Nonequilibrium Formation of Cyanophosphine (H2PCN)─The Simplest Phosphorus-Containing Nitrile─in Interstellar Ice...

The detection of phosphorus-bearing molecules along with the high abundance of nitriles in interstellar and circumstellar environments suggests that phosphorus-containing nitriles may form under astrophysical conditions; however, their formation mechanisms have remained elusive. Here, we report the first laboratory formation of cyanophosphine (H2PCN), the simplest phosphorus-containing nitrile, in low-temperature phosphine-hydrogen cyanide (PH3-HCN) interstellar ice analogues exposed to energetic electrons as proxies of galactic cosmic rays (GCRs). Tunable vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry (PI-ReToF-MS) enabled the isomer-selective identification of cyanophosphine in the gas phase during temperature-programmed desorption, establishing it as a promising target for astronomical searches. These results indicate that cyanophosphine forms through GCR-induced bond cleavage in phosphine and hydrogen cyanide to generate phosphino (PH2) and cyano (CN) radicals, followed by barrierless radical-radical recombination. These findings advance our fundamental understanding of the abiotic synthesis of phosphorus-containing nitriles via nonequilibrium pathways in extraterrestrial environments.