Theoretical Study on the Mechanisms and Kinetics of Atmospheric Oxidation of Tetrafluoropropyne and Its Analogues.

Tetrafluoropropyne (C 3 F 4 ) is a potential dielectric in various electrical insulating equipment to replace the most potent industrial greenhouse gas, sulfur hexafluoride. Atmospheric oxidation of C 3 F 4 by OH radicals in the presence of molecular O 2 has been investigated theoretically in order to clarify the lifetime and degradation products at mechanistic and kinetic aspects. Energetic minimum-energy pathways for the C 3 F 4 + OH/O 2 reactions were calculated in detail using various theoretical methods including density functional M06-2X and CCSD for geometries, CBS-QB3, CCSD(T), and multireference RS2 with extrapolation to the complete basis-set limit for energies. It has been demonstrated that the C 3 F 4 + OH reaction takes place via the bifurcated C-O addition/elimination routes leading to CF 3 C(OH)═CF and CF 3 C═C(OH)F radical adducts, where the latter is more preferable in view of the difference in barrier heights (1.3 vs 0.3 kcal/mol), followed by H-migration, HF-elimination, and C-C and C-F bond fission. The atmospheric lifetime of C 3 F 4 was estimated to be about 13 days, which is indicative of a very short-lived substance in the atmosphere. Further degradation of the energy-rich C 3 F 4 OH* intermediates by O 2 takes place spontaneously in view of the successive barrier-free and highly exothermic pathways, producing a variety of fluorinated acids, anhydrides, biacetyls, and regenerating OH radicals. For comparison, the reactions of C 3 H 4 , CF 3 CCH, and CH 3 CCF with OH radicals were examined to clarify the F-substitution effect. It is revealed that the reactivity of fluoropropynes could be either reduced by CF 3 or enhanced by atomic F attached to the acetylenic carbon. The present work provides a fundamental understanding of the reactions of fluoroalkynes with OH/O 2 . The use of C 3 F 4 as a promising eco-friendly gaseous dielectric alternative to SF 6 has been supported.