Spectroscopic and Kinetic Studies of the ClSO Radical from Cl₂SO Photolysis

Thionyl chloride (Cl₂SO) serves as a common Cl atom source in widespread applications of chlorine chemistry though little is known about the reactivity and spectroscopy of the ClSO radical after a Cl-S bond cleavage. We performed a Pulsed Laser Photolysis experiment to detect ClSO from Cl₂SO photolysis at 248 nm in a gas-flow reactor by time-resolved UV-vis transient absorption spectroscopy. A few chemical tests, using I₂ and NO₂, suggested the structured absorption band between 260 and 320 nm belonged to ClSO radical and that the termolecular ClSO + Cl + M → Cl₂SO association reaction occurred. From EOMIP-CCSD/ano-pVQZ calculations, the ClSO band was assigned to the 1²A″ ← X²A″ transition involving the π* ← π transition of the SO bond and the vibrational progression to the SO stretching mode of the 1²A″ state, with a maximum cross-section = (2.0 ± 0.5) × 10^-18 cm² near 286 nm (1σ uncertainty) and an average spacing of vibrational structure of 658 cm^-1. The rapid decay of the ClSO signal monitored near 303 nm could be fit to a second-order kinetic model over 10-90 Torr, which yields an effective bimolecular rate coefficient k_Cl+ClSO = (1.48 ± 0.42) × 10^-11 cm³ molecule^-1 s^-1 at 292 K and 90 Torr (1σ uncertainty). This fast recombination reaction suggests that Cl-containing SO species might act as significant Cl atom reservoirs in sulfur oxide-rich environments such as Venus' atmosphere. Moreover, the reported UV spectrum provides a new means for monitoring the ClSO radicals.