Hydrogen-Bonded Complexes of HPN⋅ and HNP⋅ Radicals with Carbon Monoxide

Phosphorus mononitride (PN) is a carrier of phosphorus in the interstellar medium. As the simplest derivatives of PN, the radical species HPN⋅ and HNP⋅ have remained elusive. Herein, we report the generation, characterization, and photochemistry of HPN⋅ and HNP⋅ in N₂-matrix at 3 K. Specifically, HPN⋅ was formed as a weakly bonded complex with CO in the matrix by 254 nm photolysis of the novel phosphinyl radical HPNCO⋅. The ⋅NPH-CO complex is extremely unstable, as it undergoes spontaneous isomerization to the lower-energy isomer ⋅PNH-CO through fast quantum mechanical tunneling (QMT) with a half-life of 6.1 min at 3 K. Upon further irradiation at 254 nm, the reverse conversion of ⋅PNH-CO to ⋅NPH-CO along with dehydrogenation to yield PN was observed. The characterization ⋅NPH-CO and ⋅PNH-CO with matrix-isolation IR spectroscopy is supported by D, ¹⁵N, and ¹³C isotope labeling and quantum chemical calculations at the XYGJ-OS/AVTZ level of theory, and the mechanism by hydrogen atom tunneling is consistent with multidimensional instanton theory calculations.