HCl elimination in the photolysis of chlorobenzene at 266 nm: An FT-IR spectroscopy and quantum chemical study

Abstract

The photolysis of chlorobenzene (C6H5Cl) in the gas phase has been studied at 266 nm using repetitive scan FT-IR spectroscopy and density functional theory (DFT) to understand the degradation mechanism relevant to combustion and atmospheric chemistry. Following 266 nm photolysis of C6H5Cl, ro-vibrational lines were observed in the region 3060–2625 cm−1, at 3317.8/ 3262.7 cm−1 and 1346.2/1301.2 cm−1, and at 3341.2 and 1232.7 cm−1. These infrared features are assigned to the hydrochloric acid (HCl), acetylene (C2H2), and 1,3-butadiyne (C4H2), respectively. Identification of C2H2 and C4H2 but not expected HCl co-product ortho-benzyne (o-C6H4) indicates, possibly, o-C6H4 further degraded into C2H2 + C4H2. B3PW91/aug-cc-pVTZ and CBS-QB3 calculated potential energy surfaces for the possible degradation channels of C6H5Cl shows that HCl elimination and C–Cl bond fission are major degradation paths. Their experimental branching ratio was determined to be 1:1. The RRKM rate of HCl elimination of C6H5Cl at 266 nm was found to be 3.8 × 102 s−1 and compared with the HF elimination rate in C6H5F. The possible degradation mechanism is discussed.