## 3 Results and Discussion
### Theoretical Calculations
The minimum point structures located on the potential surface scan (PES) for 5-(4-methoxyphenyl)-2-methylthio-5,12-dihydroxy-driborbenzo[\(g\)]pyrimidine0[4,5-\(b\)]quinoline-4,6,11(3\(H\))-tritone **4c** was submitted for optimization using the B3 LYP/6-31G(d,p) computational levels and theoretical approximations were performed in the gas phase. From the rotation of different groups, the minimum energy conformation and valuable structural information about compound are obtained. In order to reveal all possible conformational of the title compound, a detailed potential energy curve for \(\tau\)1 (N\({}_{1}\)-C\({}_{2}\)-S\({}_{19}\)-C\({}_{20}\)), \(\tau\)2 (C\({}_{14}\)-C\({}_{5}\)-C\({}_{24}\)-C\({}_{29}\)), and \(\tau\)4 (C\({}_{28}\)-C\({}_{27}\)-C\({}_{44}\)-C\({}_{45}\)) dihedral angles was performed in steps of 10\({}^{\circ}\) from 0\({}^{\circ}\) to 360\({}^{\circ}\) and are described in Figure 4.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Dihedral Angles (\({}^{\circ}\))** & **Experimental X-ray** & **6-31G(d,p)** & **Dihedral Angles (\({}^{\circ}\))** & **Experimental X-ray** & **6-31G(d,p)** \\ \hline N\({}_{1}\)-C\({}_{2}\)-N\({}_{3}\)-C\({}_{4}\) & \(-\)0.7 (5) & 1.3 & C\({}_{16}\)-C\({}_{17}\)-C\({}_{11}\)-C\({}_{18}\) & \(-\)0.9 (5) & 4.3 \\ C\({}_{13}\)-N\({}_{1}\)-C\({}_{2}\)-NStructure of highest and lowest energy conformers for \(\tau\)1, \(\tau\)2 and \(\tau\)4 dihedral angles and the computed values of these dihedral angles are given in Figure 4. The PES graphs are plotted using the conformation energies (kJ/mol) in Figure 5. The minimum energy curves for \(\tau\)1 (N\({}_{1}\)-C\({}_{2}\)-S\({}_{19}\)-C\({}_{20}\)) dihedral angle were obtained at \(-0.78758^{\circ}\) (\(-17.9\) Kcal/mol), \(\tau\)2 (C\({}_{14}\)-C\({}_{5}\)-C\({}_{24}\)-C\({}_{29}\)) \(-90.71251^{\circ}\) (\(-18.0\) Kcal/mol), and \(\tau\)4 (C\({}_{15}\)-C\({}_{5}\)-C\({}_{24}\)-C\({}_{29}\)) at \(178.04179^{\circ}\) (\(-17.9\) Kcal/mol) for B3 LYP level. Vibrational spectral assignments (Table 4) were performed on the recorded FT-IR spectra, based on the theoretically predicted wavenumbers by DFT methods, using 6-31G(d,p) basis set [50]. The observed spectra are in good agreement with the simulated spectra. Figure 4: One-dimensional potential energy surface (PES) scan of the calculated energies vs. dihedral angles (\(\tau\)) using DFT/B3 LYP/6-31G(d,p) for 5-(4-methoxyphenylyphenyl)-2-methylthio-5,12-dihydrobenzo [g]pyrimid0[4,5-b]quinoline-4,6,11(3\(H\))-tritone **4c**. Figure 5: Highest and lowest energy conformations using DFT/B3 LYP/6-31G(d,p) for 5-(4-methoxyphenylyphenyl)-2-methylthio-5,12-dihydrobenzo[g]pyrimid0[4,5-b]quinoline-4,6,11(3\(H\))-tritone **4c**. The assignment of experimental vibrational bands to normal vibration modes is based on the comparison with related molecules and with results of the calculations obtained. We consider the B3 LYP/6-31G(d,p) calculations because the scale factors used are well defined for this base set and also because the compared molecule was optimized at the same calculation level (0.961 \(\pm\) 0.045) [51]. _N-H vibrations_  
The bands at 3459 cm\({}^{-1}\) and 3450 cm\({}^{-1}\) are assigned to the symmetrical stretches of the N-H groups, N\({}_{12}\) and N\({}_{3}\), respectively. These bands appear overlapping in the experimental spectrum at 3272 cm\({}^{-1}\). _C=O groups vibrations_  
The C=O stretches are observed as medium intensity bands at 1674 cm\({}^{-1}\), 1650 cm\({}^{-1}\) and 1620 cm\({}^{-1}\). The DFT calculation gives the stretch wave number at 1784 cm\({}^{-1}\) for stretching the C\({}_{4}\)=O\({}_{21}\) group, 1756 cm\({}^{-1}\) for C\({}_{11}\)=O\({}_{23}\) and 1745 cm\({}^{-1}\) for C\({}_{6}\)=O\({}_{22}\) after scaling. The difference between the calculated and experimental wave numbers can be attributed to the conjugation of C=O bonds to the phenyl ring, which is expected to decrease the wave numbers of the stretches. _C=N vibrations_  
Stretch bands C=N expected in the range of 1672-1566 cm\({}^{-1}\).