12:30 PM - 12:45 PM
[PPS24-07] Hydrogenation and deuteration of solid aromatic hydrocarbon by quantum tunneling
Keywords:Aromatic hydrocarbons, hydrogenation, deuterium enrichment, quantum tunneling, molecular clouds
C6H6 + H(D) ➞ C6H7(C6H6D)Ea = 18.2 kJ mol-1,[R1]
C6H7 + H(D) ➞ C6H8(C6H6D2),[R2]
C6H8 + H(D) ➞ C6H9(C6H6D3)Ea = 6.3 kJ mol-1,[R3]
C6H9 + H(D) ➞ C6H10(C6H6D4),[R4]
C6H10 + H(D) ➞ C6H11(C6H6D5)Ea = 10.5 kJ mol-1,[R5]
C6H11 + H(D) ➞ C6H12(C6H6D6).[R6]
Ea is the activation barrier for H-atom addition in the gas phase. The radical recombination reactions R2, R4, and R6 are barrierless on the surface. We experimentally demonstrate that cold H and D atoms can efficiently add to solid benzene by tunneling at temperatures as low as 10-50 K. The present study is the first report on a nonenergetic deuteration process of aromatic hydrocarbons at low temperatures. In comparison to C6H6, PAHs tend to have lower activation barriers to H or D addition owing to the higher flexibility. Therefore, we suggest that interstellar aromatic hydrocarbons including PAHs and C6H6 can be hydrogenated or deuterated by the tunneling of H or D atoms at low temperatures. The deuteration of interstellar aromatic hydrocarbons is of particular important, because these molecules represent a major carrier of deuterium enrichment observed in carbonaceous meteorites and interplanetary dust particles. As the gaseous atomic D/H ratio in molecular clouds can be also strongly enhanced for elemental ratios of 1.5 × 10-5 to 10-2-10-1, our results suggest that tunneling might represent a major deuteration mechanism for interstellar aromatic hydrocarbons, because surface tunneling is especially facilitated in the cold dense interstellar environments.