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Is the Reaction of C3N- with C2H2 a Possible Process for Chain Elongation in Titan's Ionosphere?
Stockholm University, Faculty of Science, Department of Physics.
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Number of Authors: 11
2016 (English)In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 120, no 27, p. 5337-5347Article in journal (Refereed) Published
Abstract [en]

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 enenrgy barriers and the small observed cross sections, the title reaction is not very likely to play major role in the buildup of large anions in cold environments like the interstellar medium or planetary and satellite ionospheres.

Place, publisher, year, edition, pages
2016. Vol. 120, no 27, p. 5337-5347
National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
URN: urn:nbn:se:su:diva-133397DOI: 10.1021/acs.jpca.6b01746ISI: 000379988900083PubMedID: 27135984OAI: oai:DiVA.org:su-133397DiVA: diva2:958245
Available from: 2016-09-06 Created: 2016-09-06 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Mechanisms of Anion Reactions from the lab to ionospheres
Open this publication in new window or tab >>Mechanisms of Anion Reactions from the lab to ionospheres
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A multitude of heavy neutral and ionic molecules have been discovered by the Cassini Plasma Spectrometer in the ionosphere of Saturn's largest moon Titan. However, only three cyano anions were explicitly identified there, namely CN-, C3N- and C5N-.  The identity of the heavier anions, which show an abundance maximum at m/z 1000, could, however, not be elucidated and   there is, so far, no clear explanation how these were generated.

We investigated the reaction of the cyanide anion with methyl iodide using a velocity map imaging spectrometer setup and ab initio calculations. The data indicate a dominant direct rebound mechanism and a high internal excitation of the neutral product. According to the ab initio calculation two possible reaction pathways were expected, but in the experiment the two channels turned out to be indistinguishable due to low resolution.

We also studied the reaction between C3N- and acetylene using three different experimental setups: a triple quadrupole mass spectrometer, a tandem quadrupole mass spectrometer, and the ''CERISES'' guided ion beam apparatus.

The reaction showed three primary reaction pathways leading to C2H-, CN-, and C5N-. The production of C2H- could either happen via proton transfer or via formation of an adduct. The appearance of CN- could be explained by a reaction sequence involving an intermediate adduct but also via collision induced dissociation. Even though ab initio calculations predict two exoergic pathways leading to CN- and C5N-, all products are only accessible via energy barriers above 1 eV.

In addition, we investigated the reaction between C5N- and acetylene. Also in this case the experimental and theoretical studies revealed that all reaction pathways proceed via energy barriers well above 1 eV. The sole exoergic pathway leading to C7N- has an energy barrier of 1.91 eV.  Since the chemistry in dark interstellar clouds and planetary ionospheres is restricted to exoergic reactions with energy barriers less than 20 meV or proceed in a barrier-less manner (Vuitton et al. Planetary and Space Science 57, 1558-1572 (2009)), none of the observed pathways are feasible growth mechanism in those environments.

We also performed investigations of reactions between charged clusters with and without barriers using electrostatic models.  This led to the development of both approximate and exact expressions, which describe the sphere-sphere interaction and the electron transfer from a (neutral or charged) dielectric sphere to another charged dielectric sphere.  The exact solutions include sums that describe polarization effects to infinite orders. However, we have shown that these infinite sums can be simplified, and that these approximations can be applied to calculate the charge transfer cross-sections and Langevin-type cross-sections.

 

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2018
Keyword
ab initio, anion, complex molecules, cross section, electron transfer, heavy ions, ionosphere, nitriles, quantum chemistry, spectroscopy
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-149531 (URN)978-91-7797-073-6 (ISBN)978-91-7797-074-3 (ISBN)
Public defence
2018-02-05, FA32, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 14:00 (English)
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Available from: 2018-01-11 Created: 2017-12-05 Last updated: 2018-01-08Bibliographically approved

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