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Molecules 2014, 19(11), 17305-17313;

The Dynamical Behavior of the s-Trioxane Radical Cation—A Low-Temperature EPR and Theoretical Study

Ingenieurgesellschaft Auto und Verkehr (IAV GmbH), Rockwellstraße 16, Gifhorn D-38518, Germany
Leibniz-Institute for Surface Modification, Permoserstraße 15, Leipzig D-04318, Gemany
Department of Physics, University of Leipzig, Linnéstraße 5, Leipzig D-04103, Gemany
These authors contributed equally to this work.
Author to whom correspondence should be addressed.
Received: 1 September 2014 / Revised: 15 October 2014 / Accepted: 21 October 2014 / Published: 28 October 2014
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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The radical cation of s-trioxane, radiolytically generated in a freon (CF3CCl3) matrix, was studied in the 10–140 K temperature region. Reversible changes of the EPR spectra were observed, arising from both ring puckering and ring inversion through the molecular plane. The ESREXN program based on the Liouville density matrix equation, allowing the treatment of dynamical exchange, has been used to analyze the experimental results. Two limiting conformer structures of the s-trioxane radical cation were taken into account, namely “rigid” half-boat and averaged planar ones, differing strongly in their electron distribution. The spectrum due to the “rigid” half-boat conformer can be observed only at very low (<60 K) temperatures, when the exchange of conformers is very slow. Two transition states for interconversion by puckering and ring-inversion were identified, close in activation energy (2.3 and 3.0 kJ/mol calculated). Since the energy difference is very small, both processes set on at a comparable temperature. In the case of nearly complete equilibration (fast exchange) between six energetically equivalent structures at T > 120 K in CF3CCl3, a septet due to six equivalent protons (hfs splitting constant 5.9 mT) is observed, characteristic of the dynamically averaged planar geometry of the radical cation. DFT quantum chemical calculations and spectral simulation including intramolecular dynamical exchange support the interpretation. View Full-Text
Keywords: radical cations; s-trioxane; EPR; DFT; dynamical exchange radical cations; s-trioxane; EPR; DFT; dynamical exchange

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Naumov, S.S.; Knolle, W.; Naumov, S.P.; Pöppl, A.; Janovský, I. The Dynamical Behavior of the s-Trioxane Radical Cation—A Low-Temperature EPR and Theoretical Study. Molecules 2014, 19, 17305-17313.

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