School of Chemistry, University of Sydney, New South Wales 2006, Australia.
Nat Chem. 2011 Jun;3(6):443-8. doi: 10.1038/nchem.1052.
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.
测量氢原子与氘原子的同位素丰度是研究化学反应途径的一种关键方法。H/D“交换”是氢原子的不稳定同位素在分子内的重排，当它通过意外的途径发生时，会使这些实验的解释复杂化。在这里，我们在打破甲酰基 C-H 键的能量阈值下研究乙醛中的 H/D 交换，并揭示了一种意想不到的单分子机制。CD₃CHO 的激光光解实验表明，高达 17%的产物经历了 H/D 交换，生成 CD₂H + DCO。过渡态理论计算表明，主要的机制涉及到四个连续的 H 或 D 转移，形成 CD₂HCDO，然后再进行常规的 C-C 键断裂。在最低能量下，分子在形成产物之前经历了平均 20 次 H 或 D 转移，表明 H 和 D 原子发生了显著的交换。类似的光化学诱导异构化和同位素交换在大气化学和燃烧反应中可能都很重要。
