3.5.6. Gibbs Free Energy Change (ΔG◦)

As listed in Table 1, the higher negative values suggest that the CT complexes formed between PA and the acceptors are exergonic. Generally, the values of ΔG◦ become more negative as the value of KCT increases, where the CT interaction between the donor and acceptors becomes strong. Thus, the complex composition is subject to a lower degree of freedom, and the values of ΔG◦ become higher negative values. The negative value of ΔG◦ pointed out that the interaction between the donor (PA) and the acceptors (ChA and DDQ) is spontaneous. Their values are −18 and −11 kJmol−<sup>1</sup> for PA-ChA and PA-DDQ, respectively.

*3.6. Spectroscopy* 3.6.1. Infrared (IR) Spectra IR Spectra of PA-ChA Complex

From the comparison of the FTIR spectra of PA, ChA, and the PA-ChA complex (Figure 5), a characteristic C-Cl band at 571 cm−<sup>1</sup> for ChA and 572 cm−<sup>1</sup> for PA-ChA was observed, which confirms the complex formation. Furthermore, a red shift of 1664 cm−<sup>1</sup> for ChA was observed at 1530 cm−<sup>1</sup> for PA-ChA [24]. Other important peaks are summarized in Table 2 [25,26]. It should be noted that the vibrational bands for O-H, C-H, aromatic C=O, and C-O for ChA to PA-ChA have been shifted from 3560 to 3523, 3235 to 3151, 1664 to 1637, and 1207 to 1173, respectively.



IR Spectra of PA-DDQ Complex

As shown in Figure 6, from the comparison of the IR spectra of PA, DDQ, and the PA-DDQ complex, a characteristic C≡N band at 2233 cm−<sup>1</sup> for DDQ and 2216 cm−<sup>1</sup> for the PA-DDQ complex was observed, which confirms the complex formation [25,26]. Similarly, a red shift of 1674 cm−<sup>1</sup> for DDQ was observed at 1481 cm−<sup>1</sup> for DDQ-ChA [24,27]. Other important peaks are summarized in Table 3.

**Figure 5.** Comparison of IR spectra of procainamide (PA), chloranilic acid (ChA), and PA-ChA complex: (**A**) whole spectra; (**B**) showing the characteristic C-Cl band of ChA and PA-ChA complex.

**Figure 6.** Comparison of IR spectra of PA, DDQ, and PA-DDQ complex: (**A**) whole spectra; (**B**) showing the characteristic -C≡N band of DDQ and PA-DDQ complex.


**Table 3.** IR spectral bands of PA, DDQ, and their complex (PA-DDQ).

#### 3.6.2. NMR Spectra

NMR Spectra of PA-ChA Complex

From the comparison of the proton NMR (1H-NMR) spectra of procainamide (PA) and the PA-Chloranilic acid (PA-ChA) complex (Figure 7), the formation of the PA-ChA complex is confirmed. Aromatic protons in positions 2 and 6 of the PA-ChA complex slightly (0.13 ppm) shifted downfield from 6.52 to 6.65 ppm, and another two aromatic protons in positions 3 and 5 slightly (0.01 ppm) shifted upfield from 7.59 to 7.60 ppm.

**Figure 7.** Comparison of proton-NMR (1H-NMR) spectra of Procainamide and Procainamide-ChA complex: (**A**) aromatic region of Procainamide-ChA complex; (**B**) aromatic region of Procainamide (PA).

On the other hand, as shown in Figure 8, -CH2 protons adjacent to -CONH in the aliphatic region were upfield-shifted (0.04 ppm) from 3.56 to 3.52 ppm, -CH2 protons adjacent to tertiary amine were downfield-shifted (0.06 ppm), and methyl protons (-CH3) were also upfield-shifted (0.02 ppm) (Figure 8). In addition to this, -NH2 peaks were upfield-shifted from 10.29 to 9.10 ppm. These changes in chemical shifts might be due to changes in the structural configuration of the complex formation.

**Figure 8.** Comparison of proton-NMR (1H-NMR) spectra of PA and PA-ChA complex: (**A**) aliphatic region of PA-ChA complex; (**B**) aliphatic region of PA.

#### NMR Spectra of PA-DDQ Complex

In the case of the PA-DDQ complex, the chemical shifts of aromatic protons were dramatically downfield-shifted and split into four different chemical shifts. As shown in Figure 9, aromatic protons of PA were given the chemical shifts at 7.60 ppm as a doublet for the protons in positions 3 and 5 and 6.52 ppm as a doublet for the protons in positions 2 and 6; on the other hand, aromatic protons of the PA-DDQ complex were given the chemicals shifts at 7.98, 7.88, 7.45, and 7.20 ppm for the protons in positions 3, 5, 2, and 6, respectively. It should be noted that the protons in positions 3 and 5 were downfield-shifted to 0.38 and 0.28 ppm, and the protons in positions 2 and 6 were downfield-shifted to 0.92 and 0.88 ppm, respectively.

On the other hand, in the aliphatic region, there is little change in chemical shifts, similar to the PA-ChA complex. As shown in Figure 10, -CH2 protons adjacent to -CONH in the aliphatic region were downfield-shifted (0.08 ppm) from 3.56 to 3.64 ppm, -CH2 protons adjacent to tertiary amine were downfield-shifted (0.09 ppm), and in the case of methyl protons (-CH3), they were also downfield-shifted (0.02 ppm). Interestingly, -NH2/-OH peaks were given in 11.01, 10.05, 9.56, 9.00, and 8.89 ppm. These changes in chemical shifts are obviously due to changes in the structural configuration of the complex formation.

**Figure 9.** Comparison of proton-NMR (1H-NMR) spectra of PA and PA-DDQ complex. (**A**) aromatic region of PA-DDQ complex; (**B**) aromatic region of PA.

**Figure 10.** Comparison of proton-NMR (1H-NMR) spectra of PA and PA-DDQ complex. (**A**) Aliphatic region of PA-DDQ complex; (**B**) aliphatic region of PA.
