4-8 October 2015
Hans Harnack Haus
Europe/Berlin timezone
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Contribution Contributed Talk

Hans Harnack Haus -
FROM DENSE CORES TO PROTOSTARS 2

Low-temperature hydrogenation and deuteration of solid aromatic hydrocarbon by tunneling

Speakers

  • Dr. Tetsuya HAMA

Primary authors

Co-authors

  • Dr. Hirokazu UETA (National Institute for Materials Science, Tsukuba 305–0047, Japan.)
  • Prof. Akira KOUCHI (Institute of Low Temperature Science, Hokkaido University, Sapporo, 060–0819 Japan)
  • Prof. Naoki WATANABE (Institute ol Low Temperature Science, Hokkaido University)

Content

Aromatic hydrocarbon is one of the main component of interstellar and circumstellar dust. The hydrogenation and deuteration of interstellar aromatic hydrocarbons is 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. We exposed amorphous C6H6 samples to cold H or D atoms at 120 K. In situ infrared spectroscopy revealed that cyclohexane (C6H12) or deuterated cyclohexane (C6H6D6) are efficiently formed by H or D atom addition. Given the activation barriers and low temperatures, these reactions proceeded via tunnelling.

In addition, we observed only small KIEs. 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 KIE, despite tunneling’s providing a classically anomalous reaction efficiency.

In comparison to C6H6, polycyclic aromatic hydrocarbons (PAHs) tend to have lower activation barriers to H or D addition owing to the higher flexibility. Therefore, the present results suggest that interstellar PAHs can be hydrogenated or deuterated by the tunneling of H or D atoms at low temperatures, and that the tunneling KIE would not strongly inhibit the deuteration of interstellar aromatic hydrocarbons.