positive charge within the imidazolium ring, with a slight increase in positive charge localized at the C2-H proton. This observation hints at the possibility of a more acidic character within the DiPh cation. To further explore this, we subjected the DiPh cation to exposure with the indenyl anion as an indicator base. Following this, 1H NMR spectroscopy was employed to quantitatively gauge the acidity of the DiPh cation (for additional information, refer to Supporting Information Section V). The analysis yielded a measured pKa value of 20.1 for DiPh in DMSO. Comparing this value with the reported pKa value for the MM cation in DMSO (~22.0)70 support the more acidic character of DiPh cation. Consequently, one might anticipate an increased activity for the DiPh cation; however, this expectation is not corroborated by the LSV results shown in Figure 4.6. Unexpectedly, we observe a significantly more negative onset potential for CO2 reduction with the DiPh cation. To rationalize the unforeseen outcomes of the LSV results, we may consider the impact of cation bulkiness. It's apparent that the DiPh cation boasts a larger molecular size compared to the MM cation. The increased bulk of the DiPh cation could potentially influence the interaction between the cation and the Au electrode, as well as the surface-adsorbed CO2 species. A similar adverse effect was also witnessed with the sizeable 1-nonyl-3-methyl (NM) imidazolium cation (as discussed in Chapter 7). Despite having only marginal acidity differences in comparison to the MM cation, a noticeable decrease in activity was observed with NM cation. To further validate the impact of cation size, future investigations should consider employing techniques such as impedance measurements and molecular simulations.
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