Building chemical complexity with ion-molecule reactions: from protostellar stages to Solar System planets and exoplanets

Introduction:

Charged species have been detected at various stages of stellar evolution, from pre- and protostellar objects to planet-forming disks, where they can be used as signatures of the build-up in chemical complexity [1, 2, 3, 4, 5]. On the other hand, on planets and planetary bodies with atmospheres exposed to photons and energetic particles, ionic reactions have been suggested as playing a significant role in the build-up of chemical complexity. A good example is the atmosphere of Titan, where the CASSINI mission [6] detected, by mass spectrometry, a wide range of N-containing organic ions, and photochemical models [7, 8] predict that these ions trigger a rich gas-phase chemistry, eventually forming tholins, a key ingredient of Titan’s haze [9]. Similar effects may be expected in the atmospheres of exoplanets, for which simulation experiments indicate that complex photochemistry can take place leading to formation of large molecules [10, 11]. It has been suggested that complex organic molecules (COMs) could be synthesised through reactions on dust grains, a process which might be facilitated by the acceleration of cations in the gas phase towards negatively charged dust grains [12]. It is therefore important to understand the gas-phase ion chemistry of these environments in order to develop reliable chemical models.

 

Results:

In our laboratories, we measure kinetic parameters (cross sections, branching ratios and their dependences on collision energy) for the reaction of charged molecules with neutrals, using tandem mass spectrometric techniques and RF octupolar trapping of parent and product ions.

In this contribution, we examine the reactions of two isomers of [CNH₃]⁺ that are believed to contribute to the m/z 29 peak observed in the aforementioned mass spectra of Titan’s atmosphere, namely HCNH₂⁺ and H₂CNH⁺, with a range of neutrals including both saturated; CH₄; and unsaturated; C₂H₂, C₂H₄, C₃H₄ (allene), C₃H₄ (propyne) and C₃H₆; hydrocarbons as well as some simple nitriles; CH₃CN and C₂H₃CN; and CH₃OH with the objective of identifying the reaction pathways present as well as their respective rate constants and branching ratios.

 

Experimental Methods: The data presented here for the HCNH₂⁺ and H₂CNH⁺ ions were collected using the CERISES instrument attached to the DESIRS beamline of the French SOLEIL synchrotron [13], using VUV photons to generate ions through either direct photoionization or dissociative photoionization of suitable neutral precursors. The HCNH₂⁺ ion was generated exclusively through dissociative photoionization of cyclopropylamine (C₃H₇N)[14] while H₂CNH⁺ was generated both through dissociative photoionization of azetidine (C₃H₇N also) and via direct photoionization of neutral methanimine, produced via a gas-solid reaction between aminoacetonitrile in gas phase and powdered potassium hydroxide [15]. Working in this way allows us to present data as a function of the energy of the photons used in the ionisation; which in the case of direct ionisation is a proxy for the internal energy of the ions; as well as as a function of the collision energy. Ab initio calculations have also been performed for some of these reactions, with work underway on the others, in order to aid the interpretation of the mechanisms involved in the formation of the various products and their respective energy profiles.  

 

Conclusions: Our experiments demonstrate that not only can the two isomers of [CNH₃]⁺ be generated selectively, but that they show wide-ranging and varied chemistries with a wide range of different neutral molecules. In addition to undergoing charge transfer, proton-transfer and H-abstraction reactions, these isomers also appear to undergo a range of more complex pathways proceeding via the adduct, demonstrating that these species could indeed play a role in the complex gas-phase chemistry of planetary atmospheres with C and N-containing species.

 

Acknowledgements: D.A. and V.R. acknowledge funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 811312 for the project "Astro-Chemical Origins” (ACO). We acknowledge SOLEIL for provision of synchrotron radiation facilities and the DESIRS beamline team for their assistance, and J-CG would like to thank the Centre National d’Etudes Spatiales (CNES) for their financial support.

 

             

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