title: Near-threshold H/D exchange in CD₃CHO photodissociation.
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.
authors: School of Chemistry, University of Sydney, New South Wales 2006, Australia.
authors: Brianna R
authors: BR
authors: Heazlewood
authors: Alan T
authors: AT
authors: Maccarone
authors: Duncan U
authors: DU
authors: Andrews
authors: David L
authors: DL
authors: Osborn
authors: Lawrence B
authors: LB
authors: Harding
authors: Stephen J
authors: SJ
authors: Klippenstein
authors: Meredith J T
authors: MJ
authors: Jordan
authors: Scott H
authors: SH
authors: Kable
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