Hydrogen tunneling with an atypically small KIE measured in the mediated decomposition of the Co(CH₃COOH)⁺ complex

Quantum mechanical tunneling (QMT) is a well-documented phenomenon in the C-H bond activation mechanism and is commonly identified by large KIE values. Herein we present surprising findings in the kinetic study of hydrogen tunneling in the Co⁺ mediated decomposition of acetic acid and its perdeuterated isotopologue, conducted with the energy resolved single photon initiated dissociative rearrangement reaction (SPIDRR) technique. Following laser activation, the reaction proceeds along parallel product channels Co(CH₄O)⁺ + CO and Co(C₂H₂O)⁺ + H₂O. An energetic threshold is observed in the energy dependence of the unimolecular microcanonical rate constants, k(E). This is interpreted as the reacting population surmounting a rate-limiting Eyring barrier in the reaction's potential energy surface. Measurements of the heavier isotopologue's reaction kinetics supports this interpretation. Kinetic signatures measured at energies below the Eyring barrier are attributed to H/D QMT. The below-the-barrier tunneling kinetics presents an unusually linear energy dependence and a staggeringly small tunneling KIE of ∼1.4 over a wide energy range. We explain this surprising observation in terms of a narrow tunneling barrier, wherein the electronic structure of the Co⁺ metal plays a pivotal role in enhanced reactivity by promoting efficient tunneling. These results suggest that hydrogen tunneling could play important functions in transition metal chemistry, such as that found in enzymatic mechanisms, even if small KIE values are measured.