3.4.2. FT-IR Spectrum

Considered as a reasonable and reliable technique to detect the formation of the multicomponent crystal, Fourier transform infrared (FT-IR) spectroscopy is a very important tool to determine typical carboxylate anion and confirm the proton transfer when carboxylic acid is used as a coformer.

FT-IR spectra were obtained for commercial DPA, prepared DPA crystal, PA and DPA\_PA salt (Figure S2). FT-IR spectrum of commercial DPA and prepared DPA crystal showed same characteristic peaks, which were observed as 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> , whereas in DPA\_PA amide N-H stretching was observed at 3354.57 cm−<sup>1</sup> , amide C=O stretch and NH<sup>2</sup> deformation at 1680, 1625 cm−<sup>1</sup> , respectively [58]. The blue shift of amide-derived peaks suggests the possible changes in the hydrogen bonding interaction between molecules due to the formation of a new solid form [59,60]. In general, for tertiary amine salts, a broad band at 2300–2700 cm−<sup>1</sup> was due to the NH<sup>+</sup> stretching, which was also observed in DPA-PA salt, revealing the protonation of tertiary amine in DPA [60,61]. In addition, the peaks at 1590 and 1568 cm−<sup>1</sup> in DPA were probably attributed to benzene and pyridine rings. It was observed that similar peaks appeared with minor differences in DPA\_PA salt. Hence, it appears there is no chemical interaction on benzene and pyridine rings.

Due to the hydrogen bond of the PA dimer, pure PA existed over a broad band around 2800 cm−<sup>1</sup> , attributed to the OH group, which shifts to 3191 cm−<sup>1</sup> in DPA\_PA salt. The carbonyl stretches of two carboxylic acid groups of PA were observed at 1666 and 1583 cm−<sup>1</sup> , which appeared at 1679 cm−<sup>1</sup> for the COOH group and 1556 cm−<sup>1</sup> for the COO¯ group in DPA-PA salt [62].

In short, the appearance of a broad band at 2300–2700 cm−<sup>1</sup> and a peak at 1556 cm−<sup>1</sup> , where the ionized tertiary amine and carboxylate group could be observed, respectively, indicate a proton transfer from the salt former PA to DPA, confirming the salt formation between DPA and PA.
