HPCO—A Phosphorus‐Containing Analogue of Isocyanic Acid

We describe the isolation and spectroscopic characterization of the heavier phosphorus‐containing analogue of isocyanic acid (HPCO), and its isotopologue (DPCO). This fundamental small molecule, which has been postulated to exist in interstellar space, has thus far only been observed at low gas phase concentrations or in inert gas matrices. In this report we describe its synthesis, spectroscopic properties, and reactivity in solution. Four atom puzzle: Protonation of the 2‐phosphaethynolate anion exclusively afforded HPCO. Metastable solutions of the free acid in toluene and dichloromethane could be generated, while in THF oligomerization was observed.