and formate salt of the imidazolium cation (as detailed in Supporting Information Section III). Figure 6.3 presents a comparison between spectra obtained using 12CO2 and 13CO2 in acetonitrile – to corroborate assignments for species formed resulting from CO2 reduction. Dissolved CO2 is readily observed by the positively growing band at 2341 cm-1 assigned to the antisymmetric stretching mode of CO2 21, 126. It is accompanied by a negative band situated around ~2250 cm-1. This negative band suggests the potential replacement of MeCN molecules with CO2 at the interface and/or adsorbed on the surface. Importantly, this band does not shift in position in the experiment with 13CO2, which provides additional substantiation for assignment to acetonitrile. The negative band corresponding to the symmetric C-N stretch is accompanied by another negative band at 2280/90 cm-1, which can also be assigned to CH3CN. These negative bands at these specific wavenumbers were previously reported by Desilvestro and Pons45 during CO2 reduction at a Pt electrode under negative potential. The peaks observed Figure 6.3. Comparison of ATR-FTIR Spectra of Au Thin Film Electrode in 0.1 Molar BMMIM NTf2 in Anhydrous MeCN under 12CO2 and 13CO2 Purge. The spectra were acquired while maintaining a constant negative potential of -2.25 V (vs. Fc/Fc +). Both spectra were recorded after 5 mins of electrolysis. 2341 2235
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