## 3 Results and Discussion
### Quantum Chemical Properties  
The quantum chemical parameters depend on the localization and energy of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). The energy gap influences the ionization potential (I), electron affinity (A), hardness (n), chemical potential (\(\mu\)), electronegativity (\(\chi\)), and electrophilicity index (\(\omega\)) [44; 45]. As seen in Figure 3, the delocalization of the HOMO orbital depends on the type of a substituent at the C2 position of the 5,8-quinolinedione scaffold. The HOMO orbital for compound **3** is localized close to the formyl group at the C2 position, while for derivatives **6**-**7**, it is delocalized at the 5,8-quinolinedione scaffold (Figure 3). The LUMO orbitals for all compounds are delocalized at the 5,8-quinolinedione scaffold. The arrangement of orbitals across the entire molecule indicates that the molecular system has good charge transfer capabilities (Figure 3). Figure 3: The arrangement of orbitals for compounds: (**a**) 3, (**b**) 6, (**c**) 7. The tested compounds show the energy gap (\(\Delta\)E) ranging from \(-3.306\) to \(-3.716\) eV, which means that they can exhibit high reactivity towards the biological target [46]. The type of substituent at the C2 position of 5,8-quinolinedione slightly influences the energy gap (\(\Delta\)E). The quantum chemical descriptors are presented in Table 3.  
The quantum chemical descriptors show that compounds **3** and **6-7** are characterized by high softness and flexibility in gaining electrons. The high values of the electronegativity (\(\chi\)) and electrophilicity index (\(\omega\)) indicate that these molecules are good acceptors for the nucleophilic target. The distribution of the electrophilic and nucleophilic regions in the molecule is described by the MEP map [47; 48]. Different colors on the MEP map represent the different charges. The red, blue, and green regions are negative, positive, or neutral, respectively, [49]. As seen in Figure 4, the negative areas are localized near the oxygen and nitrogen atoms. The positively charged region is localized near the hydrogen atom at the 5,8-quinolinedione moiety, and the neutral regions are visible near the chlorine atoms. Figure 4: The MEP map for compounds: (**a**) **3**, (**b**) **6**, (**c**) **7**. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Compound** & **E**Homo & **E**lumo & **\(\Delta\)E** & **I** & **A** & \(\eta\) & \(\mu\) & \(\chi\) & \(\omega\) \\ \hline
**3** & \(-7.865\) & \(-4.559\) & \(-3.306\) & \(7.865\) & \(4.559\) & \(1.653\) & \(-6.212\) & \(6.212\) & \(11.672\) \\
**6** & \(-7.860\) & \(-4.230\) & \(-3.630\) & \(7.860\) & \(4.230\) & \(1.815\) & \(-6.045\) & \(6.045\) & \(10.066\) \\
**7** & \(-8.281\) & \(-4.564\) & \(-3.716\) & \(8.281\) & \(4.564\) & \(1.858\) & \(-6.422\) & \(6.422\) & \(11.100\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The quantum chemical descriptors for compounds **3** and **6–7**. For all compounds, the three main areas of negative electrostatic potential are observed. The first area includes the substituent at the C2 position, nitrogen atom, and formyl group at the C8 position. In this area, the type of substituent significantly influences an electrostatic potential. According to the literature [28], for 6,7-dichloro-2-methylo-5,8-quinolinedione **2**, three potential minima near the nitrogen atom and oxygen of the formyl group at the C8 position were observed. Replacing the methyl group by the chlorine atom does not affect the arrangement of the electrostatic potential. The introduction of a substituent with an oxygen atom, such as hydroxyl (**6**) and formyl (**3**) groups, led to an additional local potential minimum near the substituent at the C2 position. For molecule **3**, two local potential minima are observed, both near the formyl group at the C2 position, and they are equal to \(-1.905\) and \(-2.286\) eV, respectively. For compound **6**, the potential minimum is located near the hydroxyl group reaching a value of \(-1.959\) eV.