Chlorine–sulfur isomers as parents of ClS2 and SCl2 on Venus: Spectroscopy and photochemistry of ClSSCl, SSCl2, and (ClS)2

Abstract

Chlorine–sulfur photochemistry has emerged as a key component of Venus’s complex atmospheric chemistry and a promising avenue for explaining the planet’s sulfur cycle. A theoretical study of the ClSSCl, SSCl2, and (ClS)2 isomers has been performed to elucidate their stability, spectroscopy, and photochemistry, with implications for their potential presence in Venus's upper atmosphere. The ClSSCl and SSCl2 isomers are thermodynamically stable, with significant Cl–S and S–S bond dissociation energies (>47 kcal/mol), suggesting resistance to thermal dissociation. In contrast, the cyclic (ClS)2 isomer is a metastable species with a weak Cl–S bond, indicating it is likely a transient intermediate or pre-reaction complex. Excited states and photoabsorption cross section analysis reveal that ClSSCl exhibits a strong UV absorption around 240 nm, resulting in specific, rapid photodissociation channels. Conversely, SSCl2 displays broad absorption across the near-UV–visible range (∼340 nm), with a high density of interacting states, leading to complex and slow photodissociation dynamics. These results establish ClSSCl and SSCl2 as plausible candidates for detection in the Venusian atmosphere and, critically, as potential photochemical parent molecules for the ClS2 and SCl2 species, providing an accurate spectroscopic and photochemical roadmap for their future observation and simulation.