KINETICS AND MECHANISM OF THE REACTION OF CL ATOMS WITH CH2CO (KETENE)

The kinetics and mechanism of the gas-phase reaction of Cl atoms with CH2CO have been studied with a FTIR spectrometer/smog chamber apparatu s. Using relative rate methods the rate of reaction of Cl atoms with k etene was found to be independent of total pressure over the range 1-7 00 torr of air diluent with a rate constant of (2.7 +/- 0.5) x 10(-10) cm(3) molecule(-1) s(-1) at 295 K. The reaction proceeds via an addit ion mechanism to give a chloroacetyl radical (CH2ClCO) which has a hig h degree of internal excitation and undergoes rapid unimolecular decom position to give a CH2Cl radical and CO. Chloroacetyl radicals were al so produced by the reaction of CI atoms with CH2ClCHO; no decompositio n was observed in this case. The rates of addition reactions are usual ly pressure dependent with the rate increasing with pressure reflectin g increased collisional stabilization of the adduct. The absence of su ch behavior in the reaction of Cl atoms with CH2CO combined with the f act that the reaction rate is close to the gas kinetic limit is attrib uted to preferential decomposition of excited CH2ClCO radicals to CH2C l radicals and CO as products as opposed to decomposition to reform th e reactants. As part of this work ab initio quantum mechanical calcula tions (MP2/6-31G(d,p)) were used to derive Delta(f)H(298)(CH2ClCO) = - (5.4 +/- 4.0) kcal mol(-1). (C) 1996 John Wiley & Sons. Inc.