2026A&A...706A.135H

Aims. Methylated polycyclic aromatic hydrocarbons (PAHs) have been hypothesised to be present in the interstellar medium (ISM) through their 3.4 and 6.9 μm absorption bands. To investigate the hydrogenation of these methylated PAHs, we exposed toluene (CH₃C₆H₅), as a simple analogue of such species, to H atoms. Such measurements enable us to determine how the presence of a methyl group might change the reactivity towards atomic hydrogen as compared to benzene and larger aromatic species such as PAHs and how this may alter its chemistry in the ISM.
Methods. We deposited toluene onto a graphite surface in an ultrahigh vacuum (UHV) chamber and then exposed it to an H-atom beam. Temperature programmed desorption (TPD) measurements were used to investigate the reaction between H atoms and toluene. The masses of hydrogenation products were measured with a quadrupole mass spectrometer (QMS).
Results. H-atom exposure of toluene leads to superhydrogenation of toluene and the formation of methyl-cyclohexane (CH₃C₆H₁₁) at long exposure times. The initial cross-section of the H-addition is smaller than that for larger PAH molecules. Methyl-cyclohexane can be further hydrogenated, leading to the detachment of the methyl group and production of cyclohexane (C₆H₁₂) and methane (CH₄).
Conclusions. Toluene may be fully hydrogenated through its interaction with H atoms, although it has a smaller initial cross-section for H-atom addition compared to larger PAHs. This likely reflects it having a smaller geometric cross-section and the low flexibility of the benzene ring when undergoing sp³ hybridisation. The removal of the methyl group at high H-atom fluences provides a top-down formation route to smaller molecules, with the possibility of the formation of a cyclohexyl radical combining with other species in an interstellar environment to form prebiotic molecules.