Non-Equilibrium Preparation of (E)-Phosphinimine (HPNH, X 1A′) - A Precursor to Prebiotic Phosphorylating Agents

Abstract

Phosphorus-nitrogen compounds play crucial roles as intermediates in aza-Wittig reactions and serve as key precursors to prebiotic phosphorylating agents such as amidophosphonates linked to the Origins of Life. However, their formation mechanisms under astrophysical conditions remain largely unexplored. Here, we report the first formation of (E)-phosphinimine (HPNH, X ¹A^′) - the simplest iminophosphane - in low-temperature interstellar model ices composed of phosphine (PH₃) and ammonia (NH₃) exposed to galactic cosmic ray proxies in the form of high-energy electrons. Utilizing vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry in tandem with computed adiabatic ionization energies permits the very first identification of (E)-phosphinimine in the gas phase during the temperature-programmed desorption of irradiated ices. Our study reveals a critical formation pathway for iminophosphanes via non-equilibrium chemistry through nitrogen- and phosphorus-centered radicals in interstellar environments preceding the formation of stars and planets thus affording useful insights into the interstellar phosphorus-nitrogen chemistry essential for the origin of prebiotic molecules.