Structure and Spectroscopic Insights for CH3PCO Isomers: A High-Level Quantum Chemical Study

The exploration of phosphorus-bearing species stands as a prolific field in current astrochemical research, particularly within the context of prebiotic chemistry. Herein, we have employed high-level quantum chemistry methodologies to predict the structure and spectroscopic properties of isomers composed of a methyl group and three P, C, and O atoms. We have computed relative and dissociation energies, as well as rotational, rovibrational, and torsional parameters using the B2PLYPD3 functional and the explicitly correlated coupled cluster CCSD(T)-F12b method. Based upon our study, all the isomers exhibit a bent heavy atom skeleton with CH3PCO being the most stable structure, regardless of the level theory employed. Following in energy, we found four high-energy isomers, namely, CH3OCP, CH3CPO, CH3COP, and CH3OPC. The computed adiabatic dissociation energies support the stability of all [CH3, P, C, O] isomers against fragmentation into CH3 and [P, C, O]. Torsional barrier heights associated with the methyl internal rotation for each structure have been computed to evaluate the occurrence of possible A-E splittings in the rotational spectra. For the most stable isomer, CH3PCO, we found a V3 barrier of 82 cm-1, which is slightly larger than that obtained experimentally for the N-counterpart, CH3NCO, yet still very low. Therefore, the analysis of its rotational spectrum can be anticipated as a challenging task owing to the effect of the CH3 internal rotation. The complete set of spectroscopic constants and transition frequencies reported here for the most stable isomer, CH3PCO, is intended to facilitate eventual laboratory searches.

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 (PH₃) decomposition and formation pathways. The reactions studied include phosphine thermal dissociation, conversion into PO(²Π), PN(¹Σ₊), and reactions in the presence of H₂O⁺. 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, PH₃ is stable to thermal decomposition at T < 100 K and can be formed promptly by a reaction network involving PH(³Σ_-), PO(²Π), and PN(¹Σ₊). In the presence of radiation or ions, PH₃ 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 PH₃ formation/destruction is discussed. Also, the implications of the present outcomes on phosphorus astrochemistry are highlighted.

Formation of phosphorus monoxide through the [Formula: see text] reaction

Phosphorus is a key and vital element for a diverse set of important biological molecules, being indispensable for life as we know. A deeper comprehension of its role in astrochemistry and atmospheric chemistry may aid in finding answers to how this element became available on Earth. The PO molecule is one of the main reservoirs of phosphorus in the interstellar medium (ISM), and a better understanding of the mechanisms and rate coefficients for its formation in the ISM is important for modelling its abundances. In this work, we perform multireference configuration interaction calculations on the formation of PO via the [Formula: see text] reaction, analyzing its potential energy surface and rate coefficients for the global reaction on both doublet and quartet states. We also perform DFT (M06-2X) and CCSD(T) calculations, in order to compare the results. We found that the OPO system possesses a high multiconfigurational character, making DFT and CCSD methodologies not suitable for its potential energy landscape calculation. The rate coefficients have been calculated using the master equation system solver (MESS) package, and the results compared to recent experimental data. It is shown that the quartet state contributes for temperatures higher than 700K. The computed rate coefficient can be described by a modified Arrhenius equation [[Formula: see text]] with [Formula: see text], [Formula: see text] and [Formula: see text] K.

Collisional excitation of interstellar PN by H2: New interaction potential and scattering calculations

Rotational excitation of interstellar PN molecules induced by collisions with H₂ is investigated. We present the first ab initio four-dimensional potential energy surface (PES) for the PN-H₂ van der Waals system. The PES was obtained using an explicitly correlated coupled cluster approach with single, double, and perturbative triple excitations [CCSD(T)-F12b]. The method of interpolating moving least squares was used to construct an analytical PES from these data. The equilibrium structure of the complex was found to be linear, with H₂ aligned at the N end of the PN molecule, at an intermolecular separation of 4.2 Å. The corresponding well-depth is 224.3 cm^-1. The dissociation energies were found to be 40.19 cm^-1 and 75.05 cm^-1 for complexes of PN with ortho-H₂ and para-H₂, respectively. Integral cross sections for rotational excitation in PN-H₂ collisions were calculated using the new PES and were found to be strongly dependent on the rotational level of the H₂ molecule. These new collisional data will be crucial to improve the estimation of PN abundance in the interstellar medium from observational spectra.

Theoretical rotation-vibration spectroscopy of cis- and trans-diphosphene (P2H2) and the deuterated species P2HD

Growing astronomical interest in phosphorous (P) chemistry is stimulating the search for new interstellar P-bearing molecules, a task requiring detailed knowledge of the microwave and infrared molecular spectrum. In this work, we present comprehensive rotation-vibration line lists of the cis- and trans-isomers of diphosphene (P₂H₂). The line lists have been generated using robust, first-principles methodologies based on newly computed, high-level ab initio potential energy and dipole moment surfaces. Transitions are considered between states with energies up to 8000 cm^-1 and total angular momentum J ≤ 25. These are the first-ever line lists to be reported for P₂H₂, and they should significantly facilitate future spectroscopic characterization of this system. The deuterated species trans-P₂HD and the effect of its dynamic dipole moment on the rovibrational spectrum are also discussed.

On the formation of phosphorous polycyclic aromatics hydrocarbons (PAPHs) in astrophysical environments

The formation of phosphorous-containing polycyclic aromatic hydrocarbons (PAPHs) in astrophysical contexts is proposed and analyzed by means of computational methods [B3LYP-D3BJ/ma-def2-TZVPP, MP2-F12, CCSD-F12b and CCSD(T)-F12b levels of theory]. A "bottom-up" approach based on a radical-neutral reaction scheme between acetylene (C₂H₂) and the CP radical was used investigating: (a) the synthesis of the first PAPH (C₅H₅P) "phosphinine"; (b) PAPH growth by addition of C₂H₂ to the C₅H₄P radical; (c) PAPH synthesis by addition reactions of one CP radical and nC₂H₂ to a neutral PAH. Results show: (I) the formation of the phosphinine radical has a strong thermodynamic tendency (-133.3 kcal mol^-1) and kinetic barriers ≤5.4 kcal mol^-1; (II) PAPH growth by nC₂H₂ addition on the radical phosphinine easily and exothermically produces radicals (1a- or 1-phospha-naphtalenes with kinetic barriers ≤7.1 kcal mol^-1 and reaction free energies ≤-102.5 kcal mol^-1); (III) the addition of a single CP + nC₂H₂ to a neutral benzene generates a complex chemistry where the main product is 2-phospha-naphtalene; (IV) because of the CP radical character, its barrierless addition to a PAH produces a resonant stabilized PAPH, becoming excellent candidates for addition reactions with neutral or radical hydrocarbons and PAHs; (V) the same energy trend between all four levels of theory continues a well-calibrated computational protocol to analyze complex organic reactions with astrochemical interest using electronic structure theory.