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Influence of the leaving group on the dynamics of a gas-phase SN2 reaction

Nature Chemistry volume 8, pages 151–156 (2016)Cite this article

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Abstract

In addition to the nucleophile and solvent, the leaving group has a significant influence on SN2 nucleophilic substitution reactions. Its role is frequently discussed with respect to reactivity, but its influence on the reaction dynamics remains unclear. Here, we uncover the influence of the leaving group on the gas-phase dynamics of SN2 reactions in a combined approach of crossed-beam imaging and dynamics simulations. We have studied the reaction F− + CH3Cl and compared it to F− + CH3I. For the two leaving groups, Cl and I, we find very similar structures and energetics, but the dynamics show qualitatively different features. Simple scaling of the leaving group mass does not explain these differences. Instead, the relevant impact parameters for the reaction mechanisms are found to be crucial and the differences are attributed to the relative orientation of the approaching reactants. This effect occurs on short timescales and may also prevail in solution-phase conditions.

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Figure 1: Calculated minimum energy path of reactions (1) and (2) along the reaction pathway.
Figure 2: Differential scattering cross-sections and extracted angular and energy distributions of the SN2 reactions F− + CH3Cl and F− + CH3I.
Figure 3: Direct rebound fraction for F− + CH3Cl in comparison to previously studied systems.
Figure 4: Opacity functions for different reaction mechanisms.

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Acknowledgements

This work is supported by the Austrian Science Fund (FWF), project P 25956-N20. E.C. acknowledges support from a DOC-Fellowship by the Austrian Academy of Sciences (ÖAW). G.C. was supported by the Scientific Research Fund of Hungary (OTKA, PD-111900) and the János Bolyai Research Scholarship of the Hungarian Academy of Sciences.

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Author notes

  1. Martin A. Kainz & Aditya H. Kelkar

    Present address: Present addresses: Photonics Institute, TU Wien, Gußhausstraße 27–29, Vienna 1040, Austria (M.A.K.); Department of Physics, Indian Institute of Technology Kanpur, Kanpur-208016, Kanpur, India (A.H.K.),

Authors and Affiliations

  1. Institute for Ion Physics and Applied Physics, Universität Innsbruck, Technikerstraße 25, Innsbruck, 6020, Austria

    Martin Stei, Eduardo Carrascosa, Martin A. Kainz, Aditya H. Kelkar, Jennifer Meyer & Roland Wester

  2. Laboratory of Molecular Structure and Dynamics, Institute of Chemistry, Eötvös University, PO Box 32, Budapest 112, 1518, Hungary

    István Szabó

  3. Department of Physical Chemistry and Materials Science, University of Szeged, Rerrich Béla tér 1, Szeged, H-6720, Hungary

    István Szabó & Gábor Czakó

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Contributions

M.S., E.C., A.K. and R.W. conceived the experiment. M.S., E.C., A.K. and M.K performed the measurements. M.S. analysed the data. I.S. and G.C. carried out the simulations. All authors discussed the results. A.K, M.S., E.C., J.M., G.C. and R.W. wrote the paper.

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Correspondence to Gábor Czakó or Roland Wester.

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Stei, M., Carrascosa, E., Kainz, M. et al. Influence of the leaving group on the dynamics of a gas-phase SN2 reaction. Nature Chem 8, 151–156 (2016). https://doi.org/10.1038/nchem.2400

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