Gas-Phase Photodissociation of CH3CHBrCOCl at 248 nm: Detection of Molecular Fragments by Time-Resolved FT-IR Spectroscopy

By employing time-resolved Fourier transform infrared emission spectroscopy, the fragments HCl (v = 1-3), HBr (v = 1), and CO (v = 1-3) are detected in one-photon dissociation of 2-bromopropionyl chloride (CH3CHBrCOCl) at 248 nm. Ar gas is added to induce internal conversion and to enhance the fragment yields. The time-resolved high-resolution spectra of HCl and CO were analyzed to determine the rovibrational energy deposition of 10.0 +/- 0.2 and 7.4 +/- 0.6 kcal mol(-1), respectively, while the rotational energy in HBr is evaluated to be 0.9 +/- 0.1 kcalmol(-1). The branching ratio of HCl(v>0)/HBr(v>0) is estimated to be 1:0.53. The bond selectivity of halide formation in the photolysis follows the same trend as the halogen atom elimination. The probability of HCl contribution from a hot Cl reaction with the precursor is negligible according to the measurements of HCl amount by adding an active reagent, Br-2, in the system. The HCl elimination channel under Ar addition is verified to be slower by two orders of magnitude than the Cl elimination channel. With the aid of ab initio calculations, the observed fragments are dissociated from the hot ground state CH3CHBrCOCl. A two-body dissociation channel is favored leading to either HCl + CH3CBrCO or HBr + CH2CHCOCl, in which the CH3CBrCO moiety may further undergo secondary dissociation to release CO.