Ultrafast energy flow in the wake of solution-phase bimolecular reactions

Abstract

Vibrational energy flow into reactants, and out of products, plays a key role in chemical reactivity, so understanding the microscopic detail of the pathways and rates associated with this phenomenon is of considerable interest. Here, we use molecular dynamics simulations to model the vibrational relaxation that occurs during the reaction CN + c-C6H12 → HCN + c-C6H11 in CH2Cl2, which produces vibrationally hot HCN. The calculations reproduce the observed energy distribution, and show that HCN relaxation follows multiple timescales. Initial rapid decay occurs through energy transfer to the cyclohexyl co-product within the solvent cage, and slower relaxation follows once the products diffuse apart. Re-analysis of the ultrafast experimental data also provides evidence for the dual timescales. These results, which represent a formal violation of conventional linear response theory, provide a detailed picture of the interplay between fluctuations in organic solvent structure and thermal solution-phase chemistry.

Figure 1: MD snapshot of the products from reaction (R1).
Figure 2: Simple kinetic model used to rationalize experimentally measured time profiles.
Figure 3: Time-dependence of the HCN C–H stretching energy obtained from averaging over 250 non-equilibrium MD simulations.
Figure 4: Comparison of the total vibrational energy of the nascent c-C6H11 in both gas-phase and solution-phase reactive dynamics simulations.
Figure 5: Kinetic model demonstrating improved agreement with experiments.

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Acknowledgements

T.A.A. Oliver, M.N.R. Ashfold, I.P. Clark, G.P. Greetham, A.W. Parker and M. Towrie are thanked for their contributions to the experimental work. Funding was provided by the Engineering and Physical Sciences Research Council Programme (grant EP/G00224X). The authors thank the Leverhulme Trust for an Early Career Research Fellowship (S.J.G.) and the Royal Society and the Wolfson Foundation for a Research Merit Award (A.J.O.E).

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D.R.G. and J.N.H. conceived and designed the simulations. D.R.G. wrote the simulation code, performed the simulations and analysed data. A.J.O.E. conceived the experimental study, and R.A.R, S.J.G. and A.J.O.E. obtained and analysed the experimental data and performed the initial kinetic modelling. D.R.G. wrote the manuscript, with comments and discussions from all the authors.

Corresponding authors

Correspondence to David R. Glowacki or Jeremy N. Harvey.

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The authors declare no competing financial interests.

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Glowacki, D., Rose, R., Greaves, S. et al. Ultrafast energy flow in the wake of solution-phase bimolecular reactions. Nature Chem 3, 850–855 (2011). https://doi.org/10.1038/nchem.1154

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