Gas-phase reactions of oxide and superoxide anions with CF4, CF3Cl, CF3Br, CF3I, and C2F4 at 298 and 500 K

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Abstract
Rate constants and product branching fractions have been measured for the gas‐phase reactions of oxide (O−) and superoxide (O2−) anions with the halocarbons CF4, CF3Cl, CF3Br, CF3I, and C2F4 using a variable temperature–selected ion flow tube (VT–SIFT) instrument operated at 298 and 500 K. The reactions of O− with CF3X (X=Cl, Br, I) are fast and produce F−, XF−, and XO− for all X. For CF3Cl and CF3Br, X− is also formed. For CF3I, CF3− and IOF− are minor products. O− reacts rapidly with C2F4 producing F− as the major ionic product, along with contributions from reactive detachment and minor formation of FCO−, CF3−, and C2F3O−. The reaction of O2− with CF3Cl is slow, and both clustering and X− formation were observed. For CF3Br and CF3I, the reactions with O2− are fast, and nondissociative charge transfer was observed in addition to X− formation. O2− reacts rapidly with C2F4 by reactive detachment, in addition to producing F− as the major ionic product with smaller amounts of F2−, FCO−, FCO2−, CF3O−, and C2F4O−. O− and O2− were both found to be unreactive with CF4 at 298 and 500 K. The efficiencies of the reactions of both O− and O2− with CF3X are greater for the heavier halides at both 298 and 500 K. The rate constants for the reactions of O2− with CF3X appear to correlate both with the rates of thermal electron attachment to CF3X and with the electron affinities of CF3X, indicating that the O2−+CF3X reaction mechanism may involve initial electron transfer followed by dissociation. Thus the negative electron affinity of CF3Cl may explain the very slow rate for reaction with O2− despite the available exothermic pathways.