Cl₂ Elimination in 248 nm Photolysis of (COCl)₂ Probed with Cavity Ring-Down Absorption Spectroscopy

Abstract

Cavity ring-down absorption spectroscopy (CRDS) is employed to investigate one-photon dissociation of (COCl) at 248 nm obtaining a primary Cl elimination channel. A ratio of vibrational population is estimated to be 1:(0.12 ± 0.03):(0.011 ± 0.003) for the v = 0, 1, and 2 levels. The quantum yield of Cl molecular channel is obtained to be 0.8 ± 0.4 initiated from the X̃ A ground state surface (COCl) via internal conversion. The obtained total quantum yield is attributed to both primary ((COCl) + hν → 2CO + Cl) and secondary reactions (dominated by Cl + COCl → Cl + CO). The former is estimated to share a yield of >0.14, while the latter contributes up to 0.66. The photodissociation pathway to the molecular products is calculated to proceed via a four-center transition state (TS) from which Cl is eliminated synchronously. Installation of the mirrors with reflectivity of 99.995% in the CRDS apparatus prolongs the ring-down time to 70 μs, thus allowing for the contribution from 17% up to 66% of the total Cl yield from secondary reaction depending on the reaction temperature. Despite uncertainty in determining the product yield, the primary Cl dissociation channel eliminated from (COCl) is observed for the first time.

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

“…While earlier studies on the UV photolysis of (ClCO) 2 were compatible with 𝜙 tot (Cl) = 2, the results of our work strongly suggest that direct formation of Cl 2 is an active process in the UV photodecomposition of (ClCO) 2 . However, our photolysis quantum yields are lower than the value of 𝜙(Cl 2 ) > 14% provided by Huang et al 17 for 248 nm photolysis, which in retrospect should probably be regarded as an upper limit. It is expected that 𝜙(Cl 2 ) strongly depends on the accessible decomposition pathways at a given excess energy and hence photolysis wavelength.…”

“…Initially, CCSD(T)/cc-pVTZ//B3LYP/cc-pVTZ calculations have been performed for the stationary points of the PES, hence the same level of theory used by Huang et al 17 However, since this method yielded a ClCO bond dissociation energy of 13.4 kJ∕mol, which is not compatible with the value of 31.3 kJ∕mol calculated from accurate thermodynamic data taken from the ATcT database, 13 we instead proceeded with the G4 composite method. A comparison of G4 and CCSD(T) energies of reactants, intermediates, transition states, and products can be found in Table S3 in the Supplementary Material.…”

“…Secondly, the direct Cl 2 formation channel of (ClCO) 2 photolysis raised by Huang et al 17 was studied using measured Cl 2 concentrations in (ClCO) 2 ∕Ar mixtures after UV photolysis at 213, 266, and 355 nm. A comprehensive kinetic analysis of the Cl 2 profiles yielded photolysis quantum yields 𝜙(Cl 2 ) of (5.0 ± 1.6)% at 213 nm, (10.0 ± 3.3)% at 266 and (5.6 ± 2.0)% at 355 nm.…”

“…However, in a more recent study, 17 Huang et al investigated Cl 2 formation after single-photon UV photolysis of (ClCO) 2 at 248 nm in (ClCO) 2 ∕Ar mixtures. They detected Cl 2 on its weak B 3 Π + ou ← X 1 Σ + g electronic transition at around 500 nm by cavity ring-down spectroscopy (CRDS).…”

“…In fact, Huang et al reported high total Cl 2 concentrations, formally corresponding to a total Cl 2 yield of 0.8 ± 0.4. Such high yields could not be fully attributed to secondary chemistry, most importantly to Cl 2 from the reaction Cl + ClCO → Cl 2 + CO. Quantum-chemical calculations of the ground state potential energy surface (PES) of (ClCO) 2 were therefore performed to identify potential direct In the present work, we aim to provide further insight into two key aspects of (ClCO) 2 photolysis by revisiting ClCO formation followed by decomposition into Cl + CO and by investigating the role of direct Cl 2 formation proposed by Huang et al 17 To this end, two separate sets of experiments on the UV photolysis of (ClCO) 2 were conducted. Firstly, Cl atom formation and ClCO decomposition in (ClCO) 2 /C 2 H 6 ∕Ar mixtures after UV photolysis at 266 and 355 nm was investigated by means of midinfrared frequency modulation (MIR-FM) detection of HCl.…”