UV photolysis of oxalyl chloride: ClCO radical decomposition and direct Cl 2 ${\rm Cl}_2 {\rm }$ formation pathways

Oxalyl chloride, ${\text{(ClCO)}}_2$, is widely used as a photolytic source of Cl atoms in reaction kinetics studies. ${\text{(ClCO)}}_2$ photolysis is typically assumed to produce Cl atoms with an overall yield of 2 via three-body dissociation, ${\text{(ClCO)}}_2+h\nu \to \text{Cl}+\text{CO}+{\text{ClCO}}^{\ast }$, followed by fast subsequent ClCO unimolecular decomposition of either the energetically excited ${\mathrm{ClCO}}^{\ast }$ fragment, ${\text{ClCO}}^{\ast }\to \text{Cl}+\text{CO}$, or the thermalized ClCO radical, $\text{ClCO}+M\to \text{Cl}+\text{CO}+M$. However, a study by Huang et al. (J. Phys. Chem. A 121 (2017) 2888–2895) found that UV photolysis of ${\text{(ClCO)}}_2$ at $248\mathrm{nm}$ directly yields ${\mathrm{Cl}}_2$ with a photolysis quantum yield of $\varphi \left({\text{Cl}}_2\right)>14\%$. This new product pathway may complicate the use of ${\text{(ClCO)}}_2$ 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 ${\mathrm{ClCO}}^{\ast }$ and ClCO radicals has been investigated in ${\text{(ClCO)}}_2$/$C_2{H}_6/\mathrm{Ar}$ gas mixtures after UV photolysis at $266$ and $355\mathrm{nm}$. Cl atoms were captured by $C_2{H}_6$ 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 $k=(1.79\pm 0.17)\times {10}^{-14}{\text{cm}}^3{\text{molecule}}^{-1}{s}^{-1}$ at $295K$, which is in very good agreement with previously reported literature values. Secondly, the photolysis quantum yield of direct ${\mathrm{Cl}}_2$ formation from ${\text{(ClCO)}}_2$ photofragmentation was studied with time-of-flight mass spectrometry using a photoelectron photoion coincidence setup. Calibrated ${\mathrm{Cl}}_2$ concentration-time profiles were recorded and analyzed using kinetic simulations accounting for both direct and secondary formation of ${\mathrm{Cl}}_2$ from photolysis and reactions involving Cl, ClCO, and ${\text{(ClCO)}}_2$, respectively. Direct ${\mathrm{Cl}}_2$ formation could be confirmed, where wavelength-dependent quantum yields of $\varphi \left({\text{Cl}}_2\right)=(5.0\pm 1.6)\%$, $(10.0\pm 3.3)\%$, and $(5.6\pm 2.0)\%$ at $213\mathrm{nm}$, $266\mathrm{nm}$, and $355\mathrm{nm}$ 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, ${\mathrm{Cl}}_2\mathrm{CO}$. We suggest that subsequent ${\mathrm{Cl}}_2$ and Cl formation from energetically excited ${\mathrm{Cl}}_2\mathrm{CO}$ may actually play a role for the overall photodissociation of ${\text{(ClCO)}}_2$.