# The First Stages of Chemical and Electrochemical Aniline Oxidation--Spectroscopic Comparative Study
## 3 Results
### FTIR Kinetic Study of the Chemical Oxidation of Aniline
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Acid** & **Assignment** & **Product** & **References** \\ \hline
3264 & \(\
u\)(N–H) & Hydrogen bond to C=O of \(p\)-benzoquinone & [35] \\ \hline
3200 & \(\
u\)(N–H) & Hydrogen bonded to \(\mathrm{sulfonic}\) group & [35] \\ \hline
1622 & \(\delta\)(NH) of primary aromatic amines; \(\
u\)(C=O) & AO, monomer & [38] \\ \hline
1581 & \(\
u\)(–C=C–) of the Q ring & linear chain & [39; 40] \\ \hline
1546 & \(\
u\)(C–C) of the B ring or phenazine-like unit & AO & [39] \\ \hline
1509 & \(\
u\)(–C=C–) of the B ring & linear chain & [39] \\ \hline
1445 & \(\
u\)(ring) of the substituted aromatic ring & Phenazine-like structures & [39; 41; 42] \\ \hline
1414 & \(\
u\)(phenazine ring), \(\
u\)(C–C) next to the imine group & Phenazine-like structures & [39; 41; 42] \\ \hline
1365 & \(\
u\)(C=N) next to the aromatic ring & 1,2,4-substituted ring & [38] \\ \hline
1299 & \(\
u\)(norm. amines) & \(p\)-benzoquinone, \(p\)-benzo-like structures & [38] \\ \hline
1266 & \(\
u\)(C–N), \(\delta\)(C–H) in the Q ring, \(\
u\)(C–O) & primary amines, \(p\)-benzoquinone, phenol & [17; 38; 39] \\ \hline
1208 & \(\
u\)(C–N) & primary amines & [17] \\ \hline
1185/1174 & \(\delta\)(C–H) in plane & quinone dimine structures & [39] \\ \hline
1100 & \(\
u\)(SO\({}_{4}\)\({}^{2}\)) & ammonium sulfate & [43; 44] \\ \hline
1043 & \(\
u\)(SO\({}_{4}\)\({}^{2}\)) & sulfated aromatic rings & [43; 44] \\ \hline
861 & \(\gamma\)(C–H) out of plane & tri-substituted aromatic ring & [45] \\ \hline
847 & \(\gamma\)(C–H) out of plane & di-substituted aromatic ring & [45] \\ \hline
758 & \(\gamma\)(C–H) & linear chain & [46] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Assignment of the FTIR bands in the in-situ FTIR experiment at pH 7. Figure 2: Selected spectra representing the evolution of attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectra of aniline oxidation at pH 7 on ZnSe crystal. The spectrum of the reaction mixture was subtracted from each spectrum. are connected with water (1630 cm\({}^{-1}\)), aniline (1499 cm\({}^{-1}\)) and APS (1048 cm\({}^{-1}\)). The fact that the bands of reactants and water are present, even in the differential spectra, can be assigned to two factors: (1) The droplet of the reaction mixture spreads wider on the ATR crystal during first minute of the oxidation reaction, which influences the subtraction, and (2) the reactants are adsorbed on the crystal surface. Both factors play their role. The presence of water bands is exclusively caused by the higher area of the droplet-crystal interface. On the other hand, due to the adsorption of the reactants, the intensities of the bands from aniline and APS are considerably higher than expected based on the reactant spectra. We do not observe this behavior in the evolution of spectra measured in the salt solution (pH 7), as the first products are formed fast enough to cover the crystal-solution and solution-air interfaces, and stabilize the shape of the droplet. The first distinguishable bands which are not connected with the reactants are observed after 2 min of oxidation. The band at about 1110 cm\({}^{-1}\) is assigned to the vibration modes of ammonium sulfate, the product of the APS decomposition. The broad, weak band found at about 1250 cm\({}^{-1}\) is most probably connected with the vibration of amine groups in the aromatic amines. After 2 min, the band at 1571 cm\({}^{-1}\) (stretching vibration of quinonoid rings), the weak, broad band at about 1500 cm\({}^{-1}\) (vibration of benzenoid ring in the PANI structure), and bands at 1376 cm\({}^{-1}\) (imine structure in penriganiline), 1285 cm\({}^{-1}\) and 1260 cm\({}^{-1}\), appear. The broad band at about 1100 cm\({}^{-1}\) and a sharp band at 1047 cm\({}^{-1}\) are connected with formation of ammonium sulfate. The intensity ratio of the quinonoid and benzenoid ring stretching bands corresponds to the higher content of the quinonoid rings observed in the first stages of chemical aniline oxidations [17].