X. Extra notes on the inverse kinetic isotope effect While for most coupled electron-proton transfer reactions a normal KIE effect (>1)is expected73, 83-84, the inverse KIE (iKIE) observed in this work highlights a unique feature of the imidazolium-mediated anhydrous CO2 reduction. The observed inverse kinetic isotope effect shares similarities with the proton transfer in catalytic reactions involving the formation of lowbarrier hydrogen bonds (LBHBs). In catalytic reactions associated with LBHBs, a weak hydrogen bond in the ground state becomes a low barrier hydrogen bond (strong and shortdistance hydrogen bond) in the transition state85-87. LBHBs are mostly observed in non-protic solvents. The energy released in forming the LBHB then contributes to lowering the activation barrier for the reaction85. In another word, this energy release can explain the preference for the CEPT pathway over the sequential pathway in anhydrous media (Figure 3.4). Developing the strong hydrogen bonds between negatively charged oxygen of *CO2 ¯ and + H-C2 of MM in the transition state, thus, should have caused the inverse KIE observed in this work.
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