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 〈fv(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 〈fv(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 〈fv(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.