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Single solvent molecules can affect the dynamics of substitution reactions

Abstract

Solvents have a profound influence on chemical reactions in solution and have long been used to control their outcome. Such effects are generally considered to be governed by thermodynamics; however, little is known about the steric effects of solvent molecules. Here, we probe the influence of individual solvent molecules on reaction dynamics and present results on the atomistic dynamics of a microsolvated chemical reaction—the fundamentally important nucleophilic substitution reaction. We study the reaction of OH with CH3I using a technique that combines crossed-beam imaging with a cold source of microsolvated reactants. Our results reveal several distinct reaction mechanisms for different degrees of solvation; surprisingly, the classical co-linear substitution mechanism only dominates the dynamics for mono-solvated reactants. We analyse the relative importance of the different mechanisms using ab initio calculations and show that the steric characteristics are at least as relevant as the energetics in understanding the influence of solvent molecules in such microsolvated reactions.

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Figure 1: Measured differential scattering cross-sections for the SN2 reaction of microsolvated OH with CH3I.
Figure 2: Representation of selection criteria for different reaction mechanisms observed in the experiment.
Figure 3: Calculated energy levels of the different stationary structures of reactions (1) to (3) along the reaction pathway.
Figure 4: Potential energy surface for the entrance channel of the unsolvated reaction OH + CH3I as a function of O–C distance and O–C–I angle.

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Acknowledgements

The authors thank Jing Xie and Bill Hase for discussions and help with the ab initio calculations. The authors also acknowledge support from the University of Freiburg, where the measurements presented here were carried out. This work was also supported by the Deutsche Forschungsgemeinschaft (contract no. WE 2592/3-2). R.O. acknowledges support from the Landesgraduiertenförderung Baden-Württemberg.

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R.O. and R.W. designed and conceived the experiment. R.O., J.B., S.T. and M.S. performed the experiments. R.O. and T.B. analysed the data and discussed it with all authors. R.O. and R.W. wrote the paper.

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Correspondence to R. Wester.

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The authors declare no competing financial interests.

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Otto, R., Brox, J., Trippel, S. et al. Single solvent molecules can affect the dynamics of substitution reactions. Nature Chem 4, 534–538 (2012). https://doi.org/10.1038/nchem.1362

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