## 3 Results and Discussion
### Theoretical Calculations  
To validate the information obtained from the \({}^{13}\)C CP/MAS NMR study (8 exp), the gauge including the atomic orbital (GIAO) method at a B3 LYP/6-31G(d,p) level of theory was used to calculate the isotropic magnetic shielding (\(\alpha\)iso) and predict the \({}^{13}\)C NMR chemical shifts (\(\delta\)calc) of the CAT-TEO\({}_{1}\) and CAT-TEO\({}_{2}\) models (Table 3, Figure 5). The calculated chemical shifts are in agreement with the experimental results because the correlation coefficient R\({}^{2}\) (Figure S3 supplementary material) is 0.997 and 0.996, respectively. Density functional theory methods (DFT) allow the calculation of molecular properties of the molecular complexes. One of these properties is the optimization of the geometry in the gas phase, which gives an idea about the molecular structure of a complex [30; 31]. In the same way, natural bond orbital (NBO) analysis gives information about the electron density distribution involved in the formation of hydrogen bonds among the molecules of the complexes [32; 33]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **C1** & **C2** & **C3** & **C4** & **C5** & **C6** & **C7** \\ \hline TEO & 154.9 & 150.9 & 146.3 & 140.5 & 105.8 & 30.0 & 30.0 \\ CAT-TEO & 154.5 & 154.5 & 144.2 & 140.2 & 108.3 & 31.9 & 27.6 \\ \hline  & **C11** & **C12** & **C13** & **C14** & **C15** & **C16** \\ CAT & 142.9 & 142.9 & 114.6 & 121.5 & 121.5 & 114.6 \\ CAT-TEO & 144.7 & 144.7 & 114.7 & 119.8 & 119.8 & 114.7 \\  & & & & 112.4 & & 112.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: CP/MAS solid state \({}^{13}\)C NMR chemical shifts of CAT, TEO and the CAT-TEO cocrystal. Figure 4: Solid-state \({}^{13}\)C CP/MAS NMR spectra (\(\delta\) = ppm) of the CAT-TEO cocrystal and the starting products catechol (CAT) and theophylline (TEO). In order to obtain structural information about the intermolecular hydrogen bond patterns involved in the formation of the CAT-TEO corystral, DFT calculations at B3 LYP/6-31G(d,p) level of theory were performed in CAT-TEO\({}_{1}\) and CAT-TEO\({}_{2}\) models (Figure 5) considering the two possible C=O\(\cdots\)H-O hydrogen bond patterns in the CAT-TEO corystral. The CAT-TEO\({}_{2}\) model is the most stable structure (by only 2.404 kcal/mol) involving the C=O(2)\(\cdots\)H-O(11) and the N-H\(\cdots\)O(11) hydrogen bond interactions (Figure 5); meanwhile, the CAT-TEO\({}_{1}\) model is in accordance with the \({}^{13}\)C NMR shifts because the carbonyl C2 signal showed a A\(\delta\)\({}^{13}\)C = 3.6 ppm in the CAT-TEO corystral with respect to the free TEO. This deshielding suggests the formation of the C=O\(\cdots\)H-O hydrogen bonding in this position, contrary to the carbonyl C1, which showed a \(\Delta\delta\)\({}^{13}\)C = \(-\)0.4 ppm. This is also in accordance with previous reports about synthon hierarchies in corystals of TEO with hydroxybenzoic acids with respect to the hydrogen bonds involving phenolic O-H groups, in which the O-H\(\cdots\)O=C(urea) interaction is preferred over the O-H\(\cdots\)O=C(amide) interaction because the O=C(amide) is involved in the dimerization of TEO by the N-H\(\cdots\)O=C(amide) hydrogen bond synthon [5].