## 3 Results and Discussion
### Density Functional Theory (DFT)  
The physicochemical properties and frontier molecular orbitals (FMOs) of the new enaminone compounds play a crucial role in enhancing bioactivity. [PERSON]'s DFT approach with the B3 LYP method was used for geometry optimization [54]. The electron-donating and -withdrawing ability of a compound can be explained by its HOMO and LUMO. The higher the energy value of HOMO, the greater the electron-contributing ability of the compound. The energy difference between HOMO and LUMO is an established parameter to measure the electron conductivity or degree of intermolecular charge transfer, which also affects bioactivity [55]. In this study, to examine the urease inhibition capacity of the pyrimidine derivatives, we randomly selected compounds for comparison with DFT results. The results of HOMO-LUMO energies were plotted against the biological activity of the pyrimidine derivatives (Table 2). Good correlation was observed between biological activity and the energies of the LUMO orbitals. Compounds **3h**, **3k**, **3f**, and **3o** showed significant LUMO energy of \(-\)1.89, \(-\)2.24, \(-\)1.96 and \(-\)2.26 eV, respectively. The visualization of the HOMO-LUMO orbitals of **3h** reflects the localization of FMO (frontier molecular orbital) (Figure 3). The negative and positive phases of orbitals are shown in green and red, respectively. For **3h**, HOMO is localized on the pyrimidine ring, distal pyridine moiety and carbonyl group, whereas LUMO is on the pi-bond adjacent to the pyrimidine ring. In contrast, for **3o**, electrons are delocalized mainly on the benzene ring with iodine group as a substituent. The influence of LUMO energy on the inhibitory activity of the compounds might be due to the presence of halogen substitution and the pyridine ring. The halogen substitution at _ortho_ and _para_ positions made a great contribution to the urease inhibition capacity of the derivatives. Notably, neither HOMO nor LUMO were located on the methyl group, which would explain why this group contributed the least to binding with the protein. The energy gap for the most active compound **3h** of the series was \(-\)4.35. A similar energy difference was observed for **3k**, **3f**, and **3o**. The smaller the difference in HOMO-LUMO energies, the greater the chemical reactivity. For any potential interaction, electron transfer from high-lying HOMO to low-lying LUMO is always energetically favorable. Given this consideration, **3h**, **3k**, **3f**, and **3o** possess good activity, which correlated well with urease inhibitory activity. Molecular electrostatic potentials (MEP) were run for compounds **3h**, **3k**, and **3o** at B3 LYP/6-31G (d, p), providing information about the sites reactive towards nucleophilic and electrophilic attack, together with hydrogen-bond interactions. The potential of electrostatic interaction at the surface is shown by blue, red and green, representing the sites for positive, negative and no electrostatic potential, respectively (Figure 4). Moreover, the positive and negative regions of the maps were responsible for the nucleophilic and electrophilic reactivity of the compounds. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Compound** & **IC50** & **HOMO (eV)** & **LUMO (eV)** & **Egap (eV)** \\ \hline
**3a** & \(9\pm 2\) & \(-\)5.79330622969 & \(-\)1.21770997548 & \(-\)4.57559625421 \\ \hline
**3d** & \(8\pm 0.3\) & \(-\)5.88881823 & \(-\)1.70370550982 & \(-\)4.76988562237 \\ \hline
**3f** & \(10\pm 0.6\) & \(-\)6.2202530345 & \(-\)1.96248588673 & \(-\)4.25776714777 \\ \hline
**3h** & \(6\pm 0.3\) & \(-\)6.24583174683 & \(-\)1.88955934518 & \(-\)4.35627240165 \\ \hline
**3k** & \(11\pm 1\) & \(-\)5.70922301574 & \(-\)2.24439595033 & \(-\)3.46482706541 \\ \hline
**3o** & \(9\pm 0.3\) & \(-\)5.75357759138 & \(-\)2.25963433214 & \(-\)3.49394325924 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Theoretical results of pyrimidine derivatives.