**6.5 Anti-oxidant capacity**

Numerical parameters corresponding to all the possible anti-oxidant mechanism of coumestrol and selected coumestrol derivatives in gas phase are shown in **Table 4**.

Logically speaking, free energy (ΔG) decides the thermodynamically preferred mechanism. The calculated free-energy equation is ΔG = ΔH TΔS. In accordance with this equation ΔG is defined by ΔH and ΔS. However the absolute values of the entropic term, TΔS, reach only a few units or tens of kJ/mol, in the case of studied reactions. Free energies are thus predominantly influenced by the enthalpy term ΔH. The mechanisms of HAT, SET-PT and SPLET are primarily regulated by BDEs, IPs and PAs, respectively, and the BDEs, IPs and PAs can therefore specify the thermodynamically preferred reaction pathway involved in the free radical

**Figure 5.** *FEDAM of Coumestrol substituted at C-16th position in gas phase.*

*Theoretical Studies on Anti-Oxidant Activity of the Phytochemical, Coumestrol and Its… DOI: http://dx.doi.org/10.5772/intechopen.96967*


#### **Table 4.**

*Numerical parameters corresponding to all the possible antioxidant mechanism of coumestrol and selected coumestrol derivatives in gas phase.*

scavenging method. From the table, the measured IPs and PAs of coumestrol and its derivatives in the gas phase have been found to be substantially higher than BDEs and thus, from a thermodynamic point of view, HAT is the most desirable method in the gas phase.

BDE is the numerical parameter that characterises the stability of hydroxyl group and it is related to HAT mechanism. The lower BDE value indicate the lower the stability of the O-H bond, and high antioxidant capacity. Coumestrol contain two different hydroxyl groups which can transfer hydrogen to the free radical present in biological systems. From the table it is observed that, the derivatives substituted with electron donating groups like 16-NH2, 16-OCH3 and 16-CH3 showed good anti-oxidant capacity. In gas phase, the antioxidant power all the selected coumestrol derivatives were higher than that of parent molecule.

For coumestrol, the BDE value observed at 18-OH was less than that of 20-OH which means that 18-OH forms most stable radical. The more stable radical can imply the stronger antioxidant abilities of the compound. Among the three selected coumestrol derivatives, 16-NH2 showed lowest BDE value implies its higher antioxidant potential. In the case of 16-NH2 Coumestrol, the bond 20-OH showed lowest BDE value compared to that of 18-OH. It may be due to the presence of intra molecular hydrogen bonding between -NH2 with nearby oxygen radical. 16-OCH3 coumestrolshowed low BDE value than Coumestrol but higher than 16-NH2 coumestrol because there is no hydrogen bonding interaction possible between oxygen radical and -OCH3 group near to it. 16-CH3Coumestrol, also showed a lower BDE value than parent molecule. -CH3 group being a weakly electron donating one, only a slight difference in BDE value was observed at 20-OH and 18-OH.

#### **6.6 Frontier molecular orbital analysis**

Energy and distribution of frontier orbitals are also significant parameters that correlate with the antioxidant activity of the polyphenols. The calculated frontier orbital distributions and energies in the gas phase for Coumestrol and its derivatives like 16-NH2Coumestrol, 16-OCH3Coumestrol, and 16-CH3Coumestrol are present in **Figure 6**.

The molecule's electron donation potential is linked to the energies of HOMO. Higher HOMO orbital energy molecules have a greater capacity to donate electrons [16, 17]. It can be observed from **Figure 6** that 16-CH3 coumestrol provided the highest HOMO energy (5.797eV), followed by 16-NH2 coumestrol (5.815eV), 16-OCH3 coumestrol (5.833 eV), and coumestrol (5.855eV). This demonstrates
