## 3 Results and Discussion
### DFT Study
#### 3.5.5 Antioxidant Mechanisms of the Target Molecules **1**-**14**  
It is well established that the HAT mechanism is determined by the BDE values, which indicate the capacity of an XH (X=C, O, or N) moiety to donate its hydrogen atom and so create a radical. Increasing antioxidant potency is associated with decreased BDE of the relevant X-H bond [50]. Thus, in this part, the BDE values of the weakest X-H bonds of each molecule were calculated and presented in Table 5.  
Obviously, based on the data presented in Table 5, BDE (X-H)s where X denotes the elements C, O, or N normally ranged between 75.61 and 95.81 kcal.mol\({}^{-1}\). The phenolic methoxy group in **5**, **9**, and **11** in the _ortho_ positions may have played a key role in the antioxidant activity. Therefore, the O-H (phenolic) bonds have the lowest BDE values, ranging from 75.61 to 80.319 kcal/mol, indicating their low stability. Compounds **4** and **10** are characterized by the phenolic alkyl group, which played as an activator for the bond dissolution process and enhanced positively in the antioxidant properties. This is in agreement with the in vitro study of antioxidants. In contrast, the presence of nitro and bromo substituents in **6**, **12**, and **7** influenced negatively on the activity (acting as a deactivator for bond dissolution) resulting in decreasing the antioxidant activity. Thus, the O-H (phenolic) bonds with higher BDE are considered more stable. Based on the gas phase BDE values, the H-donation abilities of the targets were ordered as **14_RH** > 3 > 2 > **12**\(\approx\)**14_ArH** > **10**\(>\)**1**\(\approx\)**6**\(>\)**13**\(\approx\)**8**\(\approx\)**4**\(\approx\)**7**\(>\)**5**\(>\)**11**\(>\)**9**. The solvent effect tests were performed on these compounds using methanol and water. Compounds **5**, **9**, and **11** have the lowest BDE values for O-H at 80.322, 75.61 and 77.49 kcal.mol\({}^{-1}\), respectively (Table 5). Solvation effect assessments were performed on these compounds  
Figure 5: Molecular electrostatic potential of **1**–**14**. using methanol and water. Shifting BDE values obviously demonstrated that the polarity of the solvents affects the hydrogen-donating ability. Thus, methanol and water solvents were utilized in this investigation as the environment in which BDEs were calculated. The antioxidant activity and radical trapping abilities were performed in methanol [51; 52]. The obtained findings of **5**, **9**, and **11** showed a decrease in BDEs (X-H, X = O, N) values in the solvents in the range of 4.2 to 11.4 kcal.mol\({}^{-1}\) when the compounds are exposed to methanol solvent. The O-H bonds are still the easiest dissociation links in the selected **5**, **9**, and **11** (Table 6). As stated in the SETPT mechanism, the ionization of an antioxidant molecule is the initial step, and hence the term \"AIP\" is used to determine the antioxidant's ability (acting as an electron donor). The higher the antioxidant activity, the easier it is to move electrons, and the lower the AIP value (Table 5). The sequences of vertical AIP values in gas phase were ordered as \(\textbf{9}\approx\textbf{4}\approx\textbf{5}\approx\textbf{11}<\textbf{8}< \textbf{7}<\textbf{2}<\textbf{1}<\textbf{12}\approx\textbf{13}<\textbf{6}< \textbf{14}\_\textbf{ArH}=\textbf{14}\_\textbf{RH}<\textbf{3}<\textbf{10}\) (Table 5). The loss of a proton from the cation radical created in the first stage is the final step in the SETPT mechanism. The PDE parameter was calculated to determine the thermodynamically favorable X-H (X=O, and N) group for deprotonation. Table 5 presented the calculated results. The easiest deprotonation of the X-H (X=O, and N) bond is usually found in compounds having the lowest PDE values. In the investigated compounds, the lowest PDE values are found in **10**, **3**, **13**, and **6** (203.74, 214.244, 225.74, and 226.378 kcal.mol\({}^{-1}\), respectively). Moreover, the antioxidant potential is determined by the sum of PDE and IP [53].