Phosphine Reactivity and Its Implications for Pyrolysis Experiments and Astrochemistry.

Despite the importance of phosphorus-bearing molecules for life and their abundance outside Earth, the chemistry of those compounds still is poorly described. The present study investigates phosphine (PH ₃ ) decomposition and formation pathways. The reactions studied include phosphine thermal dissociation, conversion into PO( ² Π), PN( ¹ Σ ₊ ), and reactions in the presence of H ₂ O ⁺ . The thermodynamic and rate coefficients of all reactions are calculated in the range of 50-2000 K considering the CCSD(T)/6-311G(3df,3pd)//ωB97xD/6-311G(3df,3pd) electronic structure data. The rate coefficients were calculated by RRKM and semiclassical transition-state theory (SCTST). According to our results, PH ₃ is stable to thermal decomposition at T < 100 K and can be formed promptly by a reaction network involving PH( ³ Σ _- ), PO( ² Π), and PN( ¹ Σ ₊ ). In the presence of radiation or ions, PH ₃ is readily decomposed. For this reason, it should be mainly associated with dust grains or icy mantles to be observed under the physical conditions prevailing in the interstellar medium (ISM). The intersystem crossing associated with the dissociation of the isomers PON, NPO, and PNO is accessed by multireference methods, and its importance for the gas-phase PH ₃ formation/destruction is discussed. Also, the implications of the present outcomes on phosphorus astrochemistry are highlighted.