Near-threshold H/D exchange in CD3CHO photodissociation
Author: ["Brianna R. Heazlewood","Alan T. Maccarone","Duncan U. Andrews","David L. Osborn","Lawrence B. Harding","Stephen J. Klippenstein","Meredith J. T. Jordan","Scott H. Kable"]
Publication: Nature Chemistry
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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. Unexpected and significant isotope exchange is observed in the near-threshold photodissociation of isopically labelled acetaldehyde. Theoretical modelling indicates that, at the lowest energies considered, an average of 20 H- or D-shifts occur before dissociation — evidence for extensive isomerization.
Cite this article
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
