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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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.
The authors declare no competing financial interests.
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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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