In addition to the observed changes in the ν(CN) and ν(CO) region when the potential is scanned passed the second reduction wave under CO2 atmosphere, there are two noticeable negative peaks at 3542 cm -1 and 3632 cm -1 which corresponds to OH stretching mode that can be attributed to presence of residual water in the solvent (Figure ). This suggests protonation reactions of BTDN 2-by residual water. The product of BTDN 2-protonation, monitored via the ν(CN) band, will not have strong CO2 concentration dependence, which is consistent with the observed band at 2214 cm -1 . This assignment is further supported by DFT calculations shown below. Another possible protonation reaction pathway is the protonation of carboxylate group formed when CO2 is bound to reduced BTDN as the pKa of such carboxylate group is expected to be lower compared to water.