2016 年 50 巻 2 号 p. 33-50
Aromatic hydrocarbon is abundant constituent of interstellar and circumstellar dust. The hydrogenation and deuteration of interstellar aromatic hydrocarbons are of particular interest for the formation of aliphatic hydrocarbons and their deuterated isotopologues. Here, we investigated the hydrogenation/deuteration reactions of amorphous solid benzene (C6H6) at low temperatures of 10–50 K. In situ infrared spectroscopy revealed that cyclohexane or deuterated cyclohexane are efficiently formed by H or D atom addition. Given the activation barriers and low temperatures, these reactions proceeded via tunneling. However, the ratio of the hydrogenation and deuteration rates (H/D) was 1–1.5 at 15–25 K, whereas deuteration by tunneling typically occurs at a rate more than two orders of magnitude smaller than that of the comparable hydrogenation. This indicates that the isotopically insensitive surface processes (e.g., adsorption and diffusion) of the atoms physisorbed on solid C6H6 masked the tunneling kinetic isotope effect. The present results suggest that interstellar aromatic hydrocarbons can be hydrogenated or deuterated by the tunneling of H or D atoms at low temperatures, and that the tunneling kinetic isotope effect would not strongly inhibit the deuteration of interstellar aromatic hydrocarbons.