Interconversion mechanisms of PN and PO in the interstellar medium through simple atom–diatom collisions

Abstract

ABSTRACT Models for the abundances of phosphorus-bearing molecules, such as PO and PN, rely on rate coefficients guessed from similar reactions of NO. Given the importance of these molecules in pre-biotic chemistry, in this work we carry out accurate multireference configuration interaction calculations on the N+PO and O+PN reactions, unravelling their underlying mechanisms and potential energy barriers. The collisions may occur in two triplet electronic states and involve six potential wells lying under the N+PO limit. The interaction between atomic nitrogen and PO is shown to occur barrierlessly only when dynamical correlation is included in the calculations. The theoretical results confirm the assumptions used in the astrochemical models that depletion of PO by nitrogen atoms occurs fast, with a branching ratio largely favouring O+PN over the more exothermic P+NO. Among other results, we predict that PN should be stable with respect to collisions with oxygen atoms under low-temperature environments, but will be depleted to P + NO in shocks and other conditions with higher translational energies.