Is the Reaction of C₃N^– with C₂H₂ a Possible Process for Chain Elongation in Titan’s Ionosphere?

Abstract

The reaction of C3N(-) with acetylene was studied using three different experimental setups, a triple quadrupole mass spectrometer (Trento), a tandem quadrupole mass spectrometer (Prague), and the "CERISES" guided ion beam apparatus at Orsay. The process is of astrophysical interest because it can function as a chain elongation mechanism to produce larger anions that have been detected in Titan's ionosphere by the Cassini Plasma Spectrometer. Three major products of primary processes, C2H(-), CN(-), and C5N(-), have been identified, whereby the production of the cyanide anion is probably partly due to collisional induced dissociation. The formations of all these products show considerable reaction thresholds and also display comparatively small cross sections. Also, no strong signals of anionic products for collision energies lower than 1 eV have been observed. Ab initio calculations have been performed to identify possible pathways leading to the observed products of the title reaction and to elucidate the thermodynamics of these processes. Although the productions of CN(-) and C5N(-) are exoergic, all reaction pathways have considerable barriers. Overall, the results of these computations are in agreement with the observed reaction thresholds. Due to the existence of considerable reaction energy barriers and the small observed cross sections, the title reaction is not very likely to play a major role in the buildup of large anions in cold environments like the interstellar medium or planetary and satellite ionospheres.

Ion‐molecule reactions of mass‐selected ions

“…Finally, IMR continues to be a viable tool for modelling and studying interstellar processes. Recent advances include potential reaction pathways for creating larger compounds from simple building blocks (Biennier et al, 2014; Bourgalais et al, 2016; Linden et al, 2016; Lindén et al, 2018). New thrust looks at the interplay of chemistry in the gas phase versus on the grain surfaces (e.g., ice crystals) (Öberg, 2016).…”

“…Using the experimental technique called Cinétique de Réaction en Ecoulement Supersonique Uniforme (CRESU), with the apparatus shown in Figure 32, the authors were able to measure the reaction rate constant in the temperature range of 49-294 K. The reaction proceeded via proton transfer leading to the larger anion C 3 Nand representing one of the first steps towards the formation of larger ions in Titan's ionosphere, aiding the growth of molecules and aerosols in the atmosphere. More atom-growing reactions relevant to Titan's ionosphere were reported by the Geppert group (Linden et al, 2016;Lindén et al, 2018).…”

The reactivity of cyclopropyl cyanide in titan's atmosphere: a possible pre-biotic mechanism

“…Experiments have been carried out using a Triple Quadrupole Mass Spectrometer (model API3000TM LC/MS/MS, ABSciex, USA) at the University of Trento, equipped with atmospheric pressure ion sources such as APCI (Atmospheric Pressure Chemical Ionisation) and ESI (ElectroSpray ionization), and previously used to study reactivity of cations and anions with stable neutrals 31,32,33 . A schematic diagram of the set-up is given in Figure 1.…”

Theoretical Investigation of Bimolecular Carbon Chain Growth Reactions in the Interstellar Media

“…The experiments were performed at collision energies in the range of 0.3–3 eV and rate constants, branching ratios, etc., were extracted with the aid of three-dimensional velocity distributions and product ion flight information. Similar chain growth reactions between C 3 N – and C 2 H 2 were investigated by Geppert and co-workers using ab initio electronic structure calculations and multiple experimental techniques . The explicitly correlated CCSD­(T)-F12a method was used to study the bimolecular reactions between C 2 n H – ( n = 2–4) and C 2 H 2 .…”

“…Similar chain growth reactions between C 3 N − and C 2 H 2 were investigated by Geppert and co-workers using ab initio electronic structure calculations and multiple experimental techniques. 29 The explicitly correlated CCSD-(T)-F12a method was used to study the bimolecular reactions between C 2n H − (n = 2−4) and C 2 H 2 . 30 The reactions were found to be endoergic, and the reaction energies increased with chain size.…”