## 3 Results and Discussion
### Theoretical Calculations
For compound **4c**, the stretching mode C=N is assigned to 1496 cm\({}^{-1}\) and 1450 cm\({}^{-1}\) in FT-IR spectrum and theoretically at 1495 (1436) and 1467 (1409) cm\({}^{-1}\). _C-N vibrations_  
The C-N stretching vibrations are moderately to strongly in the 1275 \(\pm\) 55 cm\({}^{-1}\) region. According to reports in the literature, assign stretches of C-N are reported at 1184, 1367, and 1373 cm\({}^{-1}\) for different compounds [52; 53; 54]. For compound **4c** the bands at 1390 and 1332 cm\({}^{-1}\) are assigned to the C-N stretching modes and theoretically (DFT) at 1126 (1082), 1185 (1138) and 1350 (1297) cm\({}^{-1}\). Two factors may be responsible for the discrepancies between the experimental and computed wavenumbers of the compound. The first is caused by the environment (gas and solid phase) and the second is because the experimental values are inharmonic wavenumbers while the calculated values are harmonic ones. Therefore, the calculated values are very close to experimental measures. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{\(\mathbf{v}\)} & \multirow{2}{*}{**Experimental**} & \multicolumn{3}{c}{**Calculate**} & \multicolumn{1}{c}{**Assignment**} \\ \cline{3-5}  & & **Un-Scaled** & **Scaled** & \\ \hline
**1** & 3272 & 3459 & 3334 & N\({}_{12}\)-H stretching \\  & & 3450 & 3325 & N\({}_{3}\)-H stretching \\  & & 3141 & 3027 & O-CH\({}_{3}\) bending symmetric \\  & & 3127 & 3014 & O-CH\({}_{3}\) stretching asymmetric \\
**2** & 2841 & 3108 & 2996 & S-CH\({}_{3}\) stretching asymmetric \\  & & 3039 & 2929 & O-CH\({}_{3}\) stretching symmetric \\  & & 3018 & 2909 & S-CH\({}_{3}\) stretching symmetric \\
**3** & 1674 & 1851 & 1784 & C\({}_{4}\)=O\({}_{21}\) stretching \\
**4** & 1650 & 1822 & 1756 & C\({}_{11}\)=O\({}_{23}\) stretching \\
**5** & 1620 & 1811 & 1745 & C\({}_{6}\)=O\({}_{22}\) stretching \\
**6** & 1496 & 1495 & 1436 & C=N stretching \\
**7** & 1450 & 1467 & 1409 & C=N stretching \\
**8** & 1390 & 1350 & 1297 & C-N stretching \\
**9** & 1332 & 1185 & 1138 & C-N stretching \\
**10** & 1247 & 1126 & 1082 & C-N stretching \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Fourier Transform infrared spectroscopy (FT-IR) Experimental and computed vibrational bands for compound **4c** and their assignments at B3 LYP/6-31G(d,p) level. Natural bond orbital (NBO) analysis allows a detailed description of the electronic structure of compound **4c**, in terms of occupancy and composition of the group of NBOs Lewis (see Table 5) and not Lewis (see Table 6). 224 NBOs were calculated for this compound, representative NBOs are highlighted by finding three carbonyl-like bonds (C4=O21, C6=O22, C11=O23), a non-aromatic double bond (C15=C18), four heteroatom bonds (C2-S19, C27-O44, N12-H31 and N3-H30) and solitary pairs of N and O (N1, N3, N12, S19, O21, O22, O23 and O44) of the expected Lewis structure. For each of the molecular orbitals, the corresponding percentage of \(s\), \(p\), \(d\) character is included. In Lewis type orbitals, as seen in Table 5, the \(\sigma\) bond (C\({}_{4}\)-O\({}_{21}\)) is formed from the \(sp^{2.01}\) hybrid on carbon, mixture of \(s\) (33.23%) \(p\) (66.68%) \(d\) (0.09%); on the other hand, the \(\pi\) bond (C\({}_{4}\)-O\({}_{21}\)) is formed from the \(sp^{1.00}\) hybrid on carbon and oxygen, mixture of \(s\) (0.01%) \(p\) (99.81%) \(d\) (0.18%); the electronic densities or maximum occupations calculated for \(\sigma\) (C\({}_{4}\)-O\({}_{21}\)), \(\pi\) (C\({}_{4}\)-C\({}_{21}\)), \(\sigma\) (C\({}_{6}\)-C\({}_{22}\)) and \(\pi\) (C\({}_{6}\)-N\({}_{22}\)) are 1.99441, 1.98431, 1.99483 and 1.94776, respectively. Therefore, these results allow to infer that these bonds are controlled by the character \(p\) of the hybrid orbitals.