Atomic oxygen as degrading agent in space organic chemistry: the case of O(3P) + small aromatics reactions

Oxygen atoms are important players in regulating the chemical complexity of the Universe. In particular, the dual ability of atomic oxygen (AO) to increase or decrease the complexity of a system is evident in the reactions between O(³P) and unsaturated aliphatic/aromatic hydrocarbons. These reactions can lead to both the formation of more complex oxygenated organic compounds and the degradation of the organics, with the formation of CO or CO₂.

This last aspect is also of major concern in the development of polymeric materials used in aerospace applications. In fact, AO erosion has been recognized to be the most critical hazard for polymers exposed to the specific conditions of the Low Earth Orbit, where most satellites orbit. At these altitudes, AO, mainly formed via VUV photodissociation of O₂, represents 80% of the residual atmosphere and acts as a hydrocarbon-degrading agent. Similar processes may potentially take place in the Low Mars Orbit, where the photodissociation of carbon dioxide produces a considerable amount of AO. Furthermore, along with other oxidizing agents in the Martian atmosphere, O atoms and OH radicals could be the chemical oxidizing agents responsible for the depletion of organics. This may justify the failure to find organic compounds on the Martian surface. Notably, the Curiosity rover has recently detected chlorinated aromatic compounds on the Martian surface, namely, chlorobenzene and dichlorobenzene. Thiophene, another aromatic species containing sulfur, has also been identified on Mars. More in general, processes induced by AO can also involve other refractory forms of carbon (amorphous graphite or large polycyclic aromatic hydrocarbons) that are abundant in our galaxy.       

In this framework, following the study already carried out by the Perugia group on many O(³P) + aliphatic hydrocarbons reactions,¹ we have started a systematic investigation of the reactions of AO with small aromatic compounds, namely, benzene,² pyridine,³ and toluene.⁴ We have exploited the crossed molecular beam technique with mass-spectrometric detection and time-of-flight (TOF) analysis to unveil the primary reaction products, their relative yields (branching fractions, BFs), and the reaction micro-mechanism. The interpretation of the experimental results has been supported by high-level electronic structure calculations of potential energy surfaces (PESs) and RRKM/Master Equation computations of product BFs. In particular, for the reactions between O(³P) and benzene/pyridine, two main groups of mechanisms were observed: (i) the H-displacement channels, in which the oxygen atom replaces a H atom and (ii) the ring-contraction channels, leading to CO.^2,3 Remarkably, the presence of the N atom in the aromatic ring of pyridine increases the probability of the ring-contraction mechanism.³ On the contrary, the presence of a methyl group attached to the aromatic ring, as in the case of toluene, allows preserving the six-member aromatic ring in the reaction products.⁴ The possible implications for the chemistry of different extraterrestrial environments will be noted.

Acknowledgments

We acknowledge financial support under the National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 1.1, Call for tender No. 1409 published on 14.09.2022 by the Italian Ministry of University and Research (MUR), funded by the European Union – NextGenerationEU – Project Title P20223H8CK Degradation of space-technology polymers by thermospheric oxygen atoms and ions: an exploration of the reaction mechanisms at an atomistic level (ThermOPoly) – CUP J53D23014440001 – Grant Assignment Decree No. 1386 adopted on 01.09.2023 by the Italian Ministry of Ministry of University and Research (MUR).

References

[1]   H. Pan, et al., Chem. Soc. Rev., 46, 7517-7547 (2017).    

[2]   G. Vanuzzo, et al., J. Phys. Chem A, 125, 8434-8453 (2021).

[3]   P. Recio, et al., Nat. Chem., 14, 1405-1412 (2022).

[4]   N. Balucani, et al., Faraday Discuss., 2024, DOI: 10.1039/D3FD00181D.