Formation of methane and cyclohexane through the hydrogenation of toluene

TL;DR

This study investigates how toluene reacts with hydrogen atoms in space-like conditions, revealing pathways to form methane and cyclohexane, and highlighting the impact of methyl groups on hydrogenation reactivity.

Contribution

It provides experimental insights into the hydrogenation of toluene, a methylated PAH, showing how methyl groups influence reactivity and pathways to smaller molecules in interstellar environments.

Findings

Toluene undergoes superhydrogenation to methyl-cyclohexane.

Hydrogenation can lead to the detachment of the methyl group, producing cyclohexane and methane.

Toluene has a smaller initial cross-section for H-atom addition compared to other PAHs.

Abstract

Methylated polycyclic aromatic hydrocarbons (PAHs) have been hypothesised to be present in the interstellar medium (ISM) through their 3.4 and 6.9 $\mu$m absorption bands. To investigate the hydrogenation of these methylated PAHs, the simplest, toluene ($\mathrm{CH_3C_6H_5}$), was exposed to H-atoms. This demonstrated how the presence of a methyl group changes the reactivity towards atomic hydrogen as compared to benzene and other PAHs and how this may alter its chemistry in the ISM. Toluene was deposited onto a graphite surface in an ultrahigh vacuum (UHV) chamber and then exposed to a H-atom beam. Temperature programmed desorption (TPD) measurements were used to investigate the reaction between H-atoms and toluene and the masses of hydrogenation products were measured with a quadrupole mass spectrometer (QMS). H-atom exposure of toluene leads to superhydrogenation of toluene and the formation of methyl-cyclohexane ($\mathrm{CH_3C_6H_{11}}$) at long exposure times. The initial cross-section of H-addition is lower than that of other PAHs. Methyl-cyclohexane can be further hydrogenated, leading to the detachment of the methyl group and production of cyclohexane ($\mathrm{C_6H_{12}}$) and methane ($\mathrm{CH_4}$). 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 other PAHs. This likely reflects it having a smaller geometric cross-section and the low flexibility of the benzene ring when undergoing sp$^3$ hybridization. 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 radical cyclohexane combining with other species in an interstellar environment to form prebiotic molecules.