Tunneling Toward Interstellar PAHs: O and H’s Quantum Leap

Polycyclic aromatic hydrocarbons (PAHs) are widely observed in space both in the gas phase and as components of interstellar dust. PAHs account for ~20% of the carbon (C) in space. Understanding the mechanism to unlock C from PAHs is important to shed light on the formation of complex organic molecules (COMs), which play a crucial role in regulating the physics and chemistry of our universe and aiding in the origin of life [1]. While COM formation often begins with CO activation via hydrogenation on the surface of interstellar grains,  the breakdown of gas-phase PAHs could provide an alternative pathway. PAHs often host radicals such as oxygen (O) and hydrogen (H), which weaken their aromatic bonds. While superhydrogenation of PAHs is well-studied [2], their interaction with atomic O as a possible route for their fragmentation remains underexplored [3].

Using density functional theory (DFT) and instanton theory [4], we predicted intermediate formations resulting from the reaction of O with naphthalene in the gas phase (Figure 1) and computed accurate tunneling rates. Similar to PAH hydrogenation, DFT suggests a sequence of O attachment. Once atomic oxygen reacts with the first C atom of naphthalene, intersystem crossing occurs from the triplet to the singlet spin state. O initially bridges Cs in naphthalene, catalyzing the barrier-less breakdown of a C-C bond, thus forming a heterocyclic ring. Subsequent oxygenation breaks down the PAH structure. Computed instanton rates reveal that tunneling is dominant at temperatures below 50K, with both O and C atoms tunneling through the potential barrier, aiding in the opening of the aromatic ring. Once one of the PAH rings is opened, the attack of two H leads to the fragmentation of the PAH, forming formaldehyde. These findings may clarify the role of PAHs in the formation of COMs, potentially linking them to the organic inventory formation in star-forming regions [1].

 Figure 1: Oxygenation (in red) and hydrogenation (in black) tunneling of naphthalene aiding in fragmentation in the gas phase.

References
[1] Tielens, A. G. G. M. Rev. Mod. Phys. 2013, 85, 1021.
[2] Campisi, D.; et al. Phys. Chem. Chem. Phys. 2020, 22, 1557-1565.
[3] Cavallotti, C.; et. al. J. Phys. Chem. Lett. 2020, 11, 22, 9621–9628.
[4] Rommel, J. B.; et. al. J. Chem. Theory Comput. 2011, 7, 3, 690–698.