## 3 Results and Discussion
### Stability of Mono-Substituted Flavones (n-Hydroxylavones)
This stability trend is consistent with earlier gas-phase calculations by [PERSON] [10] (as shown in Figure S1) and correlates well with the distribution of dipole moments, which reflects the electronic environment and intermolecular interactions of these molecules. For example, the energy differences due to hydroxyl group rotation for 2\({}^{\prime}\)-HF and 6\({}^{\prime}\)-HF are small, calculated at \(\Delta\)E \(\approx\) 3.73 kJ/mol, while for 3\({}^{\prime}\)-HF and 5\({}^{\prime}\)-HF, the difference is as low as \(\Delta\)E \(\approx\) 0.29 kJ/mol ([PERSON], HF/6-31G(d)) [10]. These small energy differences indicate the flexibility of certain hydroxyl groups in these n-HFs, suggesting that the stabilization effect of intramolecular hydrogen bonding can be a key factor in their conformational preferences. The results support the fact that most naturally occurring flavones have a hydroxyl group at the C5 position of the A-ring, while hydroxylation at other positions, such as C7 (A-ring) or C3\({}^{\prime}\) and C4\({}^{\prime}\) (B-ring), varies across different plant species [5]. The dipole moment distributions further support the stability trend. The dipole moments of n-HFs range from 4.61 Debye for 3-HF (the smallest) to 7.81 Debye for 7-HF  
Figure 2: Comparison of stability of single substituted n-hydroylflavones. The solid columns are for total energies in kJ/mol and the patterned columns are for dipole moments in Debye. (the largest) in DMSO solvent. This significant variation in dipole moments correlates with the molecular geometry and the position of the hydroxyl group, which affect the electronic distribution within the molecule. High dipole moments, such as in 7-HF, suggest stronger interactions with the solvent and other molecules, which can enhance stability in solution. Conversely, lower dipole moments, such as in 3-HF, reflect less-pronounced charge separation, potentially resulting in lower stabilization through dipole-dipole interactions. The stability of n-HFs is primarily influenced by the position of the hydroxyl group and the ability to form intramolecular hydrogen bonds. The ability of specific hydroxyl positions to enhance the stability and dipole moments suggests opportunities for tuning molecular properties through selective substitutions. To further explore the intramolecular hydrogen-bonding networking associated with these n-HFs, Table 1 provides selected geometric properties of the flavones. While all the n-HFs are non-planar, with the phenyl B-ring deviating from the plane of the benzopyrone system (A- and C-rings), 3-HF and 4\({}^{\prime}\)-HF stand out by having the smallest dihedral angles between the B-ring and the benzopyrone, at 15.4\({}^{\circ}\) and 13.3\({}^{\circ}\), respectively. More details of the geometric parameters obtained in the present study, with literature, for 0-HF and 3-HF are given in Tables S1 and S2, respectively. The results in Table 1 demonstrate that, among the mono-substituted n-HFs, 3-HF stood out due to its unique geometric and electronic properties. While other n-HFs, such as 5-HF, 8-HF, and 2\({}^{\prime}\)-HF, exhibited intramolecular hydrogen bonding, their structural arrangements do not support the formation of a two-ring intramolecular hydrogen-bonding network as observed in 3-HF (see Figure 1). For instance, 5-HF forms a single strong hydrogen bond between the hydroxyl group at the C5 position and the carbonyl oxygen, but the absence of a secondary hydrogen bond limits the molecule's planarity. Similarly, 8-HF exhibits hydrogen bonding within the benzopyrone system; however, this interaction does not extend to the B-ring, reducing its overall stabilization (making 8-HF the most unstable mono-substituted n-HF). In the case of 2\({}^{\prime}\)-HF, the hydroxyl group on the B-ring primarily interacts within the B-ring itself, with minimal interaction with the benzopyrone system, further distinguishing it from 3-HF.