No abstract

No 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 〈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 ≤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 〈f 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 〈f v(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.

The quenching rate constants for NCl(a1Δ) by F and Cl atoms have been measured at room temperature to be (2.2 ± 0.7) × 10-11 and (1.0 + 1.0/−0.5) × 10-12 cm3 s-1, respectively, by adding F and Cl atoms to a flow reactor containing NCl(a1Δ). With knowledge of these quenching rate constants, the kinetics for the formation of NCl(a1Δ) from the Cl + N3 reaction could be investigated in the F/Cl/HN3 reaction system. The reduction in NF(a1Δ) yield from adding Cl atoms to the reactor containing F and HN3 and the relative NF(a1Δ) and NCl(a1Δ) yields for known concentrations of F and Cl atoms in this reaction system favor a total Cl + N3 rate constant of 3 ± 1 × 10-11 cm3 s-1 with a branching fraction for NCl(a1Δ) formation of ≳ 0.5. The branching fraction was deduced from comparing the relative intensities of the NCl(a−X) and NF(a−X) transitions using a lower limit to the NCl(a) radiative lifetime of 2 s. The direct formation of NCl(b1Σ+) from Cl + N3 is a minor channel; however, NCl(b1Σ+) is formed by bimolecular energy pooling of NCl(a1Δ) molecules with a rate constant of ≈1.5 × 10-13 cm3 s-1 and by energy transfer between NCl(a1Δ) and HF(v ≥ 2). The bimolecular energy-pooling process is a small fraction of the total bimolecular self-destruction rate for NCl(a1Δ).

The total quenching rate constants for NCl(a1Δ) molecules were measured at room temperature for 40 molecular reagent species and for F, H, N and O atoms. The Cl + N3 reaction was used to provide the NCl(a1Δ) molecules in a room temperature flow reactor; the azide radicals were obtained from the F + HN3 reaction. In most cases, rate constants were obtained for both NF(a1Δ) and NCl(a1Δ) so that the data could be confirmed by comparison with earlier studies of NF(a1Δ). The quenching rate constants for NCl(a1Δ) range from ∼1 × 10-15 to 4 × 10-11 cm3 molecule-1 s-1. Except for O2, F, H, CH3Cl, HCl, HBr, and HI, rate constants for the NCl(a1Δ) molecule are generally smaller than or comparable to those for NF(a1Δ), and NCl(a1Δ) is not especially reactive at 300 K. Small rate constants were obtained for common gases, such as H2, CO2, and CH4; the rate constants are larger for unsaturated hydrocarbons and they increase with the number of carbon atoms in the molecule. A correlation was found between the basicity of a series of amines and magnitudes of the quenching rate constants for NF(a1Δ) and NCl(a1Δ); the smaller NCl(a1Δ) rate constants indicate a less acidic nature for NCl(a1Δ). An unusual concentration dependent, wall-quenching process was found for (CH3)2O, CH3OH, OCS, and saturated hydrocarbons that affected the NCl(a1Δ) decay kinetics, but not those for NF(a1Δ). The interaction between NCl(a1Δ) and NF(a1Δ) was qualitatively studied; the total bimolecular destruction rate is no larger than those for NCl(a1Δ) or NF(a1Δ) alone. The energy-pooling process between NF(a1Δ) and NCl(a1Δ) mainly gives NF(b1Σ+) + NCl(X3Σ-).