Octofluorooxalane, C4F8O, has recently attracted attention as a comparatively green cleaning gas in vapor deposition chambers and has been put forward as a possible replacement of SF6 in high voltage insulation. Therefore, its reactivity with respect to free electrons was investigated in electron swarm and single-electron attachment experiments. Both the swarm experiments—where collisional stabilization is possible—and the single-electron experiments—where it is impossible—show as the most intense signal the parent anion, C4F8O-. In the single-electron experiments, the parent anion is associated with a broad peak centered at 0.9 eV (0.6 eV width); fragments stemming from complex breakup reactions, such as CF3O-, are detected starting at 1.6 eV. The highly unusual observation of a parent anion at an energy as high as 0.9 eV—normally parent anions are associated with threshold attachment—was explained with the help of B3LYP/6-31+G(2df) calculations in terms of electron attachment leading to one substantially longer CF bond and intermolecular-vibrational-relaxation (IVR) of about 1.2 eV. Here we investigate alternative explanations for the unusually long lifetime employing superior basis sets and computational methods. It turns out that the most convincing explanation for the long lifetime of C4F8O- is in fact an electron-induced dissociation reaction, namely, a ring-opening reaction that avoids fragmentation of the anion. The transition state for this reaction and the ring-opening product, a distonic chain isomer of C4F8O-, are studied in detail. Our analysis reveals that the chain isomer of the anion possesses many conformers, all of which are considerably more stable than the ring isomer, and that the time scale for the unimolecular ring opening reaction is significantly faster than 1 μs. Thus, at the experimental energy, the ring isomer of C4F8O- is predicted to convert practically completely into the chain isomer, and we argue that the long lifetime as well as the peak position are effectively determined by properties of the ring-opening transition state. Technological implications of our findings are briefly discussed.

Ring-opening attachment: How electron-attachment at energies substantially above threshold can produce long-lived anions

2019 Spring National Meeting

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