Ultraviolet Photolysis of Acetaldehyde in Clathrate Hydrate Reveals Cage-Controlled Reactivity

Clathrate hydrates (CHs), crystalline inclusion compounds composed of hydrogen-bonded water cages, have been proposed to act as "tiny reaction vessels", potentially providing unique environments for chemical transformations. Although these cages can enable distinct photochemistry by stabilizing and retaining reactive species, the interactions of CHs with ultraviolet (UV) radiation remain understudied. Using reflection absorption infrared spectroscopy, we investigated the UV photochemistry and subsequent thermal processing of the acetaldehyde clathrate hydrate (ACH), comparing its behavior with that of pure acetaldehyde and acetaldehyde + H₂O mixed ices under ultrahigh vacuum (∼10^-10 mbar) and cryogenic temperatures. Upon photolysis at 10 K, ACH retains significantly larger fractions of photoproducts, primarily CO and CH₃CH₂OH, whereas pure and mixed ices favor CO and CH₄ formation. Furthermore, upon warming to 130 K, UV-photolyzed ACH seeded new CHs of CO and CO₂, an outcome absent in the photolyzed acetaldehyde + H₂O sample. These findings demonstrate that clathrate cages not only modulate photochemical reaction pathways but also enhance product trapping, highlighting their potential role in the chemical evolution of astrophysical ices.