3.2.2. FT-IR Spectrum

FT-IR is a very important and useful techniques for detecting the formation of salt by the typical carboxylate anion present in spectra (Figure 8). Due to changes in the hydrogen bonding patterns of a molecule in salt or a co-crystal, there are resulting changes in the IR frequencies of vibrations associated with the functional groups. The changes in IR frequencies suggest changes in hydrogen bonding pattern. Examination of FT-IR spectra could confirm salt formation due to the transfer of acidic hydrogen TA to DPA.

FT-IR spectra were obtained for commercially pure DPA, prepared DPA:TA crystal, pure TA, and DPA:TA salt obtained from the slurry method. Commercially pure DPA demonstrated characteristic peaks, amide N-H stretching at 3263 cm−<sup>1</sup> , amide C=O stretching, and NH<sup>2</sup> deformation overlap peak at 1664 cm−<sup>1</sup> , and in TA spectra presently peak at 1671 due to C=O stretching and broad band around 2800 cm−<sup>1</sup> , attributed to the carboxylic OH group [22]. Whereas the spectra of salt give many characteristic peaks as 3350, 3149,

1679, and 1589 cm−<sup>1</sup> that are different from the starting component DPA and TA shown in Figure 8.

*Crystals* **2021**, *11*, x FOR PEER REVIEW 2 of 3

(**b**)

**Figure 8.** FT-IR spectra, (a) 4000-400 cm-1 and (b) 1800-1200 cm-1, of commercial DPA, TA, DPA:TA salt obtained from slurry method and DPA:TA salt obtained slow evaporation method. **Commented [M2]:** Even though the figure 8 (a) and (b) arranged like this in our proof version, but, Fig 8 (a) and (b) is upside down, please **Figure 8.** FT-IR spectra, (**a**) 4000–400 cm−<sup>1</sup> and (**b**) 1800–1200 cm−<sup>1</sup> , of commercial DPA, TA, and DPA:TA salt obtained from the slurry method and DPA:TA salt obtained from the slow evaporation method.

correct in webpage. Thus, in DPA:TA salt, the spectrum peaks at 1589 cm−<sup>1</sup> ; where the carboxylate group is observed respectively and is not present in the spectrum of the individual components, which indicates a proton transfer from the salt former TA to DPA, confirming salt formation between DPA and TA.
