Direct mapping of the angle-dependent barrier to reaction for Cl + CHD3 using polarized scattering data

Abstract

The transition state, which gates and modulates reactive flux, serves as the central concept in our understanding of activated reactions. The barrier height of the transition state can be estimated from the activation energy taken from thermal kinetics data or from the energetic threshold in the measured excitation function (the dependence of reaction cross-sections on initial collision energies). However, another critical and equally important property, the angle-dependent barrier to reaction, has not yet been amenable to experimental determination until now. Here, using the benchmark reaction of Cl + CHD3(v1 = 1) as an example, we show how to map this anisotropic property of the transition state as a function of collision energy from the preferred reactant bond alignment of the backward-scattered products—the imprints of small impact-parameter collisions. The deduced bend potential at the transition state agrees with ab initio calculations. We expect that the method should be applicable to many other direct reactions with a collinear barrier.

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Figure 1: Schematic illustration of how the transition-state structure affects chemical reactivity.
Figure 2: Imaging the dynamic outcomes in the reaction of Cl + CHD3(v1 = 1) with two distinct collisional geometries at Ec = 4.8 kcal mol−1.
Figure 3: Collisional energy dependency of the steric preference, defined as the ratio of the aligned to the unpolarized reactivity for end-on () or side-on () attack.
Figure 4: Ec-dependent evolution of differential steric preference in the HCl(v = 0) channel.
Figure 5: Strategy to deduce attack angle ρ at the saddle point from the experimentally controllable alignment angle α.

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Acknowledgements

The authors thank J.-S. Lin and O. Tkac for assisting with some imaging acquisitions. This work was supported by Academia Sinica and the Minister of Science and Technology of Taiwan (MOST-105-2113-M-001-019-MY3). F.W. also acknowledges support from the National Nature Science Foundation of China (grants 21322309 and 21673047) and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. G.C. thanks the Scientific Research Fund of Hungary (PD-111900) and the Bolyai Research Scholarship for financial support and the National Information Infrastructure Development Institute for computer resources.

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K.L. conceived and designed the experiments. H.P. and F.W. performed the experiments and analysed the data. G.C. performed theoretical calculations. All authors discussed the results and commented on the manuscript. K.L. wrote the paper.

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Correspondence to Fengyan Wang or Gábor Czakó or Kopin Liu.

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The authors declare no competing financial interests.

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Pan, H., Wang, F., Czakó, G. et al. Direct mapping of the angle-dependent barrier to reaction for Cl + CHD3 using polarized scattering data. Nature Chem 9, 1175–1180 (2017). https://doi.org/10.1038/nchem.2858

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