Astro seminar: Kinetic modeling of the C/H/O/N/S chemistry of exoplanets with ab initio calculations validated on experimental data

Recent observations with the James Webb Space Telescope recently revealed the first detections of SO2 in the atmospheres of two exoplanets, WASP-39 b and WASP-107b. 1D models of these warm exoplanets show that SO2 is not thermodynamically stable in these conditions, and thus must be a product of photochemistry [1,2]. To correctly model the effects of vertical mixing and photodissociations, a kinetic model, including a kinetic mechanism, is crucial to compute the abundance profiles out of equilibrium. However, these mechanisms usually undergo no experimental validation against experimental kinetic data and thus they remain a significant source of uncertainty. Since exoplanetary atmospheres of hot exoplanets, such as hot Jupiters or warm Neptunes, are characterized by high temperatures, these kinetic models can be validated against combustion experiments [3]. Combustion experiments and kinetic models of hydrocarbons and S or N heteroatomic compounds can be encountered in contexts such as the thermal destruction of pesticides, or combustion of bio-gas and ammonia [4], but validated models for reactive systems containing both sulfur and nitrogen atoms are still scarce in the literature. The work presented here focuses on the development of a kinetic model that includes the combustion chemistry of organosulfur compounds and their coupling with nitrogen chemistry. This was done by adding sulfur chemistry on a previously developed C/H/O/N kinetic model [5]. Multiple mechanisms from the literature for H2S, CH3SH, and CS2 combustion and pyrolysis were compared on H2S, CH3SH, CS2, and OCS combustion and pyrolysis experimental data. Ab initio calculations were performed at the CCSD(T)-F12/CBS level of theory for the energies, using geometries and vibrational frequencies calculated with the CCSD(T)-F12/cc-pVDZ-F12 method. The corrections for hindered rotors were calculated with the 1D-HR approach and the barrierless formations of Van der Waals (VdW) complexes were treated with Phase Space Theory. The computed potential energy surfaces were used to solve the master equation for crucial several systems: CH3SH + SH, CH3SH + CH3, CS + SH, CH2S + SH, H2S + NH2, OCS + H2O, and CS2 + H2O. The computed pressure-dependent rate coefficients are the first in the literature for these reactions. A complex behavior is found for the competition between H-abstraction and ipso-addition in CH2S + SH, with very low energy barriers due to VdW complexes formation that highly impacts the pyrolysis kinetics of CH3SH.

References
[1] S. M. Tsai, E. K. Lee, D. Powell et al. Nature, 617(7961), 483-487 (2023).
[2] A. Dyrek, M. Min, L. Decin, et al. Nature, 625(7993), 51-54 (2024).
[3] O. Venot, E. Hébrard, M. Agúndez et al. A&A 546, A43 (2012).
[4] A. Raj, S. Ibrahim, and A. Jagannath PECS, 80, 100848. (2020)
[5] R. Veillet, O. Venot, B. Sirjean et al. A&A, 682, A52 (2024).