## 3 Results and Discussion
### Stability of Mono-Substituted Flavones (n-Hydroxylavones)
A distinctive feature of 3-HF is its ability to form two strong intramolecular hydrogen-bonding network connecting rings B and C. As shown in Figure 1b, the hydroxyl group at the C3 position of the \(\gamma\)-pyrone C-ring interacts with the carbonyl oxygen (O3), -C3-O2H\(\cdots\)-O3=C4, and the hydrogen atom on C6\({}^{\prime}\) of the phenyl B-ring, -C3-(H)O2\(\cdots\)HC6\({}^{\prime}\). These interactions result in the formation of pentagonal and hexagonal hydrogen-bonded \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline
**Properties** & **0-HF *** & **3-HF** & **5-HF** & **6-HF** & **7-HF** & **8-HF** & **2\({}^{\prime}\)-HF** & **3\({}^{\prime}\)-HF** & **4\({}^{\prime}\)-HF** \\ \hline \(\Delta\)E/k\(\cdot\)mol\({}^{-1}\) & - & \(-1.84\) & \(-34.97\) & \(-6.34\) & \(-13.10\) & 0 & \(-2.99\) & \(-8.12\) & \(-13.70\) \\ \hline u/D & 6.20 & 4.63 & 7.01 & 4.75 & 7.81 & 6.57 & 4.69 & 6.26 & 8.25 \\ \hline C8H\(\cdots\)O1/Å & 2.55 & 2.54 & 2.57 & 2.56 & 2.54 & - & 2.56 & 2.54 & 2.55 \\ \hline C2\({}^{\prime}\)H\(\cdots\)O1/Å & 2.40 & 2.34 & 2.40 & 2.40 & 2.40 & 2.46 & - & 2.38 & 2.37 \\ \hline H\(\cdots\)O2H/Å & - & 2.21 & 2.61 & 2.55 & 2.55 & 2.59 & 2.51 & 2.56 & 2.56 \\ \hline H\(\cdots\)O3/Å & 2.58 & 1.94 & 1.64 & 2.56 & 2.59 & 2.59 & 2.57 & 2.57 & 2.57 \\  & 2.62 & 2.65 & 2.63 & 2.62 & 2.62 & 2.62 & 2.62 & 2.62 & 2.62 \\ \hline \(\angle\)CO2H(\({}^{\circ}\)) & - & 103.0 & 105.4 & 109.8 & 110.2 & 109.5 & 109.5 & 109.8 & 109.9 \\ \hline \(\angle\)321\({}^{\prime}\)6(\({}^{\circ}\)) & 20.5 & 15.4 & 20.0 & 20.9 & 20.6 & 26.4 & 27.4 & 20.0 & 13.3 \\ \hline \(\angle\)321\({}^{\prime}\)2(\({}^{\circ}\)) & \(-159.3\) & \(-165.0\) & \(-159.8\) & \(-159.0\) & \(-159.1\) & \(-153.0\) & \(-151.3\) & \(-159.8\) & \(-166.5\) \\ \hline \end{tabular}  
* The total electronic energy of flavone (0-HF) was \(-728.03939\) E\({}_{\rm h}\). This energy for the most stable 8-HF is given by \(-803.263818\) E\({}_{\rm h}\). Note that zero-point energy (ZPE) was not considered; it was assumed to be the same amongst the isomers of n-HFs. The optimized structures in DMSO solvent for all eight flavones are given in Table S4 of the Supplementary Materials.  
\end{table} 
Table 1: Comparison of geometric properties of flavone and n-hydroxylflavones in DMSO solvent (Use color to indicate that 3-HF is special). ring structures of -C3-O2H\(\cdots\)O3=C4- and -C3-(H)O2\(\cdots\)HC6\({}^{\prime}\)-C1\({}^{\prime}\)-C2-, respectively, which stabilize the molecule. The hexagon ring in 3-HF also exhibits an additional H\(\cdots\)O bond, which stops the phenyl ring B from free rotations along the C2-C1\({}^{\prime}\) bond. The O2H\(\cdots\)O3 distance of 3-HF was very short, underscoring their strength. Specifically, the O2H\(\cdots\)O3 and H\(\cdots\)O2H distances in 3-HF were among the shortest calculated, with the bond distances being 1.95 A and 2.21 A, respectively. This dual hydrogen-bonding network not only increases the geometric rigidity of the molecule but also plays a pivotal role in defining its electronic properties. It was also noted that 5-HF exhibited a dual hydrogen-bonding network, but the O2H\(\cdots\)O3 and H\(\cdots\)O2H distances in 5-HF were 1.64 A and 2.61 A, respectively. Thus, the dual hydrogen-bonding network of 5-HF may contribute to the total energy stabilization, and 5-HF was the most stable mono-flavone derivative, as indicated in Figure 2.  
The near-planarity in 3-HF required for the hydrogen-bonding network, however, may impose geometric constraints, leading to an increased strain energy. The constrained arrangement around the hydroxyl group and the carbonyl oxygen introduces torsional and angular strain, which counterbalances the stabilizing effect of the strong hydrogen bonding. For instance, the relatively small dihedral angle, \(\angle\)321\({}^{\prime}\)6\({}^{\prime}\), in 3-HF indicates a more planar structure compared to other n-HFs (except for 4\({}^{\prime}\)-HF).