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Nucleophilic substitution with two reactive centers: The CN- + CH3I case
Stockholm University, Faculty of Science, Department of Physics.
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Number of Authors: 92015 (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.

Place, publisher, year, edition, pages
2015. Vol. 143, no 18, article id 184309
National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
URN: urn:nbn:se:su:diva-124760DOI: 10.1063/1.4934993ISI: 000365042000030PubMedID: 26567664OAI: oai:DiVA.org:su-124760DiVA, id: diva2:891980
Available from: 2016-01-08 Created: 2016-01-04 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
Keywords
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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