Reaction dynamics of P(⁴S) + O₂(X³Σ⁻_g)  →  O(³P) + PO(X²Π) on a global CHIPR potential energy surface of PO₂(X²A₁): implications for atmospheric modelling

Abstract

Abstract. The reaction dynamics of P(4S) + O2(X3Σg-) → O(3P) + PO(X2Π) are thought to be important in atmospheric and interstellar chemistry. Based on the state-of-the-art ab initio energy points, we analytically constructed a global potential energy surface (PES) for the ground-state PO2(X2A1) using the combined-hyperbolic-inverse-power-representation (CHIPR) method. A total of 6471 energy points were computed by the multireference configuration interaction method with the Davidson correction and aug-cc-pV5Z basis set. The analytical CHIPR PES reproduces ab initio energies accurately with a root-mean-square deviation of 91.5 cm−1 (or 0.262 kcal mol−1). The strongly bound valence region of the PES has complicated topographical features with multiple potential wells and barriers. The attributes of the important intermediates are carefully validated with our geometry optimization results, as well as previous experimental and computational results. Finally, the reaction probability, integral cross sections, and rate constants for P(4S) + O2(X3Σg-) → O(3P) + PO(X2Π) are calculated using the quasi-classical trajectory and time-dependent wave packet methods. The trends of probability and integral cross section versus the collision energy can be divided into three stages, which are governed by the entrance barriers or exothermicity of the reaction. The rate constant demonstrates strong Arrhenius linear behaviour at relatively low temperatures but deviates from this pattern at high temperatures. The calculated cross sections and rate constants are helpful for modelling the phosphorus chemistry in atmospheric and interstellar media.

Relevance of the P+O2 Reaction for PO Formation in Astrochemical Environments: Electronic Structure Calculations and Kinetic Simulations

“…On the other hand, the relative energy of the emerged barrier of the doublet PES obtained by Gomes et al (2022) (10.1 kJ mol −1 ) is close to that computed in the present work (8.5 kJ mol −1 , which becomes 10.9 kJ mol −1 when referring to the prereactive complex), and the same applies to the other stationary points characterized in Gomes et al (2022). In Chen et al (2023), despite the high level of theory employed (MRCI(Q)/aug-cc-pV5Z), no prereactive complex was identified as well; this is probably related to the ab initio grids being not sufficiently dense in the description of the P + O 2 approach. Interestingly, Chen et al (2023) found the reaction to proceed from the intermediate denoted as 2 OPO(A') in Figure 1(a) to the products via a linear OPO transition state.…”

“…In Chen et al (2023), despite the high level of theory employed (MRCI(Q)/aug-cc-pV5Z), no prereactive complex was identified as well; this is probably related to the ab initio grids being not sufficiently dense in the description of the P + O 2 approach. Interestingly, Chen et al (2023) found the reaction to proceed from the intermediate denoted as 2 OPO(A') in Figure 1(a) to the products via a linear OPO transition state. However, we were not able to find any linear OPO structure connecting 2 OPO(A') to PO( 2 Π) + O( 3 P).…”

“…These computations pointed out an emerged barrier of 10.1 kJ mol −1 in the entrance channel, an outcome far from the theoretical estimate by Douglas et al (2019) and their interpretation of the experimental RRKM parameters. Very recently, an analytical global potential energy surface (PES) for the electronic ground state of PO 2 , characterizing in detail the P( 4 S) + O 2 ( 3 Σ − ) reaction, has been published (Chen et al 2023). In Chen et al (2023), an even higher emerged barrier is reported: 12.8 kJ mol −1 at the MRCI(Q)/aug-cc-pV5Z level.…”

“…Very recently, an analytical global potential energy surface (PES) for the electronic ground state of PO 2 , characterizing in detail the P( 4 S) + O 2 ( 3 Σ − ) reaction, has been published (Chen et al 2023). In Chen et al (2023), an even higher emerged barrier is reported: 12.8 kJ mol −1 at the MRCI(Q)/aug-cc-pV5Z level. In our opinion, these persisting disagreements call for a reexamination of the P( 4 S) + O 2 ( 3 Σ − ) reaction with the aim of obtaining a quantitative description of the PO production under the very different conditions characterizing the Earth's atmosphere and the ISM.…”

A Theoretical Study of Temperature-dependent Photodissociation Cross Sections and Rates for O2

“…Such difference corresponds to different PECs of the B 3 u Sstate, in which we considered the avoided crossing with the E 3 u Sstate. To better reproduce the spectrum of the Schumann-Runge band, we use the CHIPR program (Rocha & Varandas 2019Chen et al 2022Chen et al , 2023Li et al 2022Li et al , 2023 in combination with experimental energy levels (Krupenie 1972) to refine the PECs of the B 3 u Sand X 3 g Sstates. Here, we present a brief overview for the theory of this method.…”