Concurrent with the change in band intensity at 2342 cm -1 , there are similar changes in the ν(OH) modes at 3542 cm -1 and 3632 cm -1 . This indicates that the reversible binding of CO2 to reduced BTDN is coupled to protonation reaction. Since the intensity of the 2214 cm -1 band increases, with concurrent recovery of the 2342 cm -1 as well as the 3542 cm -1 and 3632 cm -1 bands, it is likely that the observed reversible protonation behavior is associated with protonation of a carboxylate group of reduced CO2. At the reduced state of BTDN at -2.1 V vs Fc 0/+ , the pKa of the carboxylate group is higher than water resulting in proton transfer from residual water. At -0.8 V vs Fc 0/+ , oxidation of the BTDN-carboxylic acid molecule results in lowering of its pKa, resulting in proton transfer to either hydroxide or bicarbonate molecule. This redox induced shift in pKa of carboxylic residue has been previously reported with ferrocene carboxylic acid. 32 For all DFT calculated species, the predicted ν(CO) bands are in a range that is in satisfactory agreement with experimental observations.