Formation of methane and cyclohexane through the hydrogenation of toluene

TL;DR

This study investigates how a single methyl group in toluene affects hydrogen atom reactivity relevant to interstellar chemistry by exposing a toluene monolayer on graphite to H atoms and monitoring hydrogenation products with temperature-programmed desorption and mass spectrometry. The experiments reveal progressive hydrogenation to methyl-cyclohexane, followed by demethylation to cyclohexane and methane, with the initial H-addition cross-section significantly smaller than that of larger PAHs, consistent with a smaller molecular footprint. Complementary DFT calculations show substantial energy release during hydrogenation and identify feasible scission pathways that can liberate CH4 and generate radical species, potentially enabling further chemistry on grain surfaces. Collectively, the results illuminate how methylated PAHs may evolve in the ISM, contribute to small hydrocarbon formation, and influence PAH growth via hydrogenation and demethylation processes under varying radiation environments.

Abstract

Methylated polycyclic aromatic hydrocarbons (PAHs) have been hypothesised to be present in the interstellar medium (ISM) through their 3.4 and 6.9 $μ$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.

Figures (11)

• Figure 1: Three contour plots showing the TPD traces for toluene exposed to different fluences of H-atoms. (a) Pristine toluene (92 amu), and its H-loss fragment (91 amu) being observed at around 200 K. (b) TPD of a toluene monolayer exposed to a fluence of $8.6\times10^{18}$ cm$^{-2}$ H-atoms showing a reduction in the toluene related desorption features along with the appearance of new desorption products with masses 98 and 83 amu at lower temperatures, attributed to the formation of methyl-cyclohexane. (c) TPD corresponding to a H-atom fluence of $17.3\times10^{18}$ cm$^{-2}$ where the hydrogenated species and the toluene now all desorb at lower temperatures.
• Figure 2: The integrated desorption signals for species with masses 80 to 100 amu integrated over a desorption temperature range of 150 - 215 K as a function of increasing H-atom fluence. The integrated signals have not been not normalised. Toluene, highlighted in orange decreases while the hydrogenation products, indicated by shades of blue, increase. Fully hydrogenated toluene is observed at 98 amu. Fragments at -15 amu relative to the parent ions, indicated by *, are consistent with CH3 loss during ionization. The peak at 84 amu, highlighted in red, corresponds to cyclohexane.
• Figure 3: The integrated desorption signal for toluene (92 amu) as a function of H-atom fluence. The data is fitted with the exponential decay function given in equation \ref{['decayform']}. The best fit value for the cross-section for the first H-atom addition is $\sigma=0.003^{+0.004}_{-0.001}$ Å$^2$.
• Figure 4: (a) TPD trace for toluene (92 amu) in orange following a H-atom fluence of $2.2\times10^{18}$ cm$^{-2}$. (b) TPD traces on a different Y axis scale of partially hydrogenated toluene (parent mass 96 amu, main fragment 81 amu) in light blue and methyl-cyclohexane (parent mass 98 amu, main fragment 83 amu) in dark blue. The vertical grey lines show the desorption peaks of the three different molecules. The toluene desorbs at the highest temperature at 202 K, then the partially hydrogenated toluene at 196 K and then the methyl-cyclohexane has the lowest at 191 K.
• Figure 5: (a) TPD trace for toluene (92 amu) in orange following a H-atom fluence of $25.9\times10^{18}$ cm$^{-2}$. (b) TPD traces, on a different y-axis scale, for partially hydrogenated toluene (parent mass 96 amu, main fragment 81 amu) in light blue and methyl-cyclohexane (parent mass 98 amu, main fragment 83 amu) in dark blue and cyclohexane (84 amu) in red. The cyclohexane desorbs over a range of temperatures starting from 160 K. (c) shows the shared QMS fragments of mass 55 and 56 amu that are fragments of the hydrogenated toluene species and cyclohexane.