in 13C NMR and VDD charge analysis show that the electron density is a function of the substitution, which also affects the C2-H acidity. pKa calculations/measurements in acetonitrile from previous studies also demonstrate a trend in the acidity for MM (32.5), i-Pr i-Pr (33.6), and t-Bu t-Bu (34.1) cations69-70, which is also in agreement with the trend obtained with 13C NMR spectra (Table S3.1). Interestingly, and most importantly, the electron density of the imidazolium cation translates to higher or lower performance in the electrochemical reduction of CO2, showing an almost linear trend between 13C peak position (Figure 3.2 b) and current density in the reduction of CO2, as schematically indicated in the graphical abstract of this chapter. With the establishment of this structure-activity relationship, the focus now shifts to understanding how imidazolium promotes CO2 reduction in anhydrous media. To address this, the significance of the C2-proton was first compared to that of the C4- and C5-protons of the imidazolium ring. In the literature discussing electrochemical CO2 reduction, it is well-known that the rate-determining step (RDS) is the first electron transfer from the electrode to adsorbed CO2 19, 39. According to previous studies, the Au electrode surface facilitates the initial electron transfer to CO2, resulting in the formation of a high-energy *CO2 ¯ intermediate71-72. DFT calculations revealed a notable ~70 kcal/mol difference in free energy between the adsorbed *CO2 ¯ (Au-CO2 ¯ ) and the solvated CO2 ¯ radical (CO2 ¯ (sol)), confirming the hypothesis of adsorption-induced electron transfer (*+CO2 + e ¯ = *CO2 ¯ ) in acetonitrile (Supporting Information Section V). The structure-activity relationship developed in this work suggests that ring-protons H2, H4, and H5 may play a role in promoting the kinetics by stabilizing the *CO2 ¯ intermediate. While the trend in NMR chemical shifts for C2 is more pronounced than for C4 or C5 (Figure 3.2 b), it remains inconclusive which ring proton is involved in stabilizing the *CO2 ¯ intermediate. To investigate further, DFT calculations were employed to compare the stability of *CO2 ¯ when interacting with the C2-proton (configuration i) or C4- and C5-protons
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