title: Structures and cis-to-trans photoisomerization of hexafluoro-1,3-butadiene radical cation: electron spin resonance and computational studies.
abstract: The s-cis and s-trans isomer radical cations of hexafluoro-1,3-butadiene (s-cis-HFBD+ and s-trans-HFBD+) were generated by a gamma-irradiated solid solution of the neutral HFBD molecule in solid matrix at 77 K and observed by means of electron spin resonance (ESR) and electronic spectroscopies. In comparing the experimental isotropic and anisotropic 19F hyperfine splittings with the computational ones by the DFT B3LYP and MP2 methods, the generated s-cis-HFBD+ and s-trans-HFBD+ radical cations were confirmed to be 2A2 and 2Bg electronic ground states in C2v and C2h symmetries, respectively. The present spectroscopic study revealed that the relative abundance of s-cis-HFBD+ to s-trans-HFBD+ was 4.0 immediately after being formed by gamma-irradiation, and subsequently most s-cis-HFBD+ was isomerized to s-trans-HFBD+ by visible-light illumination with 500-600 nm wavelength. The process of nonplanar HFBD ionizing to form stable planar s-cis- and s-trans-HFBD+ and the reaction mechanism of the cis-to-trans photoisomerization were discussed by (MS-)CASPT2//CASSCF calculated vertical excitation energies (Tv) and torsional potential energy curves (TPECs) of HFBD and HFBD+.
authors.affiliation: College of Chemistry, Beijing Normal University, Beijing, 100875, China.
authors.forename: Hong-Yan
authors.initials: HY
authors.lastname: Xiao
authors.forename: Jun
authors.initials: J
authors.lastname: Cao
authors.forename: Ya-Jun
authors.initials: YJ
authors.lastname: Liu
authors.forename: Wei-Hai
authors.initials: WH
authors.lastname: Fang
authors.forename: Hiroto
authors.initials: H
authors.lastname: Tachikawa
authors.forename: Masaru
authors.initials: M
authors.lastname: Shiotani