UV photolysis of oxalyl chloride: ClCO radical decomposition and direct Cl2${\rm Cl}_2 {\rm }$ formation pathways

Abstract

Oxalyl chloride, , is widely used as a photolytic source of Cl atoms in reaction kinetics studies. photolysis is typically assumed to produce Cl atoms with an overall yield of 2 via three‐body dissociation, , followed by fast subsequent ClCO unimolecular decomposition of either the energetically excited fragment, , or the thermalized ClCO radical, . However, a study by Huang et al. (J. Phys. Chem. A 121 (2017) 2888–2895) found that UV photolysis of at directly yields with a photolysis quantum yield of . This new product pathway may complicate the use of as a clean source of Cl atoms and challenges the previously accepted photodissociation scheme. The purpose of the present work was 2‐fold. Firstly, the unimolecular decomposition of and ClCO radicals has been investigated in / gas mixtures after UV photolysis at and . Cl atoms were captured by added in excess such that concentration‐time profiles of HCl measured by means of mid‐infrared frequency modulation spectroscopy reflect the temporally separated Cl formation pathways. The low‐pressure thermal ClCO decomposition rate constant was determined to be at , which is in very good agreement with previously reported literature values. Secondly, the photolysis quantum yield of direct formation from photofragmentation was studied with time‐of‐flight mass spectrometry using a photoelectron photoion coincidence setup. Calibrated concentration‐time profiles were recorded and analyzed using kinetic simulations accounting for both direct and secondary formation of from photolysis and reactions involving Cl, ClCO, and , respectively. Direct formation could be confirmed, where wavelength‐dependent quantum yields of , , and at , , and were determined. Complementary quantum‐chemical calculations of the potential energy diagram for ground‐state photodissociation reveal a low‐lying energy barrier for the formation of phosgene, . We suggest that subsequent and Cl formation from energetically excited may actually play a role for the overall photodissociation of .