Low-Temperature Synthesis of α,β-Unsaturated Carbonyl Compounds: Key Intermediates in Interstellar Organic Chemistry

Prototypical α,β-unsaturated carbonyl compounds, methyl ketene and propenal, are key intermediates in the formation of complex interstellar organic molecules and prebiotic molecules. The two important species have been detected in the interstellar medium; however, the formation mechanisms of the two species at low temperatures remain elusive. Here, we demonstrate the gas-phase synthesis of both molecules from the reaction of methylidyne (CH•) with acetaldehyde (CH3CHO) at temperatures from 25 to 100 K. Reaction rate coefficients were measured via the laser-induced fluorescence technique and products were identified by vacuum ultraviolet photoionization mass spectrometry and the photoionization efficiency method. Astrochemical modeling of the Taurus molecular cloud (TMC-1) revealed the key role of the CH• + CH3CHO reaction in the gas-phase formation of methyl ketene and propenal, replicating their astronomically observed abundances. This study highlights the significant role of gas-phase synthesis of unsaturated complex compounds in cold interstellar environments.