2023MNRAS.518.5991G

The study of reactions involving phosphorus bearing species (PBS) in star-forming regions as well as in circumstellar envelopes are important to elucidate the mechanisms in which this element is formed and destroyed, and perhaps, lead to important pre-biotic molecules. Phosphorus nitride (PN) is the most easily detected PBS in the interstellar medium (ISM), and is considered as one of the major reservoirs of this element in the ISM. However, only a few of its reactions have been analysed experimentally or computationally. Therefore, modelling PN chemistry and interpretation of the observations suffer uncertainties, affecting our astrochemical understanding of this species. In this work, we perform explicitly correlated multireference configuration interaction (MRCI+Q/AVTZ+d//CAS/AVTZ+d) calculations on the destruction of PN through the N(⁴S) + PN(¹Σ⁺) reaction. We have also performed DFT (M06-2X) and CCSD(T) calculations for benchmark purposes. Rate coefficients over a large range of temperatures were computed using standard transition state theory (TST), canonical variational TST (CVT), and also incorporating tunneling effects with the small curvature tunneling method (SCT). We found that the NPN system possesses a considerable multireference character, and the DFT approach cannot properly describe the available destruction mechanisms. Our best estimate for the rate coefficients, at the MRCI+Q/AVTZ+d level, can be described by the modified Arrhenius equation 1.09×10^–11(T/300)^–1.02exp (- 7919/T). We show for the first time that this reaction may be considerably fast in shock regions and in high temperature environments of solar-type star forming regions, and of significant importance to model the abundance of PN in such environments.