Unimolecular reaction dynamics of CH₃COCl and FCH₂COCl : An infrared chemiluminescence and ab initio study

Abstract

The F+CH₃COCl and H+ICH₂COCl reaction systems were studied by the infrared chemiluminescence method in a flow reactor. The primary reaction of F+CH₃COCl gives a nascent HF(v) distribution of P₁-P₃ =21:52:27. A linear surprisal analysis gives P₀=3 and ƒ_v(HF) = 0.60, which is typical for H abstraction reactions by F atoms. The C-H bond energy in acetyl chloride is estimated as ≤101.2 kcal mol⁻¹, from the highest HF(v, J) level populated in the primary reaction. The H + ICH₂COCl primary reaction leads to HI + CH₂COCl. The secondary F+CH₂COCl and H + CH₂COCl reactions give chemically activated FCH₂COCl/CH₃COCl molecules. The 1,2-HCl elimination channel is the dominant unimolecular pathway for both reactions under our experimental conditions. The HCl(v) distribution from CH₃COCl is P₁-P₄=39:32:20:9. Surprisal analysis was used to estimate the P₀ value as 36% and <ƒ_v(HCl)>=0.12. The reaction time had to be increased from ≤0.2 ms to ≥0.5 ms to record the HCl(v) emission from F + CH₂COCl, and the best distribution was P₁-P₄+ 68:24:5:3. The estimated ?ƒv(HCl)? was only 0.06 which is a lower limit due to HCl(v) relaxation. The CO(v =1 → 0) emission could also be observed from this reaction with an intensity that was typically less than 10% of the HCl(v) emission. Ab initio calculations for FCH₂COCl at MP2/6-31G^* level give the threshold energy for HCl elimination as 61 kcal mol⁻¹, which is 12 kcal mol⁻¹ larger than that for CH₃COCl at the same level. The threshold energies for the other reactions of FCH₂COCl are 81.0 for CO elimination, 82.5 for C-C dissociation, and 78.4 for C-Cl dissociation. RRKM and ab initio calculations indicate that CO formation results from the FCH₂COCl → FCH₂+COCl dissociation step followed by COCl → CO+ Cl. For CH₃COCl^*, with 105 kcal mol⁻¹ energy, HCl elimination accounts for 98% of the total reaction and C-C dissociation accounts for the rest. The C-Cl dissociation channel is not important for either molecule at these energies.