Phosphorus is a crucial biogenic element, yet its astrochemical role remains poorly understood due to its low cosmic abundance and the limited number of detected P-containing molecules in the interstellar medium. Given its significance for prebiotic chemistry, PCO-bearing molecules, such as the phosphorus analogs of isocyanates, are promising candidates for laboratory and interstellar studies. Herein, we present a comprehensive theoretical study on the isomeric landscape of the C 2 H 3 PO system, identifying and characterizing 24 low-lying isomers through high-level quantum chemical calculations. The study employs double-hybrid DFT and coupled-cluster methods to refine energy values and structural parameters, while topological analysis of electronic density characterizes chemical bonding. Vinylphosphinidene oxide (CH 2 CHPO) emerges as the most stable isomer, followed by methylphosphaketene (CH 3 PCO), with oxygen-bound structures playing a crucial role in stability. Comparisons with the C 2 H 3 NO system reveal structural parallels, reinforcing the importance of oxygen-bound species. Cyclic structures were also explored, with three- and four-membered P- and O-heterocycles identified, although they are generally less stable than open-chain isomers. These results provide insights into the chemical behavior and stability of C 2 H 3 PO isomers, which could help future spectroscopic studies and detection efforts in the interstellar medium.
