In situ electrochemical-ATR-FTIR spectroscopic studies on solution phase carboxylate radical anions

Pyridine- and benzene-based para- and meta-substituted diesters (1) were reduced by one-electron in CH₃CN (with 0.1 M Bu₄NPF₆ as the supporting electrolyte) in a two-compartment controlled potential electrolysis cell at negative potentials [-2.0 to -2.5 V vs. Fc/Fc⁺ (Fc = ferrocene)] to produce anion radicals (2) that subsequently decomposed to form carboxylate anions (3). The sequence of reactions beginning with 1 being reduced to 2, then 2 reacting to form 3 was monitored using in situ FTIR spectroscopy with an attenuated total reflectance (ATR) probe containing a diamond composite sensor inserted into the working electrode compartment of the electrolysis cell. Analysis of the data obtained during the reduction of several esters allowed for the first time the identification of spectral features that can be considered typical of carboxylate radical anions (2). It was observed that the C=O bond stretch that occurs in the neutral esters (1) at 1720-1730 cm^-1 was missing from the spectra of the anion radicals (2). Instead, the anion radicals (2) showed a strong absorbance in the range 1600-1575 cm^-1, which was at a slightly lower wavenumber than the asymmetric carboxylate stretch in the corresponding carboxylate anions (3) (1620-1600 cm^-1). The O-C-R stretch present in the spectra of the neutral esters at 1120-1130 cm^-1 shifted to 1075-1085 cm^-1 in the anion radicals (2), whilst the C-C(=O)-O stretch at ∼1280-1250 cm^-1 in the neutral esters shifted by only 0-10 cm^-1 lower in the spectra of the anion radicals (2).