Aromatic compounds and their reactions with atomic nitrogen in the upper atmosphere of Titan

Aromaticity, a property of ring-shaped molecular structures with resonant π bonds, confers increased stability to aromatic compounds compared to other chemical arrangements of the same groups of atoms. For this reason, aromatic rings are very stable and, upon chemical attack, are expected to evolve further while retaining their aromatic nature. This is also why polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in contexts where the first aromatic ring can form. 

Since the first laboratory experiments simulating the conditions of the atmosphere of Titan, polycyclic aromatic hydrocarbons (PAHs) were suggested as possible components of the haze aerosol [1]. The detection of benzene, the simplest aromatic hydrocarbon, supported this suggestion since benzene can easily evolve towards PAHs. Benzene is quite diffuse in Titan as it has been identified in the upper atmosphere (peak around 1000 km), in the mesosphere, in the stratosphere, and even on the surface upon the impact of the Huygens probe [2-5].

The presence of the N-heterocyclic aromatic molecules was also considered after the analysis of INMS spectra where ions at m/z 80 and 81, corresponding to protonated pyridine and pyrimidine or their isomers, were identified. Attempts to confirm their detection via ALMA failed and only upper limits were derived [6]. However, the analysis of an unidentified emission band near 3.28 microns observed in the upper daytime atmosphere [7] suggested a contribution from N-containing PAHs (such as N-heterocyclic compounds or aromatic amines) in addition to PAHs [8]. Furthermore, the CAPS-IBS data can be interpreted by considering that PAHs and N-containing PAHs are responsible for the heavy positive ions signal between 170–310 Da [9]. 

In conclusion, there must be active reaction mechanisms causing the incorporation of nitrogen into aromatic compounds. Since molecular nitrogen is not a reactive species, nitrogen ions or atoms must be responsible for that. Interestingly, photodissociation, dissociative photoionization, cosmic-ray-induced dissociation, electron impact as well as N₂⁺ dissociative recombination can produce atomic nitrogen in the first electronically excited ²D state (a metastable state with a very long radiative lifetime) in similar amounts to the ground ⁴S state [10]. N(⁴S) exhibits very low reactivity with closed-shell molecules, while N(²D) is reactive also with the abundant closed-shell molecules of Titan [10].

For this reason, we have started a systematic investigation of the reactions between N(²D) and simple aromatics, like benzene itself, pyridine, and toluene. We have employed the same combined experimental and theoretical approach described in Refs. [11,12,13]. The reaction mechanism has been elucidated for the three systems and the reaction products with their relative yield have been determined [14,15]. In all cases, the ring contraction channel is the dominant one (thus confuting the dogma on the stability of the aromatic rings towards chemical attack) but, among the minor species, interesting cyclic structures incorporating N have been observed.

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