## 3 Results and Discussion
### Quantum Mechanical Insights of the Uniqueness of 3-Hf  
The \({}^{13}\)C-NMR chemical shifts of mono-hydroxylavones are primarily influenced by their local electronic environments, including inductive effects, lone-pair resonance, and conjugation, which are accurately captured through DFT-based quantum mechanical calculations. This highlights the importance of computational approaches for reliably predicting \({}^{13}\)C-NMR shifts. While empirical methods, such as curve fitting or machine learning (ML), provide estimates, they lack the ability to fully capture the fundamental quantum mechanical principles that dictate these chemical environments. For example, a recently developed ML model failed to accurately predict \({}^{13}\)C-NMR chemical shifts for up to six classes of natural products, including flavonoids, lignans, sesquiterpenoids, steroids, alkaloids, and chromans [19]. These classes of natural products contain a delocalized conjugation framework. To highlight the unique electronic structure of 3-HF, Figure 4 presents the calculated electronic charge (natural bond orbital (NBO)) distributions of 3-HF compared to those of 5-HF, the most stable isomer, and 0-HF, the unsubstituted flavone. The analysis of NBO atomic charges and bonding in the flavones revealed significant variations in the electronic environments and roles of individual carbons and hydrogens. According to the NBO charge distribution, all hydrogens are positively charged (green), while carbons are generally negatively charged, except for those involved in C-O bonds, which exhibit positive charges (red). This distinction underscores the influence of oxygen's strong electronegativity on neighboring carbons. In the case of 0-HF (unsubstituted flavone), the carbons C3 and C9 within the \(\gamma\)-pyrone C-ring are negatively charged. However, C3 is more negatively charged than C9 due to its position between two highly electronegative oxygen atoms. In contrast, C9 is part of the aromatic A-ring, where its negative charge is partially delocalized into the aromatic system, reducing its overall negative character. Notably, 0-HF lacks any C(n)-OH bonds, which sets its NBO charge distribution apart from its hydroxylated derivatives. An interesting observation is that the positively charged (green-colored) carbons bonded directly to oxygen atoms do not form C-H bonds. In 0-HF, C3 is the only carbon within the \(\gamma\)-pyrone C-ring that forms a C-H bond, further highlighting its unique electronic environment within the \(\gamma\)-pyrone C-ring of unsubstituted flavone. In 3-HF and 5-HF, hydroxyl substitution introduces a fourth positively charged carbon, corresponding to the hydroxylated carbon (C3 in 3-HF and C5 in 5-HF, respectively). The substitution significantly alters the local electronic environment, highlighting the differences in the charge distribution compared to unsubstituted flavone. In 3-HF, the hydroxyl group at C3 enables the formation of a robust intramolecular hydrogen-bonding network. This group participates in multiple hydrogen bonds, including H\(\cdots\)O(3)=C and C(6\({}^{\prime}\))-H\(\cdots\)O(2), forming a pentagonal ring, and C(2\({}^{\prime}\))-H\(\cdots\)O(1), constituting a hexagonal ring. These hydrogen-bonding interactions contribute to the geometric rigidity of 3-HF, although they also introduce additional electronic strain, as discussed earlier. The distinct charge distributions and bonding environments in flavones are primarily determined by the presence and positioning of hydroxyl groups and the resulting C-O bonds. Hydroxyl substitution not only influences the local electronic properties of flavones, but also enhances the negative charge on specific carbons. Furthermore, it enables hydrogen bonding, which plays a crucial role in determining their structural stability and energetic properties. Such quantum mechanical effects of flavones are well predicted using pattern-recognition methods such as machine learning [19].