*3.5. Fourier Transform Infrared (FT–IR) Spectroscopy*

Due to its simplicity and reduced consumption of time and samples, FT–IR spectroscopy is a widely used technique for detecting new multicomponent materials [47]. Functional groups exhibit defined bands in the IR spectrum, and intermolecular interactions, such as hydrogen bonds, induce changes in the position of these bands. Hence, the study of the shifts can detect the formation of a cocrystal or a salt and gives information about the groups involved in the interaction [48].

Figure 6 shows the FT–IR spectra of DIC and DIC multicomponent materials. The DIC spectrum has a characteristic band at 3322 cm−1, ascribed to the stretching mode of –NH. In DIC–ADE, DIC–CYT, and DIC–ICT, this band is shifted to 3326, 3300, and 3298 cm<sup>−</sup>1, respectively. Another characteristic band of DIC is the C=O stretching vibration that appears at 1961 cm−1. This band is shifted to 1671 (DIC–ADE), 1693 (DIC– CYT), and 1678 cm−<sup>1</sup> (DIC–ICT). FT–IR data support the information observed in SCXRD, where –COOH and –NH groups from DIC and coformers, respectively, drive the formation of the crystalline structures.

**Figure 6.** Fourier transform infrared (FT–IR) spectra of DIC, DIC–ADE, DIC–CYT, and DIC–ICT.
