Structure of Tetrafluorocyclopropane, Hexafluorocyclobutene, Octofluorocyclopentene, and Related Perfluoroalkene Radical Anions Revealed by ESR Spectroscopic and Computational Studies

Isotropic and anisotropic ESR spectra were observed for the radical anions of hexafluorocyclobutene (c-C4F6-), octafluorocyclopentene (c-C5F8-) and perfluoro-2-butene (CF3CFCFCF3-) in γ-irradiated plastically crystalline neopentane, tetramethylsilane (TMS) and TMS-d12 matrices, or the rigid 2-methyltetrahydrofuran (MTHF) matrix. The isotropic spectra of c-C4F6- and c-C5F8- are characterized by three different sets of pairs of 19F nuclei with the isotropic hyperfine (hf) splittings of 15.2 (2F), 6.5 (2F), 1.1 (2F) mT for c-C4F6- and 14.7 (2F), 7.4 (2F), 1.0 (2F) mT for c-C5F8-. By comparison with the results of ab initio quantum chemical computations, the large triplet 19F hf splittings of ca. 15 mT are assigned to the two fluorines attached to the CC bond. The UHF, B3LYP and MP2 computations predict that the geometrical structures of the perfluoroalkenes are strongly distorted by one-electron reduction to form their radical anions; c-C3F4-: C2 symmetry (2A state) ← C2v (1A1), c-C4F6-: C1 (2A) ← C2v (1A1) and c-C5F8-: C1 (2A) ← Cs (1A‘). The structural distortion arises from a mixing of the π* and higher-lying σ* orbitals at the CC carbons similar to that previously found for CF2CF2- with a C2h distortion. The isotropic 19F hf splittings computed with the B3LYP method with 6-311+G(2df,p) basis set for the geometry optimized by the UHF and/or MP2 methods are within 6% error of the experimental values. The experimental anisotropic spectra of c-C4F6-, c-C5F8- and CF2CF2- were satisfactorily reproduced by the ESR spectral simulation method using the computed hf principal values and orientation of 19F nuclei. In addition, the electronic excitation energies and oscillator strengths for the CF2CF2-, c-C3F4-, c-C4F6- and c-C5F8- radical anions were computed for the first time by TD-DFT methods.