For a CEPT mechanism, an *(OC)OH intermediate develops during the rate-determining step. To determine the kinetic significance of the *(OC)OH intermediate, the difference in frequencies between O-H and deuterated O-D of the adsorbed intermediate was first calculated ()In, to be 964 cm-1 (in these calculations, complete covalent O-H and O-D bond formation were considered), which is larger than the difference in frequency of the C-H and C-D bonds in the imidazolium cations (()Gs=837 cm-1) (see Supporting Information Section IX). This difference in differential frequencies leads to a larger difference in the ZPE (zero-point energy) of the transition state than in the ZPE in the ground state, suggesting an inverse kinetic isotope effect (iKIE, when a covalent bond character is considered (see Figure 3.5 a). Thus, from frequency calculations, it is predicted that for a CEPT mechanism deuterium substitution at the C2 position of the imidazolium cation should result in a higher current density (lower activation energy) in the reduction of CO2. The inverse kinetic isotope effect was indeed experimentally identified when performing the reduction of CO2 in an RDE setup ( Figure 3.5 b and Supporting Information Figure S3.5). The deuterated version of MM shows significantly larger current densities, with an 18% deviation between the calculated, and experimentally determined iKIE. This deviation can be rationalized in the context of a partial proton transfer in the concerted mechanism (see also Supporting Information Section X for extra notes on iKIE) and there is still an opportunity to further improve performance by optimizing the *(OC)O -+ H-C2 bond in the transition state, lowering the energy of the transition state.
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