A joint computational and experimental study of the reactions between N(2D) and simple aromatic hydrocarbons

The reactivity of atomic nitrogen in its ground state (⁴S) with closed shell molecules, like hydrocarbons, is very low, while atomic nitrogen in its first electronically excited ²D state shows a significant reactivity with hydrocarbons.  N(²D) was detected in the water-poor comet C/2016 R2 (Pan-STARRS)(Raghuram et al. 2020) and in a plethora of strongly photon-irradiated environments including the Orion Nebula (M42), low-ionization H II regions (M43), planetary nebulae (i.e. the Ring Nebula), supernova remnants (i.e. the Crab Nebula), and Herbig-Haro objects (Ferland et al. 2012,  Dopita et al. 1976, Ferland et al. 1988,  Bautista 1999).Polycyclic aromatic hydrocarbons (PAHs) and related species are presumed to be omnipresent in the interstellar medium (ISM) and aromatic chemistry is widespread in the earliest stages of star formation.

Nitrogen, in its molecular form, and hydrocarbons, both aliphatic and aromatic, are also the main components of the atmosphere of Titan (Hörst 2017, Vuitton et al. 2006). This atmosphere is similar, in some aspects, to the primordial atmosphere of Earth (Vuitton et al. 2013, Balucani 2012) and for this reason has been extensively studied by several missions (Brown et al. 2010), Lai et al. 2017). Among the hydrocarbons identified on Titan there is benzene (Vuitton et al. 2008, Clark et al. 2010), while toluene is easily produced by the reaction of C₆H₅, obtained by photodissociation of benzene, and CH₃ (Loison et al. 2019). Dinitrogen in the atmosphere of Titan can dissociate into atomic nitrogen both in its ground state or ²D excited state in similar amounts (Lavvas et al. 2011, Dutuit et al. 2013) and N (²D) can easily react with other constituents of the upper atmosphere of Titan or with species present in the ISM medium (Balucani 2013, Balucani 2009, Imanaka & Smith 2010, Balucani et al. 2001, Balucani et al. 2006, Homayoon et al. 2014, Balucani et al. 2015, Israel et al. 2005).

In this contribution, we report on a theoretical characterization of the reaction involving N(²D) and simple aromatic hydrocarbons, like benzene (Balucani et al. 2018, 2019, 2023), toluene (Rosi et al. 2020. 2021) or pyridine (Mancini et al. 2024). We have already investigated the reactions of atomic nitrogen in its excited ²D state with various aliphatic hydrocarbons, like CH₄ (Balucani et al. 2009), C₂H₂ (Balucani et al. 2000A), C₂H₄ (Balucani et al. 2000B, 2012), C₂H₆ (Balucani et al. 2010), allene (Vanuzzo et al. 2022), methylacetylene (Mancini et al. 2021), alkynes (Mancini et al. 2020) in laboratory experiments by the crossed molecular beam technique with mass spectrometric detection and time-of-flight analysis at different collision energies complemented by electronic structure calculations of the stationary points along the minimum energy path and kinetics calculations. The aim is to determine the chemical behavior of N(²D) with aromatic species after the previous investigation with aliphatic molecules. In particular, we wish to establish whether the aromatic ring is preserved in this reaction and whether the N atom is incorporated in the ring of carbon atoms.

 

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