Quantum tunneling dynamics in the Ni⁺-mediated C-H activation of acetic acid

The mechanistic and dynamic properties of the Ni⁺ mediated reaction with CH₃COOH and its perdeuterated isotopologue are presented. Microcanonical kinetic measurements are made in the gas phase with the energy- and time-resolved single photon initiated dissociative rearrangement reaction (SPIDRR) technique and these are complemented with density functional theory (DFT) and multi-reference (MRCI) calculations. Experimental and theoretical evidence indicates that the formation of three product pairs - Ni(C₂H₂O)⁺ + H₂O, Ni(CH₄O)⁺ + CO and Ni(H₂O)⁺ + C₂H₂O - are rate limited by C-H bond activation. Measurements of rate-limiting microcanonical k(E) rate constants are made over the 15 000 cm^-1 to 20 000 cm^-1 (180-240 kJ mol^-1) energy range where a transition from quantum mechanical tunneling to over-barrier reaction control is observed. Rate constants, where quantum mechanical tunneling (QMT) primarily contributed to their magnitudes, possessed a large H/D QMT kinetic isotope effect (KIE = 19.0 ± 3.2) consistent with the expectations of QMT. Surprisingly, QMT rate constants in the tunneling energy regime were nearly energy independent and appear to extrapolate to very low energies. Applications of tunneling corrections to RRKM (Rice-Ramsperger-Kassel-Marcus) calculated rate constants failed to describe this unexpected QMT behavior. It is proposed that the Ni⁺ cation's electronic structure may promote QMT by providing bonding schemes where the proximity of organic fragments increase QMT probability. Such structures are proposed to exist along the multidimensional PES, providing tunneling pathways with reduced barrier widths and consequent energy dependence. These results highlight quantum dynamic properties of Ni⁺ ions in C-H bond activation reactions, an important step towards understanding the metal's ability to promote catalysis at low energy.