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Mechanisms of Anion Reactions from the lab to ionospheres
Stockholm University, Faculty of Science, Department of Physics.
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.
Keywords [en]
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: urn:nbn:se:su:diva-149531ISBN: 978-91-7797-073-6 (print)ISBN: 978-91-7797-074-3 (electronic)OAI: oai:DiVA.org:su-149531DiVA, id: diva2:1162753
Public defence
2018-02-05, FA32, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 14:00 (English)
Opponent
Supervisors
Available from: 2018-01-11 Created: 2017-12-05 Last updated: 2018-01-08Bibliographically approved
List of papers
1. Interaction and charge transfer between dielectric spheres: Exact and approximate analytical solutions
Open this publication in new window or tab >>Interaction and charge transfer between dielectric spheres: Exact and approximate analytical solutions
2016 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 145, no 19, article id 194307Article in journal (Refereed) Published
Abstract [en]

We present exact analytical solutions for charge transfer reactions between two arbitrarily charged hard dielectric spheres. These solutions, and the corresponding exact ones for sphere-sphere interaction energies, include sums that describe polarization effects to infinite orders in the inverse of the distance between the sphere centers. In addition, we show that these exact solutions may be approximated by much simpler analytical expressions that are useful for many practical applications. This is exemplified through calculations of Langevin type cross sections for forming a compound system of two colliding spheres and through calculations of electron transfer cross sections. We find that it is important to account for dielectric properties and finite sphere sizes in such calculations, which for example may be useful for describing the evolution, growth, and dynamics of nanometer sized dielectric objects such as molecular clusters or dust grains in different environments including astrophysical ones.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-137717 (URN)10.1063/1.4967701 (DOI)000388956900022 ()27875888 (PubMedID)
Available from: 2017-01-17 Created: 2017-01-10 Last updated: 2017-12-14Bibliographically approved
2. Is the Reaction of C3N- with C2H2 a Possible Process for Chain Elongation in Titan's Ionosphere?
Open this publication in new window or tab >>Is the Reaction of C3N- with C2H2 a Possible Process for Chain Elongation in Titan's Ionosphere?
Show others...
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.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-133397 (URN)10.1021/acs.jpca.6b01746 (DOI)000379988900083 ()27135984 (PubMedID)
Available from: 2016-09-06 Created: 2016-09-06 Last updated: 2017-12-14Bibliographically approved
3. The reaction of C5N- with acetylene as a possible intermediate step to produce large anions in Titan’s ionosphere
Open this publication in new window or tab >>The reaction of C5N- with acetylene as a possible intermediate step to produce large anions in Titan’s ionosphere
Show others...
2018 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 20, no 8, p. 5377-5388Article in journal (Refereed) Published
Abstract [en]

A theoretical and experimental investigation of the reaction C5N + C2H2 has been carried out. This reaction is of astrophysical interest since the growth mechanism of large anions that have been detected in Titan's upper atmosphere by the Cassini plasma spectrometer are still largely unknown. The experimental studies have been performed using a tandem quadrupole mass spectrometer which allows identification of the different reaction channels and assessment of their reaction thresholds. Results of these investigations were compared with the predictions of ab initio calculations, which identified possible pathways leading to the observed products and their thermodynamical properties. These computations yielded that the majority of these products are only accessible via energy barriers situated more than 1 eV above the reactant energies. In many cases, the thresholds predicted by the ab initio calculations are in good agreement with the experimentally observed ones. For example, the chain elongation reaction leading to C7N, although being slightly exoergic, possesses an energy barrier of 1.91 eV. Therefore, the title reaction can be regarded to be somewhat unlikely to be responsible for the formation of large anions in cold environments such as interstellar medium or planetary ionospheres.

Keywords
anion, complex molecules, quantum chemistry, spectroscopy
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-149526 (URN)10.1039/C7CP06302D (DOI)000427085400005 ()
Available from: 2017-12-05 Created: 2017-12-05 Last updated: 2018-04-03Bibliographically approved
4. Nucleophilic substitution with two reactive centers: The CN- + CH3I case
Open this publication in new window or tab >>Nucleophilic substitution with two reactive centers: The CN- + CH3I case
Show others...
2015 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 143, no 18, article id 184309Article in journal (Refereed) Published
Abstract [en]

The nucleophilic substitution reaction CN- + CH3I allows for two possible reactive approaches of the reactant ion onto the methyl halide, which lead to two different product isomers. Stationary point calculations predict a similar shape of the potential and a dominant collinear approach for both attacks. In addition, an H-bonded pre-reaction complex is identified as a possible intermediate structure. Submerged potential energy barriers hint at a statistical formation process of both CNCH3 and NCCH3 isomers at the experimental collision energies. Experimental angle-and energy differential cross sections show dominant direct rebound dynamics and high internal excitation of the neutral product. No distinct bimodal distributions can be extracted from the velocity images, which impedes the indication of a specific preference towards any of the product isomers. A forward scattering simulation based on the experimental parameters describes accurately the experimental outcome and shows how the possibility to discriminate between the two isomers is mainly hindered by the large product internal excitation.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-124760 (URN)10.1063/1.4934993 (DOI)000365042000030 ()26567664 (PubMedID)
Available from: 2016-01-08 Created: 2016-01-04 Last updated: 2017-12-14Bibliographically approved

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