Photochemical Reaction Mechanism of Phosphapropyne to Ethynylphosphinidene via Sequential Hydrogen Atom Transfers and Dehydrogenation

Phosphinidenes are transient species that have contributed significantly to synthetic chemistry, coordination chemistry, and astrochemistry. Herein, the multistate complete active space second-order perturbation theory (MS-CASPT2) method has been employed to investigate the photochemical generation of ethynylphosphinidene (HCCP) from phosphapropyne (CH₃CP). Our results show that upon photoexcitation, CH₃CP undergoes sequential hydrogen atom transfers along the C─C─P backbone to form ethynylphosphine (CHCPH₂), a reaction that proceeds sequentially through the 1-phosphaallene (CH₂CPH) and 1H-phosphirene (cyc-HCC(H)PH) isomers and ultimately yields HCCP and H₂ via dehydrogenation. Moreover, the studied photoinduced processes are kinetically accessible, with the highest energy barrier being only 0.56 eV, which can be overcome by the sufficient energy of the 254 nm excitation light. Importantly, our calculations have located several key intersection points between S₁ and S₀ states or between S₀ and T₁ states, which are involved and facilitated this photochemical process. These theoretical results not only elucidate the experimental observations (Lawzer et al., Angewandte Chemie International Edition 60 (2021): 6400–6402) but also provide a framework for advancing the mechanistic understanding of phosphorus photochemistry and elucidating the formation pathways of interstellar phosphorus-containing molecules.