The structure of trans- and cis-3-hexene radical cations stabilised in halocarbon matrices at low temperature: An ESR and MNDO/AM1 study

Abstract

The radical cations of trans- and cis-3-hexene have been studied by electron spin resonance spectroscopy in halocarbon matrices at low temperature. The experimental ESR results were compared with semiempirical MO calculations employing MNDO and AM1. The geometrical structure of the cis and trans isomers is discussed in terms of twisted double bonds. The trans-3-hexene radical cation formed by ionising irradiation was stabilised in CFCl 2, CF 3CCl 3 and CF 2ClCFCl 2 at 77 K. The experimental hyperfine splittings were attributed to the cation with a planar double bond structure at 77 K in CFCl 3. This assignment was supported by AM1 calculations which predicted a planar structure to have the lowest energy. The cation of trans-3-hexene transforms to an allyl radical in CF 2ClCFCl 2 at temperatures above 100 K. The cis-3-hexene cation, too, exhibits a structure with a planar double bond above 110 K in CFCl 3, while a twisted structure could not be excluded at lower temperature. AM1 and MNDO calculations predict cis-3-hexene + to be twisted, 22° and 23°, about the CC bond, respectively. A preferential motion of the trans-3-hexene cation was observable in CFCl 3 whereas such motion was absent in the cis isomer. Differences in the AM1 and MNDO optimised geometrical structures of both isomers are discussed.