Unimolecular Reaction Dynamics of CH3COCl and FCH2COCl:  An Infrared Chemiluminescence and ab Initio Study (original) (raw)

The F + CH 3 COCl and H + ICH 2 COCl reaction systems were studied by the infrared chemiluminescence method in a flow reactor. The primary reaction of F + CH 3 COCl gives a nascent HF(V) distribution of P 1-P 3) 21:52:27. A linear surprisal analysis gives P 0) 3 and 〈f 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 e101.2 kcal mol-1 , from the highest HF(V, J) level populated in the primary reaction. The H + ICH 2 COCl primary reaction leads to HI + CH 2 COCl. The secondary F + CH 2 COCl and H + CH 2 COCl reactions give chemically activated FCH 2 COCl*/CH 3 COCl* molecules. The 1,2-HCl elimination channel is the dominant unimolecular pathway for both reactions under our experimental conditions. The HCl(V) distribution from CH 3 COCl* is P 1-P 4) 39:32:20:9. Surprisal analysis was used to estimate the P 0 value as 36% and 〈f v (HCl)〉) 0.12. The reaction time had to be increased from e0.2 ms to g0.5 ms to record the HCl(V) emission from F + CH 2-COCl, and the best distribution was P 1-P 4) 68:24:5:3. The estimated 〈f v (HCl)〉 was only 0.06 which is a lower limit due to HCl(V) relaxation. The CO(V) 1 f 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 FCH 2-COCl 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 CH 3 COCl at the same level. The threshold energies for the other reactions of FCH 2 COCl are 81.0 for CO elimination, 82.5 for CC dissociation, and 78.4 for C-Cl dissociation. RRKM and ab initio calculations indicate that CO formation results from the FCH 2 COCl f FCH 2 + COCl dissociation step followed by COCl f CO + Cl. For CH 3 COCl*, with 105 kcal mol-1 energy, HCl elimination accounts for 98% of the total reaction and CC dissociation accounts for the rest. The C-Cl dissociation channel is not important for either molecule at these energies.