A Computational Investigation into Hydrocarbon Growth on Extraterrestrial Mineral Surfaces toward Understanding the Carbon Discrepancy in Space

The distribution and chemical form of carbon in the universe are of great interest to astrobiology; however, the measured abundance of carbon in interstellar environments is significantly less than that predicted by astrochemical modeling. Polycyclic aromatic hydrocarbons (PAHs) have been postulated to account for some of this discrepancy, given that carbon in this molecular form is difficult to measure by astronomical techniques due to their large partition functions, resulting in weak spectroscopic signatures. However, with new advances in observation in recent years, a small number of PAH-derivatives have been detected in molecular clouds, leading to the important question regarding their formation routes under a diverse range of cloud environment conditions. Several bimolecular gas-phase routes have been proposed for cold regions, but surface-mediated formation routes, where polymerization of small hydrocarbons takes place on exposed interstellar dust grains, may also play a role at warmer temperatures. In the present work, we have applied computational techniques to investigate possible reaction mechanisms, from acetylene to the smallest aromatic benzene, on model forsterite surfaces. A large (>2 eV) barrier to initial C–C bond formation between adsorbed C2H2 is found, suggesting gas-phase routes to form C4 species is likely imperative. However, significantly lower barriers (∼0.5 eV) are observed for subsequent C4–C2 bond formation and cyclization to form benzene on forsterite. While these barriers likely preclude purely surface-based polymerization in cold cloud environments where grains are coated in ice mantles anyway, this study offers support for recent laboratory studies that identified reaction bottlenecks for PAH formation, which other mechanisms may surmount.