Ring-opening attachment as an explanation for the long lifetime of the octafluorooxolane anion

Octafluorooxalane, C₄F₈O, has recently attracted attention as a possible replacement of SF₆ in high voltage insulation, and its reactivity with respect to free-electron attachment was investigated by mass spectrometry. The most intense signal peaks at 0.9 eV and corresponds to the parent anion, C₄F₈O^-; fragments stemming from complex breakup reactions are detected starting above ∼1.6 eV. Since parent anions in free-electron attachment are normally associated with threshold attachment or an embedding environment allowing excess energy deposition, this observation is highly unusual. Based on density functional calculations, it was nevertheless interpreted as attachment followed by intermolecular-vibrational-relaxation. Here, electron-attachment to octafluorooxalane is studied computationally. First, the electron affinity (EA) is characterized using density functionals and ab initio methods. Moreover, the negative vertical EA is estimated by extrapolating electron binding energies computed in the vicinity of C₄F₈O^- to the geometry of neutral octafluorooxalane. Then, alternative explanations for the 0.9 eV peak are considered. Specifically, a ring-opening reaction that yields a distonic isomer of C₄F₈O^- is identified. Our analysis reveals that the chain isomer 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 and the peak position are effectively determined by the properties of the ring-opening transition state.