Global climate change is becoming an increasingly important environmental issue. Widespread attention is being directed towards the strongest greenhouse gas trifluoromethyl sulfur pentafluoride (SF5CF3) with a radiative force of 0.59 W m-2ppbv-1 on a per molecule basis. Although its concentration of about 0.12 part per trillion (ppt) as first measured from the Antartic firn in 1999, is relatively small, its concentration is growing at a rate of 6% per year. Released as a by-product during the manufacturing of fluoro-chemicals, the only known source so far, accounts for only a small existing portion in the atmosphere. It was hypothesized by Sturges et al. (2000) that a possible source of SF5CF3 could be the SF5 radical, formed by high-voltage discharge, attacking CF3 groups on the surface of fluorpolymers in high-voltage equipment, indicating this gas could inflict a significant environmental impact in the future. Confirmation of this formation route should then direct environmental assessment in the global use of perfluorinated sulfur and hydrocarbon bearing chemicals in high voltage equipment. In the present work, the formation of SF5CF3(X1A’), through the radical - radical recombination of SF5(X2A1) and CF3(X2A1), was observed for the first time in low temperature sulfur hexafluoride – carbon tetra fluoride matrices at 12 K via infrared spectroscopy upon irradiation of the ices with energetic electrons. The v1 fundamentals of the SF5(X2A1) and CF3(X2A1) radicals were monitored at 857 cm-1 and 1110 cm-1 respectively; the newly formed trifluoromethyl sulfur pentafluoride molecule, SF5CF3(X1A’), was detected via its absorptions at 846 cm-1 and 1160 cm-1. This formation mechanism suggests that a source for this potentially dangerous greenhouse gas might be the recombination of SF5(X2A1) and CF3(X2A1) radicals on aerosol particles in the terrestrial atmosphere.
Schematic representation of the underlying reaction pathways of the SF6/CF4 ices upon electron irradiation at low electron irradiation currents.
Temporal evolution of the column densities and best fit of the trifluoromethyl sulfur penta- fluoride molecule (a) trifluoromethyl (b) and sulfur pentafluoride radicals (c) during the irradiation of the SF6/CF4 matrix at 12 K.
W. Carrier, C. Jamieson, R.I. Kaiser, Mechanistic Studies on the Formation of Trifluoromethyl Sulfur Pentafluoride, SF5CF3 - a Greenhouse Gas. Inorganic Chemistry 46, 1332-1336 (2007) (PDF file)