Electron Driven Reactions in Tetrafluoroethane: Positive and Negative Ion Formation

Abstract

In the search for alternatives to chlorine-containing gases, tetrafluoroethane, CF3CH2F (R134a), a widely used refrigerant gas, has been recognized as a promising substitute for dichlorodifluoromethane, CCl2F2 (R12). When R12 is replaced by R134a, the global warming potential drops from 8100 to 1430, the ozone depletion potential changes from 1 to 0, and the atmospheric lifetime decreases from 100 to 14 years. Electron interactions in the gas phase play a fundamental role in the atmospheric sciences. Here, we present a detailed study on electron-driven fragmentation pathways of CF3CH2F, in which we have investigated processes induced by both electron ionization and electron attachment. The measurements allow us to report the ion efficiency curves for ion formation in the energy range of 0 up to 25 eV. For positive ion formation, R134a dissociates into a wide assortment of ions, in which CF3 + is observed as the most abundant out of seven ions with a relative intensity above 2%. The results are supported by quantum chemical calculations based on bound state techniques, electron-impact ionization models, and electron-molecule scattering simulations, showing a good agreement. Moreover, the experimental first ionization potential was found at 13.10 ± 0.17 eV and the second at around 14.25 eV. For negative ion formation, C2F3 – was detected as the only anion formed, above 8.3 eV. This study demonstrates the role of electrons in the dissociation of R134a, which is relevant for an improvement of the refrigeration processes as well as in atmospheric chemistry and plasma sciences.

Triphenylboroxine stability under low-energy-electron interactions

“…Negative ion formation through DEA to the TPB molecule was investigated in the energy range from 0 to 15 eV. Considering the detection limit of the ToF mass spectrometer and the minimum anion lifetime to allow it to reach the detector (∼1 × 10 −6 s), 25 four fragment anions were observed. The ground state of the TPB molecule belongs to the D 3h point group of symmetry, therefore it does not have a permanent dipole moment.…”

“…The experimental setup used for the present studies consists of a TEM, assembled with a reflectron time-of-flight mass spectrometer in orthogonal geometry (OReToFMS), is described in detail elsewhere. 25 The TEM generates a quasi-mono-energetic electron beam with a resolution of about 180 meV. Its stability was constantly monitored using a picoampere meter connected to a Faraday plate (∼100 nA for an electron energy of 70 eV).…”

“…The dissociation thresholds were addressed within the G4(MP2) procedure, 29,30 as implemented in Gaussian 09, 31 which has been widely used to accurately predict energetic quantities. 25,32 In this work, the thresholds were evaluated as…”

“…The procedure has been described in previous studies. 25,34 4.1.5 Minor fragmentation channels. Apart from the four cations described above, with threshold energies above 16 eV for the three fragments and the ionization energy for the parent cation, there were 18 more fragments identified.…”

Comments on “Electron Driven Reactions in Tetrafluoroethane: Positive and Negative Ion Formation”

“…Recently, 1 Pereira-da-Silva et al gave a substantial contribution to the understanding of processes initiated by electronic interactions with 1,1,1,2-tetrafluoroethane (also known as CF 3 CH 2 F, elsewhere called R-134a). Although some effort was made to take into account previous literature, in several cases prior work was not properly put into context.…”