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Near-threshold H/D exchange in CD3CHO photodissociation

Nature Chemistry volume 3, pages 443–448 (2011)Cite this article

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Abstract

Measuring the isotopic abundance of hydrogen versus deuterium atoms is a key method for interrogating reaction pathways in chemistry. H/D ‘scrambling’ is the intramolecular rearrangement of labile isotopes of hydrogen atoms and when it occurs through unanticipated pathways can complicate the interpretation of such experiments. Here, we investigate H/D scrambling in acetaldehyde at the energetic threshold for breaking the formyl C–H bond and reveal an unexpected unimolecular mechanism. Laser photolysis experiments of CD3CHO show that up to 17% of the products have undergone H/D exchange to give CD2H + DCO. Transition-state theory calculations reveal that the dominant mechanism involves four sequential H- or D-shifts to form CD2HCDO, which then undergoes conventional C–C bond cleavage. At the lowest energy the molecule undergoes an average of 20 H- or D-shifts before products are formed, evincing significant scrambling of H and D atoms. Analogous photochemically induced isomerizations and isotope scrambling are probably important in both atmospheric chemistry and combustion reactions.

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Figure 1: Changing product yield of DCO for different excitation energies.
Figure 2: Percentage H/D exchange products in the photodissociation of CD3CHO:
Figure 3: Schematic of the critical points on the global PES for C2D3HO.

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Acknowledgements

This work was funded by the Australian Research Council (grant DP1094559). Parts of this research were undertaken on the NCI Computational Infrastructure National Facility in Canberra, Australia, which is supported by the Australian Commonwealth Government. B.R.H. is the recipient of a University of Sydney Postgraduate Award stipend. L.B.H. and S.J.K. are supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences, under Contract No. DE-AC02-06CH11357. D.L.O. is supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the National Nuclear Security Administration under contract No. DE-AC04-94-AL85000. We acknowledge fruitful discussions about this work with J. Bowman at Emory University.

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Authors and Affiliations

  1. School of Chemistry, University of Sydney, New South Wales 2006, Australia

    Brianna R. Heazlewood, Alan T. Maccarone, Duncan U. Andrews, Meredith J. T. Jordan & Scott H. Kable

  2. Combustion Research Facility, Sandia National Laboratories, Livermore, California, 94551, USA

    David L. Osborn

  3. Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, 60439, Illinois, USA

    Lawrence B. Harding & Stephen J. Klippenstein

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  1. Brianna R. Heazlewood
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Contributions

S.H.K. and M.J.T.J. directed the project. S.H.K. was responsible for experimental aspects of the project and M.J.T.J. led the theoretical aspects of the project. A.T.M., D.U.A. and B.R.H. conducted experiments and data analysis. Theoretical calculations were a joint effort of B.R.H., M.J.T.J., D.L.O., L.B.H. and S.J.K. All authors contributed to writing the paper.

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Correspondence to Meredith J. T. Jordan or Scott H. Kable.

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Heazlewood, B., Maccarone, A., Andrews, D. et al. Near-threshold H/D exchange in CD3CHO photodissociation. Nature Chem 3, 443–448 (2011). https://doi.org/10.1038/nchem.1052

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