# The First Stages of Chemical and Electrochemical Aniline Oxidation--Spectroscopic Comparative Study
## 3 Results
### Raman SEC of the Aniline Oxidation at pH 7
Figure 10: Raman spectra of gold SERS electrodes during the electrochemical oxidation of aniline in aqueous medium of pH 7 (excitation 785 nm (**a**) and 633 nm (**b**)). Raman SEC of the Film Prepared Electrochemically in Salt Solution, Conducted in 0.1 M H\({}_{2}\)SO\({}_{4}\)  
Even if the AOs formed in pH 7 seem not to be very electroactive, this is not the case, as we illustrate by spectroelectrochemical study at pH 1. As the AOs become protonated, changes of the Raman spectra with potential as well as oxidation and reduction peaks in CV appear. At potentials below 0.3 V, we obtain a spectrum known for protonated phenazine-like AOs (\(N\)-phenylphenazinium cation) prepared chemically (Figure 9a). As the chemical oxidation of aniline is connected with a decrease of pH, the AOs must at some point become protonated. This is not the case for electrochemical oxidation - the \(N\)-phenylphenazinium cation features thus became visible after the electrolyte change. At higher potentials, the observed spectrum is identical to the spectra obtained during the electrochemical oxidation at pH 7. This process is probably connected with the oxidation peak around 0.3 resp. 0.2 V in the first resp. second oxidation run (Figure 12b). This peak has been previously connected with carbonyl defects [23; 32], therefore, the AO probably contains both \(N\)-phenylphenazinium and benzoquinone unit in their structure (Figure 5b). In a second oxidation run, the original Raman spectrum is not obtained. The main difference is the appearance of a band at 1394 cm\({}^{-1}\) corresponding to C\(\sim\)N\({}^{+}\) stretching vibration, with a little more localized charge than in the case of the vibration peaking at 1371 cm\({}^{-1}\). The less obvious changes are visualized by the potential dependencies of the normalized intensities of selected bands (Figure 13). The intensity dependencies can be divided into three groups:  
1. The peaks at 1150 cm\({}^{-1}\) (C-H bending), 1161 cm\({}^{-1}\) (C-H bending in a semiquinonoid structure), 1417 cm\({}^{-1}\) (C\(\sim\)N stretching in a highly localized polaron structures) and 1457 cm\({}^{-1}\) (C=N stretching) reach maximum intensity around 0.2 V (Figure 13a). These bands are connected with the semiquinonoid-charged linear AO. 2. The intensity of the peaks at 1184 cm\({}^{-1}\) (C-H stretching in charged linear chain), 1569 cm\({}^{-1}\) (benzenoid ring stretching in linear chains), 1594 cm\({}^{-1}\) (semiquinonoid ring stretching) and 1627 cm\({}^{-1}\) (quinonoid ring stretching) reach their maximum around 0.4 V (Figure 13b). These bands are connected with the quinonoid-charged linear AO. 3. The intensity of the peaks at 1345 cm\({}^{-1}\) (C\(\sim\)N stretching in delocalized polaron structure), 1373 cm\({}^{-1}\) (C\(\sim\)N structure in localized polaron structure), 1394 cm\({}^{-1}\) (C-N stretching in polaron pair structure), 1481 cm\({}^{-1}\) (C=N stretching) and 1517 cm\({}^{-1}\) (N-H deformation vibrations) decrease around 0.2 V (Figure 13c). These bands are connected with the \(N\)-phenylphenazinium-based AO. The changes in band intensities are hugely influenced by the fact that Raman scattering from the \(N\)-phenylphenazinium cation-based structure is enhanced at gold SERS subst  
Figure 12: Raman spectra of gold SERS electrodes during the spectroelectrochemical study at pH 1 of the AO formed electrochemically in an aqueous medium of pH 7. Excitation 633 nm (**a**) and the corresponding CV (**b**). Red marks separate potential ranges connected with one Raman spectrum. and thereby it dominates the spectrum when present in this form--below 0.2 V. The intensity changes above this point are mainly due to the redox changes of the linear AOs, always present in the mix.