Unraveling sulfur chemistry in interstellar carbon oxide ices

Formyl radical (HCO•) and hydroxycarbonyl radical (HOCO•) are versatile building blocks in the formation of biorelevant complex organic molecules (COMs) in interstellar medium. Understanding the chemical pathways for the formation of HCO• and HOCO• starting with primordial substances (e.g., CO and CO₂) is of vital importance in building the complex network of prebiotic chemistry. Here, we report the efficient formation of HCO• and HOCO• in the photochemistry of hydroxidooxidosulfur radical (HOSO•)-a key intermediate in SO₂ photochemistry-in interstellar analogous ices of CO and CO₂ at 16 K through hydrogen atom transfer (HAT) reactions. Specifically, 266 nm laser photolysis of HOSO• embedded in solid CO ice yields the elusive hydrogen‑bonded complexes HCO•···SO₂ and HOCO•···SO, and the latter undergoes subsequent HAT to furnish CO₂···HOS• under the irradiation conditions. Similar photo-induced HAT of HOSO• in solid CO₂ ice leads to the formation of HOCO•···SO₂. The HAT reactions of HOSO• in astronomical CO and CO₂ ices by forming reactive acyl radicals may contribute to understanding the interplay between the sulfur and carbon ice-grain chemistry in cold molecular clouds and also in the planetary atmospheric chemistry.