Formation of methane and cyclohexane through the hydrogenation of toluene

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.

💡 Research Summary

The authors investigate how a methyl‑substituted aromatic hydrocarbon, toluene (CH₃C₆H₅), reacts with atomic hydrogen under conditions that simulate interstellar dust grain surfaces. A monolayer of toluene was deposited on highly oriented pyrolytic graphite (HOPG) inside an ultra‑high‑vacuum chamber (base pressure < 5 × 10⁻¹⁰ mbar). The sample was then exposed to a beam of neutral H atoms generated by a hot tungsten capillary and cooled through a quartz nozzle, delivering a flux of (1.2 ± 0.6) × 10¹⁵ cm⁻² s⁻¹. Different fluences ranging from 1.1 × 10¹⁸ to 1.73 × 10¹⁹ cm⁻² were applied. After exposure, temperature‑programmed desorption (TPD) was performed at a heating rate of 1 K s⁻¹ while a quadrupole mass spectrometer (QMS) recorded desorbing species from 30 to 180 amu.

The TPD spectra show that the parent toluene signal (m/z = 92) diminishes with increasing H‑atom fluence, while new peaks appear at m/z = 98 and 96, corresponding to fully hydrogenated toluene (methyl‑cyclohexane, CH₃C₆H₁₁) and a partially hydrogenated intermediate (four extra H atoms). Fragment ions at m/z = 83, 55, and 56 confirm the identity of methyl‑cyclohexane. At the highest fluences, additional peaks at m/z = 84 and 16 emerge, which the authors assign to cyclohexane (C₆H₁₂) and methane (CH₄), respectively. The appearance of these species indicates that after full saturation of the aromatic ring, the methyl substituent can be cleaved, releasing CH₄ and leaving a saturated six‑membered carbon ring.

Density‑functional theory calculations (B3LYP/def2‑TZVP with Grimme D4 dispersion) were used to map the reaction pathways. The first H‑atom addition breaks a C=C bond in the benzene ring, creating a radical that readily captures a second H atom without a barrier, converting the ring to a cyclohexane skeleton. Subsequent H‑atom additions to the methyl‑substituted carbon lower the C‑CH₃ bond strength; at sufficient fluence the bond ruptures, producing a CH₄ molecule and a cyclohexyl radical (C₆H₁₁·). The calculated activation barrier for methyl cleavage is ~0.9 eV, which can be overcome by the kinetic energy of the H atoms (≈0.2–0.5 eV) combined with the exothermicity of the preceding hydrogenations.

The initial H‑addition cross‑section for toluene is measured as σ₀ ≈ 1.5 × 10⁻¹⁶ cm², about 30 % lower than values reported for larger PAHs such as coronene or pentacene. The reduced cross‑section is attributed to (i) the smaller geometric size of toluene (seven carbon atoms) and (ii) the electron‑withdrawing effect of the methyl group, which slightly diminishes the reactivity of the π‑system. Nevertheless, once a critical fluence is reached, the reaction proceeds efficiently to full hydrogenation and subsequent methyl loss.

Astrophysically, the results have two major implications. First, methyl‑substituted PAHs can become super‑hydrogenated, acquiring sp³‑hybridized carbon atoms that contribute to the observed 3.4 µm and 6.9 µm aliphatic absorption features in interstellar infrared spectra. Second, the top‑down pathway demonstrated here offers a viable route for the formation of small saturated hydrocarbons—particularly methane and cyclohexane—from larger aromatic precursors in environments rich in atomic hydrogen, such as photodissociation regions (PDRs) or dense molecular clouds. The residual cyclohexyl radical could further react with other species, potentially seeding more complex prebiotic molecules.

In summary, the study shows that toluene, despite having a lower initial H‑addition probability than larger PAHs, can be fully hydrogenated under interstellar‑like H‑atom bombardment. At high fluences the methyl group is cleaved, yielding CH₄ and C₆H₁₂, thereby providing a concrete laboratory analogue for a “top‑down” chemical route that may operate on dust grain surfaces throughout the ISM.