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Roaming-mediated ultrafast isomerization of geminal tri-bromides in the gas and liquid phases

Nature Chemistry volume 7, pages 562–568 (2015)Cite this article

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Abstract

‘Roaming’ is a new and unusual class of reaction mechanism that has recently been discovered in unimolecular dissociation reactions of isolated molecules in the gas phase. It is characterized by frustrated bond cleavage, after which the two incipient fragments ‘roam’ on a flat region of the potential energy surface before reacting with one another. Here, we provide evidence that supports roaming in the liquid phase. We are now able to explain previous solution-phase experiments by comparing them with new ultrafast transient absorption data showing the photoisomerization of gas-phase CHBr3. We see that, upon S0–S1 excitation, gas-phase CHBr3 isomerizes within 100 fs into the BrHCBr–Br species, which is identical to what has been observed in solution. Similar sub-100 fs isomerization is now also observed for BBr3 and PBr3 in solution upon S1 excitation. Quantum chemical simulations of XBr3 (X = B, P or CH) suggest that photochemical reactivity in all three cases studied is governed by S1/S0 conical intersections and can best be described as occurring through roaming-mediated pathways.

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Figure 1: Experimental data illustrating direct photochemical isomerization of gas-phase CHBr3.
Figure 2: Minimum energy paths (MEPs) from S1 of the parent molecules to S0 of the isomers.
Figure 3: Roaming-mediated isomerization of CHBr3 following S1 excitation.
Figure 4: Evidence for ultrafast direct isomerization of S1-excited PBr3 and BBr3 in solution.
Figure 5: Schematic representation of ultraviolet photochemistry of boron and phosphorus tribromides in solution.
Figure 6: Roaming-mediated isomerization of BBr3 and PBr3 following S1 excitation.

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Acknowledgements

This work was supported by the National Science Foundation (CAREER CHE-0847707 and CHE-0923360). The authors acknowledge an allocation of computer time from the Ohio Supercomputer Center (PCS0204-7) and the Extreme Science and Engineering Discovery Environment (XSEDE, CHE130073). The authors thank R. M. Wilson and P.Z. El-Khoury for discussions. This work would not have been possible without discussions with M. Olivucci, as well as his guidance through the semiclassical trajectory calculations.

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Author notes

  1. Andrey S. Mereshchenko, Evgeniia V. Butaeva and Veniamin A. Borin: These authors contributed equally to this work

Authors and Affiliations

  1. Department of Chemistry and the Center for Photochemical Sciences, Bowling Green State University, Bowling Green, 43403, Ohio, USA

    Andrey S. Mereshchenko, Evgeniia V. Butaeva, Veniamin A. Borin, Anna Eyzips & Alexander N. Tarnovsky

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  1. Andrey S. Mereshchenko
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  2. Evgeniia V. Butaeva
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Contributions

A.S.M. conceived and designed the experiments, and carried them out together with E.V.B. and A.N.T. E.V.B. and V.A.B. designed and performed the high-level quantum chemical simulations. A.E. analysed the semiclassical trajectories of boron and phosphorus tribromides. A.N.T. analysed the data and supervised the project. The first three authors contributed equally to this work, and all authors contributed to writing sections of the paper.

Corresponding author

Correspondence to Alexander N. Tarnovsky.

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The authors declare no competing financial interests.

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Supplementary information (PDF 8326 kb)

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Supplementary Movie 1 (AVI 600 kb)

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Supplementary Movie 2 (AVI 470 kb)

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Supplementary Movie 3 (AVI 505 kb)

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Supplementary data 1. The xyz coordinates of the BBr3 gas-phase CASPT2 semiclassical trajectory. (TXT 24 kb)

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Supplementary data 2. The xyz coordinates of the CHBr3 gas-phase CASPT2 semiclassical trajectory. (TXT 21 kb)

Supplementary information

Supplementary data 3. The xyz coordinates of the PBr3 gas-phase CASPT2 semiclassical trajectory. (TXT 24 kb)

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Supplementary data 4. The xyz-file describing the trajectory of bromoform in the acetonitrile solvent cage: CASSCF QM/MM molecular dynamics simulation of CHBr3 in a rigid CH3CN cavity. (TXT 32421 kb)

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Mereshchenko, A., Butaeva, E., Borin, V. et al. Roaming-mediated ultrafast isomerization of geminal tri-bromides in the gas and liquid phases. Nature Chem 7, 562–568 (2015). https://doi.org/10.1038/nchem.2278

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