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

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.