Formation of N-bearing complex organic molecules in molecular clouds: Ketenimine, acetonitrile, acetaldimine, and vinylamine via the UV photolysis of C₂H₂ ice

The solid-state C₂H₂ chemistry in interstellar H₂O-rich ice has been proposed to explain astronomically observed complex organic molecules (COMs), including ketene (CH₂CO), acetaldehyde (CH₃CHO), and ethanol (CH₃CH₂OH), toward early star-forming regions. This formation mechanism is supported by recent laboratory studies and theoretical calculations for the reactions of C₂H₂+OH/H. However, the analog reaction of C₂H₂+NH₂ forming N-bearing species has been suggested to have a relatively low rate constant that is orders of magnitude lower than the value of C₂H₂+OH. This work extends our previous laboratory studies on O-bearing COM formation to investigate the interactions between C₂H₂ and NH₃ ice triggered by cosmic ray-induced secondary UV photons under molecular cloud conditions. Experiments were performed in an ultra-high vacuum chamber to investigate the UV photolysis of the C₂H₂:NH₃ ice mixture at 10 K. The studied ice chemistry of C₂H₂ with NH₂ radicals and H atoms resulting from the UV photodissociation of NH₃ leads to the formation of several N-bearing COMs, including vinylamine (CH₂CHNH₂), acetaldimine (CH₃CHNH), acetonitrile (CH₃CN), ketenimine (CH₂CNH), and tentatively ethylamine (CH₃CH₂NH₂). The experimental results show an immediate and abundant CH₂CHNH₂ yield as the first-generation product, which is further converted into other chemical derivatives. The effective destruction and formation cross-section values of parent species and COMs were derived, and we discuss the chemical links among these molecules and their astronomical relevance.