## 3 Results and Discussion
### Calculated vs. Experimental: The Explanation of Electrochemical Behavior
On the other hand, the medium at pH 11.98 contained only the deprotonated, zwitter-ionic form (**9D**). As pH values approached alkaline values, the share of **9D** increased, along with current values which were higher than those obtained for the oxidation of the protonated form. In addition, the value of the initial potential decreased, along with an increase in pH value, from 0.6 V in pH 1.81 to 0.45 V in pH 11.98, indicating an easier formation of **9D** radical. Thereby, it can be suggested that the deprotonated form was more electrochemically active than the protonated one. In other words, the share of the deprotonated form improved the electrochemical activity of compound **9**. Moreover, DFT calculations were also employed to calculate the zero-point-corrected total energy (E0), vertical ionization energies (VIE) and adiabatic ionization energies (AIE) of the protonated and deprotonated forms of the electrochemically active pyridine **9**. VIE and AIE are energies required for the ejection of an electron, without and with a consecutive structural/conformational reorganization of the molecule, respectively [39]. These energies represent the difference between the E0 ground and E0 radical. Considering that the ratio of the **C** form of compounds **9** was negligible, the calculations obtained for this form were not taken into account. The DFT data obtained for compound **9** in water using the M06-2X method are presented in Table 2, whereas data obtained for all compounds (**5-8**) in gas phase and water are shown in Tables S2-S4 (Supplementary Materials). As seen in Table 2, the difference in the energies of the ground state and the corresponding radical is lowest in the deprotonated **9D** form, indicating that the formation of this radical is energetically the most favorable. On the other hand, the VIE of all forms is higher in value than AIE, because electron transfer is accompanied by the molecule's structural/conformational reorganization in order to achieve the most efficient electron density distribution and the most stable system. VIE and AIE energies have the lowest values for the **9D** form of the molecule, as well as the lowest difference between these values, which indicates that the anionic form of the molecule has the highest propensity to absorb excess electron density. The calculated values are in accordance with the electrochemical behavior of **9** (Figure 3), confirming that the deprotonated anionic form of **9** is in fact the most electrochemically active. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Compound Form** & **E0(radical)/Hartree** & **E0(ground)/Hartree** & **VIE/eV** & **AIE/eV** & **AIE */eV** \\ \hline
**9A** & –684.99165 & –685.00377 & 8.65 & 8.32 & 0.33 \\
**9B** & –685.05465 & –685.06112 & 6.87 & 6.69 & 0.18 \\
**9D** & –684.67384 & –684.67969 & 5.58 & 5.42 & 0.16 \\ \hline \hline \end{tabular}  
* difference in ionization energies (VIE-AIE = AIE).  
\end{table} 
Table 2: The vertical (VIE) and adiabatic ionization energies (AIE) in water for compound **9**, calculated with the M06-2X method. In order to compare the activity of **9** with the analogue pyridones, their structures should also be taken into account. Namely, theoretical calculations showed that pyridones **5** and **8** also exist in three protonated forms (**A**, **B** and **C**) and one deprotonated form (**D**) (Figure S4, Supplementary Materials), while the \(N\)-substituted one (**6** and **7**), due to the lack of proton in the nitrogen atom, may exist in two protonated forms (**A** and **B**) and one deprotonated form (**D**). The relative energies of the possible protonated forms of **5**-**8** as well as their amounts are presented in Table S5 (Supplementary Materials). According to the data obtained with the M06-2X method in water, it is clear that for all pyridones the most stable structure is that of form **A**, i.e., protonated at the C5 atom, which is analogous to **9**. Figure S5 (Supplementary Materials) depicts the experimental and calculated UV-Vis spectra of **5** in water.