## 3 Results and Discussion
### C-NMR Chemical Shifts Indicating Intermolecular Interactions of n-Hydroxylflavones
These effects, which became apparent in the \({}^{13}\)C substituent chemical shift analyses, caused the associated chemical shifts to vary in a systematic and predictable manner. Figure 3 plots the \({}^{13}\)C-NMR spectra of these n-HFs with respect to 0-HF. The full width at half maximum (FWHF) of the spectra was given by 0.20 ppm. The \({}^{13}\)C-NMR spectra of all the n-HF isomers are given in Figure S3 in the Supplementary Materials. The carbons with the largest chemical shifts across all flavones were typically those forming C=O or C-O bonds, including C(4), C(2), and C(9), as well as the hydroxyl-substituted C(n) carbons. This indicates that chemical shifts are more influenced by the carbon-oxygen bonding environment than by their positions within the A-, B-, or C-rings. For the unsubstituted flavone (0-HF), only three chemical shifts occurred above 135 ppm, corresponding to its three C-O bonds. However, all the n-HF derivatives showed four  
Figure 3: \({}^{13}\)C-NMR spectra of the n-HFs that exhibited intramolecular hydrogen bonding and formed pentagon and/or hexagon rings, together with the unsubstituted flavone. chemical shifts above 135 ppm, which is attributable to the presence of an additional C-OH bond introduced by hydroxyl substitution. This further emphasizes the strong impact of oxygen bonding on the chemical shifts of carbons in flavones. As expected, all the flavones showed the C(4)=O carbonyl group as the one with the highest chemical shift, which aligns with its highly deshielded nature due to strong electron-withdrawing effects. The C-NMR chemical shifts (\(\delta_{\rm C}\)) of C4 in the most stable isomer, 5-HF, exhibited the largest \(\delta_{\rm C}\) for its carbonyl carbon C(4), consistent with its strong electron-withdrawing effects and stability and the hydrogen-bonding interaction of C5-O-H-\(\cdots\)O3=C4. The hydroxyl-substituted C-OH carbons exhibited \(\delta_{\rm C}\) values that were intermediate, but closer to the higher end of the chemical shift range, reflecting their partially deshielded environment. These carbons consistently had \(\delta_{\rm C}\) values above 135 ppm, which aligns with their attachment to electron-withdrawing oxygen atoms. The specific \(\delta_{\rm C}\) ordering for various carbons provides insight into the electronic environments. For unsubstituted flavone, 0-HF, the chemical shift order is as follows:  
\[\delta_{\rm C}\ ({\rm C(4)=O3})>\delta_{\rm C}\ ({\rm C(2)-O1})>\delta_{\rm C}\ ({\rm C(9)-O1})\]  
For most n-HF derivatives, the order is slightly modified to the following:  
\[\delta_{\rm C}\ ({\rm C(4)=O3})>\delta_{\rm C}\ ({\rm C(2)-O1})>\delta_{\rm C}\ ({\rm C(n)-O2})>\delta_{\rm C}\ ({\rm C(9)-O1})\]  
where n = 5, 8, ad 2\({}^{\prime}\). However, in 3-HF, the order is unique:  
\[\delta_{\rm C}\ ({\rm C(4)=O3})>\delta_{\rm C}\ ({\rm C(9)-O1})>\delta_{\rm C}\ ({\rm C(2)-O1})>\delta_{\rm C}\ ({\rm C(3)-O2})\]  
This unique \(\delta_{\rm C}\) ordering in 3-HF further highlights the distinct electronic effects of its extended hydrogen-bonding network and near-planar geometry. The deshielding effect on C(9) surpasses that on C(2) in 3-HF, reflecting the strong influence of hydrogen bonding and electronic delocalization in this isomer.