*3.7. FT–IR Spectroscopy*

The IR spectra of cocrystals **1** and **3** are depicted in Figures S13 and S14, respectively. A comparison with the spectra of its coformers is represented in Figure 14. Regions 3500–3000 and 1700–600 cm−<sup>1</sup> are important sources of information about the molecular interactions, because these two regions are associated with hydrogen bonding interaction. In solid DL-mandelic acid, these bands correspond to 3397 cm<sup>−</sup>1, ν(OH), 1716 cm<sup>−</sup>1, ν(C=O) and 1078–1064 cm−1, ν(C-O) [44], while that in pyridine carboxamides appear at 3414 and 3164 cm<sup>−</sup>1, ν(NH2), 1658 cm−1, ν(C=O), 1603 cm−1, δ(NH2), 1386 cm−1, ν(CN) and 640–629 cm<sup>−</sup>1, α(CCC), in picolinamide; and 3362 and 3178 cm−1, ν(NH2), 1655 cm<sup>−</sup>1, ν(C=O), 1622 cm<sup>−</sup>1, δ(NH2), 1390 cm−1, ν(CN) and 668–614 cm−11, α(CCC), in isonicotinamide [45]. Consequently, in cocrystals, the absorb peaks around 3400 and 3200 cm−<sup>1</sup> attribute to stretching vibrations of OH and NH2 groups of the acid and of pyridine carboxamides, where the wavenumbers for the asymmetric stretching vibration of NH2 in cocrystal **1** are about 3435 cm<sup>−</sup>1, while cocrystal **3** shows absorb peaks at about 3379 cm−<sup>1</sup> due to stronger hydrogen bond interactions than in **1**. In the ν(NH2) symmetric stretching vibrations, the same behavior is observed for **3** respective to **1**. For the two cocrystals, the wavenumbers at around 3450 cm−<sup>1</sup> indicate the O−H···O hydrogen bond interactions between them, while the slight differences of the wavenumbers for NH2 groups may attribute to the different hydrogen bond interaction experienced by it. While for the region around 1700–600 cm−1, which are corresponding to C=O group, the almost identical plots for the two cocrystals, at 1700–1600 cm−1, indicate similar hydrogen bond interactions around the C=O groups for **pic**, and **inam** in cocrystals **1** and **3**, respectively, but in **3** the band of mean intensity at 1729 cm−<sup>1</sup> corresponds to the C=O of **H2ma** that does not take part in hydrogen bonding, which are in accordance with the structural analysis. We also point out the presence of two new bands at 2506 and 1913 cm−<sup>1</sup> for **1** and 2465 and 1891 cm−<sup>1</sup> for **3** that result from the O−H···Npy hydrogen bond. This provides clear proof that the hydroxyl or carboxylic group of **H2ma** interacts with the aromatic nitrogen of pyridine carboxamides [46]. A similar behavior is deduced when observing the position of in-plane and out-of-plane ring deformation bands of pyridinecarboxyamides in the cocrystals, which reflect a greater strength of the O−H···N interaction in **3** than in **1** (Table S1), since while in **1** it is O−H(hydroxyl)···N(py) with a distance O···N of 3.071 Å, in **3** is 2.624 Å for that distance in O−H(carboxylic)···N(py).

**Figure 14.** IR spectra of the cocrystals **1** (**left**) and **3** (**right**).
