# The First Stages of Chemical and Electrochemical Aniline Oxidation--Spectroscopic Comparative Study
## 3 Results
### UV--Visible Absorption Spectroscopy of Chemically Oxidated Aniline  
In contrast to FTIR, UV--visible absorption and SERS kinetics of the chemical oxidation of aniline could not be conducted in situ, as the solution becomes opaque during the reaction, and the bulk material rather than the film is analyzed. The chemical oxidation of aniline was conducted in a wide beaker. Glass slides and SERS substrates were introduced in the medium at the very beginning. Based on the kinetics, certain times were selected for removal of the substrates from the reaction medium. The spectra were normalized, and in the case of pH 7 also shifted for clarity, the total absorbance of the sample was strongly dependent on the film thickness. Significant differences are observed between UV--Vis absorption spectra obtained on glass substrates removed from the reaction mixtures at pH 1 and 7 at various times (Figure 4). Whereas in the strongly acidic medium, PANI in the emeraldine (15 min, final) [41; 50; 51; 52; 53] and pernigraniline (2 min, 10 min) [48; 52] salt form is observed. The evolution of the UV--Visible absorption spectra of the product formed at pH 7 are more complex. After 15 s of reaction, a spectral shape corresponding to emeraldine salt is observed. After 100 s, a relatively narrow band at 425 nm appears, which corresponds to the charged phenazine-like AOs [54; 55]. \begin{table}
\begin{tabular}{c c c c} \hline
**Acid** & **Assignment** & **Product** & **References** \\ \hline
1571 & \(\
u\)(–C=C–) of the Q rings & neutral AO & [39; 40] \\ \hline
1517 & \(\
u\)(C–C) + \(\delta\)(C–H) & doped dimer & [39] \\ \hline
1499 & \(\
u\)(–C=C–) of the B rings & neutral AO & [40] \\ \hline
1490 & \(\
u\)(–C=C–) of the B rings & protonated PANI chain & [40] \\ \hline
1460 sh & \(\
u\)(ring) substituted aromatic ring & pernigraniline & [47; 48] \\ \hline
1376 & \(\
u\)(C=N) next to the aromatic ring & pernigraniline & [47; 48] \\ \hline
1312/1338 & \(\
u\)(C\(\ast\)N) & PANI chain/AO (delocalization length) & [39] \\ \hline
1302 & \(\
u\)(C–N) & secondary aromatic amines & [45] \\ \hline
1285 & \(\delta\)(N\(-\)H) + \(\
u\)(C–N) & Primary amino group & [38] \\ \hline
1260 & \(\delta\)(C–H) in the Q ring, \(\
u\)(C–N), \(\
u\)(C–O) & Primary amino group in AO, \(p\)-benzquinone & [17; 38; 39] \\ \hline
1245 & \(\
u\)(C–N\({}^{+}\)) & PANI salt & [49] \\ \hline
1148 & \(\
u\)(=NH\({}^{+}\)-) & protonate PANI chain & [38; 39] \\ \hline
1100 & \(\
u\)(SO\({}_{4}\)\({}^{2}\)) & ammonium sulfate & [43; 44] \\ \hline
1049 & \(\
u\)(SO\({}_{4}\)\({}^{2}\)) & sulfated aromatic rings & [43; 44] \\ \hline
835 & \(\gamma\)(C–H) & 1,2-disubstituted ring & [45] \\ \hline
800 & \(\gamma\)(C–H) & 1,2-disubstituted ring & [39] \\ \hline \end{tabular}
\end{table} 
Table 2: Assignment of the FTIR bands in the in-situ FTIR experiment at pH 1. This band is then present till the end of the reaction, with only minor shifts in position. The shoulder at 480 nm observed in the spectra obtained after 15 and 20 min of reaction is attributed to radical cations of aniline-type dimer [54]. The soluble dimer intermediates were clearly not washed off properly after the substrate removal from the reaction mixture. Raman Spectra Obtained on SERS Electrodes Removed from the Media at Different Times of Chemical Aniline Oxidation  
Raman spectra of the products of aniline oxidation on SERS electrodes at our starting pH 7 excited with the 785 nm laser line (Figure 5a, Table 3), vaguely resemble those of emeraldine salt. The 785 nm excitation line is in resonance with charged emeraldine and pernigraniline-like structures, making the semiquinonoid (1600 cm\({}^{-1}\)) and quinonoid rings (1625 cm\({}^{-1}\)) with charged nitrogen structure (\(\sim\)1370 cm\({}^{-1}\)) dominant in the spectrum. The phenazinium cation-based AOs are resonantly enhanced by the laser line at 633 nm [28]. Their typical spectral profile strongly dominated by the bands at 1388/1363 and 608/575 cm\({}^{-1}\) is clearly observed (Figure 5b, Table 3) [28; 34].