## 3 Results and Discussion
### Quantum Mechanical Insights of the Uniqueness of 3-Hf
The chemical shifts \(\delta_{\rm C}\) of the carbons with C-O bonding for these n-HFs are further compared in Figure S4. For the three \(\delta_{\rm C}\)s with single C-O bonding (that is, C2, C9, and Cn for n-HFs), they all follow the same trend that C9 has the smallest \(\delta_{\rm C}\) among these carbons, except for 3-HF, where C9 has the second largest \(\delta_{\rm C}\) after \(\delta_{\rm C}\) (C4=O). This is in good agreement with the measurement of [PERSON] et al. [6]. From a structural point of view, the unsubstituted 0-HF and mono-substituted 3-HF share the same molecular skeleton with the same number carbon atoms, but 3-HF has one more oxygen atom. The uniqueness of 3-hydroxyflavone (3-HF) compared to flavone (0-HF)  
Figure 4: Natural bond orbital (NBO) of 0-HF (**a**), 3-HF (**b**), and 5-HF (**c**). The carbons in the compounds that directly bond with oxygens are positively charged (in green color in the figure), such as C(4), C(9), C(2), and C(n)-OH, as the carbons are deshielded. The rest of the carbon atoms in the flavones/hydroylflavones are negatively charged (red color in the figure), and are shielded. The calculated NBO values are given in Table S3 in the Supplementary Materials. can be attributed to the significant changes in the carbon core electron density distributions (C1s) caused by the presence of the hydroxyl group at the C3 position. Table 4 compares the top six C1s orbitals of 3-HF and 0-HF (out of 15 C1s core orbitals). The DFT calculations reveal that the local energy C1s orbitals, 4a-12a for 3-HF and 3a-11a for 0-HF, exhibited minimal changes in their local density distributions. This is because these core C1s electrons are primarily dominated by the aromatic phenyl rings in the A-ring and B-ring, as shown in Figure S5. However, the top six C1s core electrons in Table 4 demonstrate noticeable reordering in 3-HF (13a-18a) compared to 0-HF (12a-17a). All these top six carbon C1s core orbitals are located in the \(\gamma\)-pyrone C-ring, plus the C1\({}^{\prime}\) in the B-ring and C2 in the \(\gamma\)-pyrone C-ring. Specifically, the hydroxyl group at C3 introduces a significant electronic perturbation, causing a pair-swapping in the order of the six top C1s electron densities (see Table 4 by matching colors). For instance, the sixth highest C1s state in 0-HF (12a on C3, blue) shifts to become the third highest C1s state (17a on C3) in 3-HF. This reordering is directly attributed to the C3-OH group, which alters the local electronic environment and increases the electron density around the C3 carbon, as indicated in their \({}^{13}\)C-NMR chemical shifts. These changes reflect the strong electronic influence of the hydroxyl group in 3-HF, distinguishing it from unsubstituted flavone and highlighting its unique electronic structure and reactivity. The mono-substitution on C3 for 3-HF also caused the C1s electron density reordering of the highest C1s orbitals on C4 and C2 in 0-HF, which was switched in 3-HF as C2 and C4 in 3-HF. It was also noted that a certain amount of electron density sharing between C4 and C3 of 3-HF was observed, and this was not seen in 0-HF under the same conditions.