## 3 Results and Discussion  
### IR Spectroscopy  
ATR-IR spectroscopy study was performed to determine the formation of the new crystalline solid-phase solid by monitoring the shifts of the hydroxyl (O-H), and carbonyl (C=O) stretching frequencies in the CAT-TEO (Figure 2) ground solid with respect to the starting products (CAT and TEO, Figure 2). The IR spectra were acquired directly after the CAT-TEO ground process, and from the bulk CAT and TEO. TEO and CAT were ground in cycles of 3 min, until a total of 15 min of grinding time. IR spectra of CAT and TEO were assigned according to previous reports [16; 18]. In the IR spectrum of CAT-TEO obtained after 15 min of grinding (Figure 2), the O-H stretching bands (Table 1) were shifted with respect to the starting products and the physical mixture (CAT-TEO-MIX, Figure 2). This suggests the formation of a C=O\(\cdots\)H-O hydrogen bond between CAT and TEO. The free form of CAT showed two O-H stretching bands (Figure 2)--one in the 3500 to 3400 cm\({}^{-1}\) range and the other in the 3300 cm\({}^{-1}\) range of the phenolic O-H stretching frequency. As a consequence of the rearrangement in the intermolecular hydrogen bonding pattern from O-H\(\cdots\)O-H in the free form to the O-H\(\cdots\)O=C in the cocrystal, the band in the 3500 to 3400 cm\({}^{-1}\) range was shifted to lower frequencies with \(\Delta\)(\(\
u\)O-H) = \(-\)293 cm\({}^{-1}\). Meanwhile, the band in the 3300 cm\({}^{-1}\) range shifted to higher frequencies with \(\Delta\)(\(\
u\)O-H) = 105 cm\({}^{-1}\). This shift is in agreement with previous reports about molecular complexes formed by O-H\(\cdots\)O=C hydrogen bond interactions between catechol with oxal  
Figure 2: IR spectra of CAT-H (catechol ground with water), CAT (catechol), TEO (theophylline), TEO-H (theophylline ground with water), CAT-TEO-H (catechol and theophylline ground with water), CAT-TEO (catechol and theophylline ground with dichloromethane), and CAT-TEO-MIX (physical mixture of catechol and theophylline). The carbonyl C=O stretching bands of TEO (Figure 2) were shifted in the IR spectrum of the CAT-TEO ground mixture as a consequence of the formation of the cocrystal (Table 1), with absolute values of \(\Delta\)(vC=O) less than 20 cm\({}^{-1}\). This shift is in accordance with the formation of the neutral C=O\(\cdots\)H-O hydrogen bond interaction, discarding the proton transfer [16]. The solid form of TEO presents polymorphism, and each polymorph gives a different vibrational spectrum [26]. On the other hand, there are not any reports about polymorphs of CAT. To discard polymorphic transitions as a consequence of the grinding, CAT and TEO were ground individually with dichloromethane. After 15 min of grinding, the IR spectra were similar to those obtained before grinding, which discards polymorphic transformations (Figure S1 of supplementary material) as a consequence of the grinding. Possible effects of water absorption from the air during mechanochemical grinding over the starting products and the mixtures were evaluated by grinding CAT and TEO with water (CAT-H and TEO-H, Figure 2); the CAT-TEO mixture was ground with water (CAT-TEO-H, Figure 2). The IR spectrum of TEO ground with water, recorded after 15 min of grinding (TEO-H, Figure 2), was different with respect to the anhydrous form (TEO, Figure 2), showing shifts in the N-H, C=O and C-H(alkyl) stretching frequencies, and the appearance of the broad O-H band in the range of the 3350 to 3360 cm\({}^{-1}\). The spectrum (TEO-H, Figure 2) is similar to that reported for the TEO hydrate [18]. Two characteristic bands in the IR spectrum of TEO-H are the N-H stretching frequency at 3139 cm\({}^{-1}\) and the C-H alkyl stretching frequency at 3106 cm\({}^{-1}\). Contrarily to TEO, the IR frequencies of CAT ground with water (CAT-H, Figure 2) did not show any significant shift with respect to the anhydrous form. The O-H band of the water appeared to overlap with the O-H bands of CAT.