Two HCl-Elimination Channels and Two CO-Formation Channels Detected with Time-Resolved Infrared Emission upon Photolysis of Acryloyl Chloride [CH₂CHC(O)Cl] at 193 nm

Following photodissociation of gaseous acryloyl chloride, CH₂CHC(O)Cl, at 193 nm, temporally resolved vibration-rotational emission spectra of HCl (v ≤ 7, J ≤ 35) in region 2350-3250 cm^-1 and of CO (v ≤ 4, J ≤ 67) in region 1865-2300 cm^-1 were recorded with a step-scan Fourier-transform spectrometer. The HCl emission shows a minor low-J component for v ≤ 4 with average rotational energy E_rot = 9 ± 3 kJ mol^-1 and vibrational energy E_vib = 28 ± 7 kJ mol^-1 and a major high-J component for v ≤ 7 with average rotational energy E_rot = 36 ± 6 kJ mol^-1 and vibrational energy E_vib = 49 ± 9 kJ mol^-1; the branching ratio of these two channels is ∼0.2:0.8. Using electronic structure calculations to characterize the transition states and each intrinsic reaction coordinate, we find that the minor pathway corresponds to the four-center HCl-elimination of CH₂ClCHCO following a 1,3-Cl-shift of CH₂CHC(O)Cl, whereas the major pathway corresponds to the direct four-center HCl-elimination of CH₂CHC(O)Cl. Although several channels are expected for CO produced from the secondary dissociation of C₂H₃CO and H₂C=C=C=O, each produced from two possible dissociation channels of CH₂CHC(O)Cl, the CO emission shows a near-Boltzmann rotational distribution with average rotational energy E_rot = 21 ± 4 kJ mol^-1 and average vibrational energy E_vib = 10 ± 4 kJ mol^-1. Consideration of the branching fractions suggests that the CO observed with greater vibrational excitation might result from secondary decomposition of H₂C=C=C=O that was produced via the minor low-J HCl-elimination channel, while the internal state distributions of CO produced from the other three channels are indistinguishable. We also introduce a method for choosing the correct point along the intrinsic reaction coordinate for a roaming HCl elimination channel to generate a Franck-Condon prediction for the HCl vibrational energy.