Formation of phosphorus monoxide through the $$\mathbf {P}(^{4}S)+\mathbf {O}_{\mathbf {2}}(^{3}\Sigma ^{-})\rightarrow \mathbf {O}(^{3}P)+\mathbf {PO}(^{2}\Pi )$$ reaction

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

“…Since the system under study has a high multiconfigurational character (Gomes et al 2022), the structures of all the stationary points on the ground doublet (D0) as well as quartet (Q1 and Q2) PESs have been optimized without any constraint using the fully internally contracted CASPT2 (FIC-CASPT2) model (Andersson et al 1990) in conjunction with the Dunning aug-cc-pVQZ basis set (Dunning 1989;Kendall et al 1992;Woon & Dunning 1993). In all cases, the CAS wave functions have been obtained without state averaging because all test calculations performed indicated a significant energy gap between the ground and excited states.…”

“…The overall process is exothermic, showing a reaction energy of −83.2 kJ mol −1 at the CASPT2/CBS level. Note that we carefully checked that the reaction on the D0 PES proceeds from 2 TS2_P-O 2 (A') to 2 OPO(A') and not directly to the bimolecular products, as previously suggested (Gomes et al 2022).…”

“…Furthermore, they found that the second transition state of the doublet PES ( 2 TS2_P-O 2 (A') in our PES) leads directly to the products through the dissociation of the O-O bond without passing through the second intermediate (denoted as 2 OPO(A') in Figure 1(a)). The impossibility of optimizing a smooth saddle point connecting 2 POO to 2 OPO pointed out in Gomes et al (2022) might be related to their use of geometries optimized at the CASSCF level in place of the more physically sound CASPT2 structures employed in the present work. This difference in the doublet PES is reflected in the relative energy of 2 TS2_P-O 2 (A'), which is −29.1 kJ mol −1 in our study, to be compared to a value of −40.3 kJ mol −1 in Gomes et al (2022).…”

“…In Douglas et al (2019), the pressure-and temperature-dependent global rate constants were evaluated with the MESMER software (Glowacki et al 2012) by fitting the RRKM parameters to the experimental data. Since a quantitative description of the strong static correlation present in this system can be hardly achieved by hybrid density functionals, Gomes et al (2022) reconsidered the P + O 2 reaction, employing multireference ab initio methods (CASSCF and MRCI(Q) for geometry optimizations and energy refinements, respectively). 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.…”

“…For this reason, in the present work, we perform a detailed study of the PES governing the P + O 2 reaction both in the doublet and quartet spin states. These can both be accessed following O 2 ( 3 Σ − ) recombination with P( 4 S) (Gomes et al 2022). In addition, we also investigated the reaction between P( 4 S) and O 2 ( 1 Δ), which is expected to take place on the excited quartet PES.…”