Atmospheric Chemistry of Perfluoro-3-methyl-2-butanone [CF3C(O)CF(CF3)2]: Photodissociation and Reaction with OH Radicals.

Abstract

Perfluoro-3-methyl-2-butanone (CF3C(O)CF(CF3)2, abbreviated as C5) is a potentially excellent fire suppression alternative to halons and a promising dielectric gas for SF6 replacement. As a prototypical perfluorinated asymmetrical ketone, photodissociation and reaction with hydroxyl radicals of C5 have been investigated theoretically to gain insights into its atmospheric chemistry and environmental impact. C5 has a broad UV absorption band in the range 260-360 nm with a maximum at 302 nm and the maximal photolysis rate coefficient is 8.3 × 10-5 s-1. Photoexcitation from S0 through the perpendicular n → π* transition produces the excited S1 species, which can either dissociate straightforwardly via the bifurcated α-CC bond cleavage or be trickled down to T1 via the S1/T1 intersystem crossing (ISC) pathway. In the Franck-Condon region of the S1 surface, the long-lived S1 species exists and the slow ISC pathway is dominant, followed by the α-cleavage through T1 barriers to form perfluoroalkyl and perfluoroacetyl radicals. While the excitation energy exceeds 286 nm, the direct dissociation of C5 though the S1 barriers takes over before the ISC occurs. Several pathways for regeneration of the ground-state S0 from S1 and T1 via seams of crossing or internal conversion were revealed. The C5 + OH reaction occurs via direct carbonyl addition mechanism followed by the rapid displacement of one of the alkyl groups. Although it can be accelerated considerably by the H2O-mediated catalysis or the intercepted vibrationally excited quantum states in the hot S0*, the degradation of C5 by OH radicals is too slow to compete with the photolysis pathways.