Astrochemical significance and spectroscopy of tetratomic [H, P, S, O]

¹ Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6243, USA
e-mail: vje12@sas.upenn.edu, frjoseph@sas.upenn.edu
² Department of Earth and Environment Science, University of Pennsylvania, Philadelphia, PA 19104-6243, USA
e-mail: trabelsi@sas.upenn.edu

Received: 19 November 2021
Accepted: 18 January 2022

Abstract

Context. Phosphorus is integral to life on Earth, and its role in the chemistry of the interstellar medium is highly debated and unknown. Only a handful of phosphorus-bearing species have been detected thus far, with the most recent confirmed detection taking place in 2014. The simultaneous detection of molecules such as PO, SH, and OH indicate the possibility of reactive intermediate species existing in the interstellar medium and circumstellar envelopes of evolved stars. To explore this possibility, the [H, P, S, O] tetratomic isomer family was characterized using high level ab initio methods.

Aims. The aim of this study is to provide rotational, vibrational, and electronic spectroscopic data to drive experimental and observational detection of new phosphorus and sulfur-bearing molecules. Additionally, chemical pathways are explored to explain possible reservoirs for the as of yet undetected PH and PS diatomic molecules.

Methods. Coupled cluster quantum chemistry methods were used to calculate the equilibrium electronic structure followed by the anharmonic treatment of the cubic and quartic force fields to obtain accurate rotational and vibrational data. Møller–Plesset perturbation theory in conjunction with coupled cluster methods were used to explore bimolecular reaction pathways. Multi-reference methods were then used to characterize the photochemical pathways of the excited electronic states and simulate the electronic absorption spectrum.

Results. The reaction between detected molecules SH and PO is highly exothermic and forms the HSPO isomer. Deeply submerged transition state barriers allow for facile isomerization to other isomers, especially HOPS. The dominant photochemical process predicted for HOPS is dissociation to form OH + PS, while that of HSPO is a combination of photodissociation to form H + SPO and SH + PO, depending on the wavelength of light absorbed. If PH and PS are formed in the early outflows from evolved stars, bimolecular reactions may act as a reservoir and partially account for their lack of detection to date. The electronic absorption spectrum is predicted to be congested in the 175–200 nm region for both HOPS and HSPO. Differentiating peaks exist >400 nm, which can be used for spectral assignment. Vibrationally corrected rotational constants and anharmonic vibrational frequencies were calculated to assist in the laboratory and observation identification of the most stable molecules. The PO stretch is predicted to be the most intense vibrational mode in both HOPS isomers, and a frequency difference of 20 cm⁻¹ may prove to help differentiate the conformers in an experimental spectrum.