The interstellar chemistry of PAH cations

Abstract

Diffuse interstellar bands (DIBs) are mysterious absorption lines in the optical spectra of stars, and have been known for 75 years. Although it is widely believed that they arise from gas-phase organic molecules (rather than from dust grains) in the interstellar medium, no consensus has been reached regarding their precise cause. The realization that many emission features in astronomical infrared spectra probably arise from polycyclic aromatic hydrocarbons (PAHs), which may themselves be very abundant in the interstellar medium, has led to the suggestion that ionized PAHs might be the source of the DIBs. Laboratory investigations have revealed that small, positively charged PAHs in matrices have absorption features that bear some resemblance to DIBs, but no clear identification of any DIB with any specific PAH cation has yet been made. Here we report a laboratory study of the chemical reactivity of PAH cations (C6H6+, C10H8+ and C16H10+) in the gas phase. We find that these PAH cations are very reactive, and are therefore unlikely to survive in high abundances in the interstellar medium. Rather, such molecules will react rapidly with hydrogen, and we therefore suggest that the resulting protonated PAH cations (and species derived from them) should become the focus of future searches for a correspondence between molecular absorption features and the DIBs.

Closed-shell polycyclic aromatic hydrocarbon cations: a new category of interstellar polycyclic aromatic hydrocarbons

“…The sequential addition of a second hydrogen atom to form the 1,2-dihydronaphthalene cation is exothermic by another 178 kJ/mol but, as already discussed, faces a significant reaction barrier and proceeds at a rate some two orders of magnitude less at room temperature [23,24]. Analogous calculations have been performed for the pyrene, coronene, and circumcoronene radical cations (C 16 H 10 + , C 24 H 12 + , and C 54 H 18 + , respectively), with similar results.…”

“…The calculations indicate that the C 10 H 8 + + H reaction is exothermic by 259 kJ/mol. Thus, in the absence of a reaction barrier, the rapid reaction observed in the laboratory is understandable [23,24,62]. As already discussed, the primary driving force for this reaction lies in the pairing of the parent radical cation's odd electron.…”

“…In recent selected-ion flow tube experiments, Le Page et al [23] and Snow et al [24] explored the reactivity of ionized PAH structures with various simple atomic and molecular species of interstellar relevance. Of particular interest, they found that the radical cations of benzene, naphthalene, and pyrene reacted readily with atomic hydrogen, but were relatively unreactive toward molecular hydrogen.…”

Collision rate constants for polarizable ions

“…Because of their potential relevance in the interstellar medium, we have carried out comprehensive experimental studies of the reactions of aromatic cations and of carbon chain anions with H 2 , H, N, and O [12][13][14][15][16][17]. In these unusual systems, the polarizability of the neutral is small and that of the ion is large, which accentuates the effect of the V ␣ term on the overall potential.…”

Reaction of the C2H Radical with 1-Butyne (C4H6): Low-Temperature Kinetics and Isomer-Specific Product Detection

“…7 Similarly, C 2 H is a key radical in the formation of polycyclic aromatic hydrocarbons (PAHs), 6 which are believed to play an important role in interstellar chemistry. 8,9 Benzene (C 6 H 6 ), the simplest aromatic hydrocarbon, is considered to be a precursor for larger PAH formation and its recent identification on Titan 10 strongly suggests that much more complex PAH synthesis may proceed in the cold atmosphere of the Saturnian moon. Recently, Mebel and coworkers 11 proposed a novel Ethynyl Addition Mechanism (EAM) as a viable alternative to the high-temperature Hydrogen-Abstraction-C 2 H 2 -Addition (HACA) 12 mechanism for the formation of PAHs at very low temperatures.…”