## 1 Introduction  
Theophylline is a methyl-xanthine drug that occurs naturally in products such as coffee, tea and cocoa nuts, and is used for the treatment of asthma and chronic obstructive pulmonary disease [1]. Molecular recognition of theophylline has been exploited in solution and a solid state. In solution, it has led to the development of molecular receptors [2, 3]; meanwhile, in solid state, it has been applied in the design and synthesis of pharmaceutical cocrystals. Pharmaceutical cocrystals are multi-component crystalline solids formed by a pharmaceutical active ingredient (API) and a cocrystal former, stabilized by noncovalent interactions, principally hydrogen bonds [4]. Theophylline is a model molecule [5] for cocrystallization studies because bears in their chemical structure carbonyl groups are able to act as hydrogen bond acceptors (theophylline can also act as a donor because it has a N-H group), allowing the formation of cocrystals with different cocrystal coformers, such as: organic acids, diclofenac, nicotinamide, acesulfame, to mention some [6; 7; 8; 9]. On the other hand, catechol or 1,2-dihydroxybenzene contain hydroxyl groups, which can act as hydrogen bond donors for the synthesis of cocrystals due to their ability to form O-H-\(\cdots\)X (X = O, N) hydrogen bonds, with the advantage that they form neutral hydrogen bonds, discarding the formation of salts. Some examples of API's that have formed cocrystals with catechol are nalidix acid, tenoxicam, brexpiprazole, and isoniaid [10; 11; 12; 13]. From the reported cocrystallization methods, the most used are the solvent evaporation method and the solvent-assisted grinding method [14]. In the solvent evaporation method, the API and the cocrystal former are dissolved in a suitable solvent, which is left to evaporate to obtain the cocrystal. This method is used to obtain suitable monocrystals for single crystal X-ray diffraction; however, it depends on the solubility of the compounds, it is necessary to wait a few days for the evaporation of the solvent, and the presence of the solvent can lead to the formation of solutes. On the other hand, in the solvent-assisted method, the API and the cocrystal former are ground together with some solvent drops. The mechanochemical energy favors the rearrangement of the molecules in solid state to form the cocrystal. This method offers the advantage of reproducibility, rapid screening and independence of the solubility of the starting products [14]. Solid-state monitoring techniques such as powder and single X-ray diffraction, solid-state nuclear magnetic resonance (NMR) and infrared spectroscopy (IR) are employed in the characterization of pharmaceutical cocrystals [15]. IR spectroscopy has been used to determine the formation of cocrystals, polymorphs, salts and solvates [16; 17; 18]. Attenuated total reflection (ATR) IR spectroscopy offers the advantage of rapid determination (the sample does not need previous preparation for the analysis) of the formation of the new solid between the API and the cocrystal former by the shift of the IR absorption bands of the functional groups in the cocrystal with respect to the starting components [19; 20]. Solid state nuclear magnetic resonance is used for the characterization of pharmaceutical cocrystals and polymorphs because it gives structural and crystallographic information and is sensitive to hydrogen bonding [21; 22]. In order to exploit the ability of the dihydroxybenzenes to form O-H-\(\cdots\)O-C hydrogen bond interactions with theophylline, here we report the mechanochemical synthesis of the catechol-theophylline cocrystal (CAT-TEO) (Figure 1) obtained by the solvent-assisted grinding method. The ATR-IR spectroscopy has played an important role because it allowed to rapidly determine the formation of the powder cocrystal, discarding the hydrate formation. The characterization of the CAT-TEO cocrystal was completed using solid-state \({}^{13}\)C NMR, powder X-ray diffraction and DSC calorimetry.