Electron attachment, thermal detachment, and the electron affinity of the SF6 substitute C4F7N (heptafluoroisobutyronitrile) and related compounds

Gas phase experiments in a flowing afterglow under thermal conditions were carried out on a common substitute for SF6 in electrical devices, heptafluoroisobutyronitrile (i-C4F7N), yielding the electron attachment efficiency, electron affinity (EA), and providing information on thermal limitations to its usage. Analogous information was obtained for the chain isomer, heptafluorobutyronitrile (n-C4F7N), and the shorter pentafluoropropionitrile (C3F5N), which scavenge electrons poorly. Experiments were carried out between 295 and 1000 K. The measured electron affinities are 1.12 ± 0.06 eV for i-C4F7N and 0.56 ± 0.03 eV for the chain isomer. Calculated EAs are varied: DLPNO-CCSD(T) yields 0.98 eV (i-C4F7N), 0.29 eV (n-C4F7N), and 0.17 eV (C3F5N); G4 1.29, 0.63, and 0.48 eV, respectively. The attachment rate constant to i-C4F7N varies minimally between 295 K (1.55 ± 0.30 × 10−7 cm3 s−1) and 700 K (1.7 ± 0.4 × 10−7 cm3 s−1). At room temperature, the i-C4F7N attachment rate constant is in agreement with previously reported cross sections; however, as temperature increases, the present measurements deviate indicating internal energy of the neutral enhances attachment. At 295 K and between 1 and 2 Torr, attachment is purely associative forming the parent anion. Above 500 K, attachment becomes increasingly dissociative, producing C4F6N− and at higher temperatures CN−, F−, and C3F4N−. At room temperature, the other measured attachment rate constants are 8.5 ± 1.6 × 10−9 cm3 s−1 (n-C4F7N) and <1.6 × 10−10 cm3 s−1 (C3F5N). The experiments are supported by statistical modeling of the attachment process and by quantum chemical calculations. The thermal electron attachment of i-C4F7N is found to be remarkably similar to that of SF6. The predominant factor in the efficient electron attachment for both species is identified as the EAs of ∼1 eV, while compounds with both smaller and larger EAs are expected to show a decrease in attachment efficiency.