## 3 Results and Discussion
### C-NMR Chemical Shifts Indicating Intermolecular Interactions of n-Hydroxylflavones
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Label** & **Ring** & **0-HF** & **3-HF** & **5-HF** & **6-HF** & **7-HF** & **8-HF \#** & **2\({}^{\prime}\)-HF** & **3\({}^{\prime}\)-HF** & **4\({}^{\prime}\)-HF** \\ \hline
2 & C & 164.81 & 143.34 & 166.26 & 164.36 & 164.4 & 166.88 & 167.01 & 164.23 & 163.65 \\ \hline
3 & C & 106.42 & 139.41 & 104.56 & 105.44 & 106.31 & 107.53 & 108.04 & 106.69 & 103.53 \\ \hline
4 & C & 177.43 & 172.22 & 182.96 & 176.75 & 176.36 & 177.31 & 176.68 & 177.38 & 176.99 \\ \hline
5 & A & 125.58 & 125.65 & 163.57 & 106.28 & 127.93 & 114.52 & 125.84 & 125.61 & 125.4 \\ \hline
6 & A & 125.28 & 124.56 & 109.69 & 155.46 & 112.97 & 127.82 & 126.03 & 125.33 & 124.97 \\ \hline
7 & A & 134.4 & 134.49 & 136.37 & 121.18 & 162.55 & 117.66 & 134.66 & 134.47 & 134.05 \\ \hline
8 & A & 118.13 & 118.23 & 104.79 & 120.3 & 100.64 & 147.37 & 117.61 & 118.09 & 117.82 \\ \hline
9 & A/C & 156.67 & 155.28 & 157.01 & 150.37 & 158.2 & 144.17 & 155.74 & 156.49 & 156.43 \\ \hline
10 & A/C & 123.36 & 118.96 & 108.51 & 124.64 & 116.55 & 124.07 & 123.12 & 123.34 & 123.37 \\ \hline
1\({}^{\prime}\) & B & 132.1 & 131.66 & 131.39 & 132.28 & 132.17 & 132.54 & 115.61 & 110.7 & 122.17 \\ \hline
2\({}^{\prime}\) & B & 126.66 & 125.99 & 126.93 & 126.54 & 126.37 & 126.52 & 158.23 & 133.88 & 128.78 \\ \hline
3\({}^{\prime}\) & B & 129.38 & 128.94 & 129.47 & 129.34 & 129.32 & 129.44 & 117.1 & 158.56 & 113.83 \\ \hline
4\({}^{\prime}\) & B & 132.76 & 130.61 & 133.36 & 132.54 & 132.44 & 132.76 & 119.57 & 117.69 & 161.66 \\ \hline
5\({}^{\prime}\) & B & 129.27 & 128.76 & 129.35 & 129.22 & 129.23 & 129.49 & 134.99 & 131.01 & 114.71 \\ \hline
6\({}^{\prime}\) & B & 127.36 & 129.4 & 127.63 & 127.22 & 127.12 & 127.93 & 130.13 & 118.03 & 129.6 \\ \hline RMSD/ppm & 0.85 & 1.13 & 1.56 & 1.07 & 1.16 & 2.25 & 2.13 & 0.98 & 0.92 \\ \hline \hline \end{tabular}  
* \({}^{\star}\) DFT B3 PW91/cc-pTVZ model. The RMSD of the calculated C-NMR chemical shifts with respect to their experimental measurements in DMSO [6]. **f** The NMR \({}^{\star}\)measurement\({}^{\prime}\) of 8-hydroxyflavone (8-HF) was estimated [17], as 8-HF is not found in nature [17]. While the \({}^{13}\)C chemical shifts of 8-hydroxyflavone have been reported previously, the observed \({}^{13}\)C chemical shifts were not assigned [17].  
\end{table} 
Table 3: DFT-calculated \({}^{13}\)C-NMR chemical shifts of flavone and mono-hydroxyflavones in DMSO solvent (ppm) \({}^{\star}\) (color to emphases the pattern). For unsubstituted flavone, there were only three such carbons due to the absence of a hydroxyl group. Interestingly, the smallest \(\delta_{\rm C}\) was almost always associated with the C3 carbon in the n-HF isomers, except for 3-HF and 7-HF. In these two cases, the smallest \(\delta\)C was observed for C8. The unique position of C3 in the flavone structure contributes to its low chemical shift. It is the only carbon atom in the \(\gamma\)-pyrone C-ring that is not part of the phenyl ring (unlike C9 and C10) and does not directly bond with oxygen (unlike C2, C4, and C9), except in 3-HF. This structural uniqueness likely explains the consistently low \(\delta_{\rm C}\) values for C3 in most n-HF isomers. The remaining \(\delta_{\rm C}\) values corresponded to the phenyl carbons, which typically fell within the 110-140 ppm range, depending on their specific positions. Notably, 8-HF emerged as an outlier, as its experimental NMR values were based on literature estimates rather than direct measurements. This introduced additional uncertainty in the comparison and inflated the RMSD values. Half of the n-HF isomers, such as 3-HF, 5-HF, 8-HF, and 2\({}^{\prime}\)-HF, were able to form intramolecular hydrogen bonding with oxygens (O1 or O3 on the \(\gamma\)-pyrone C-ring) on adjacent carbon atoms, which was reflected by the chemical shifts of the isomers. The environment of a \({}^{13}\)C nucleus can also be affected in an inductive, through-space (field effect), or resonance-based manner by substituents that are bonded to its carbon framework (skeleton).