*3.4. Fourier Transform Infrared (FT-IR) Spectroscopy*

FT–IR is a helpful technique that quickly detects the formation of novel multi-component pharmaceutical solid forms [43]. Changes in vibrational frequencies due to cocrystal/salt formation can be easily monitored. When the two APIs are joined together in the solid form, the reported IR bands with diagnostic values are expected to be shifted, thus indicating the presence of intermolecular forces between functional groups—i.e., hydrogen bonds—which build the cocrystal structures [44]. Band assignments (Table 3) were performed based on the crystallographic analysis (Section 3.1) and considering the spectroscopic data available for related FUR compounds found in the literature [13].

**Table 3.** Summary of relevant FT−IR vibrational frequencies (cm<sup>−</sup>1) in the spectra of FUR, FUR–ETZ, and FUR–PRX.


FUR exhibits stretching frequencies at 3400 and 3351 cm−<sup>1</sup> (sulfonamide primary amine), 3285 cm−<sup>1</sup> (sulfonamide secondary amine), 1670 cm−<sup>1</sup> (carboxyl stretch), and 1328 and 1139 cm−<sup>1</sup> (sulfonamide S=O stretching modes). The FT–IR spectra of FUR and the multi-component forms are shown in Figure 5. In FUR–ETZ and FUR–PRX cocrystals the band corresponding to carboxyl group (1670 cm<sup>−</sup>1) appears in the same position as in FUR. In the cocrystals, the –NH2 asymmetric and symmetric stretching modes are shifted (3438 and 3291 cm−<sup>1</sup> for FUR–ETZ and 3317 and 3230 cm−<sup>1</sup> for FUR–PRX). S=O stretching modes are shifted to 1345 and 1143 cm−<sup>1</sup> in FUR–ETZ, 1339 and 1154 cm−<sup>1</sup> in the case of FUR–PRX, confirming that these functional groups interact with the coformer, as demonstrated in the crystal structures analysis. The FT–IR vibrational frequency comparisons are summarized in Table 3.

**Figure 5.** Comparison of Fourier transform infrared (FT−IR) spectra of FUR, FUR–ETZ and FUR–PRX solid forms.
