2.1.5. UATR-FTIR Spectroscopy

Universal-attenuated total reflectance Fourier transform infrared (UATR-FTIR) spectra of RSP, RM-β-CD, and their corresponding PM and KP are presented in Figure 6.

**Figure 6.** Universal-attenuated total reflectance Fourier transform infrared (UATR-FTIR) spectra of RSP, RM-β-CD, and RSP/RM-β-CD binary systems PM and KP.

The UATR-FTIR spectrum of RSP presents characteristic bands at 3063, 2936, 2812, 2759, 1648, 1534, 1449, 1414, 1352, 1130, 1027, 959, 854, and 816 cm−<sup>1</sup> that have been assigned to the functional groups from the drug structure in our previous study [22]. RMβ-CD shows a broad absorption band in the 3600–3100 cm−<sup>1</sup> region corresponding to the O-H stretching vibration from the non-methylated hydroxyl moieties and a large region below 1500 cm−<sup>1</sup> which exhibits distinct peaks, which is most probably characteristic to the cyclodextrin ring [19,51].

In the spectral patterns of binary products, several differences are noticed as compared with those of the parent compounds. Thus, the band assigned to the C-N stretching vibration shifted from 1352 cm−<sup>1</sup> in the RSP spectrum to 1364 cm−<sup>1</sup> in both PM and KP spectra. In addition, the C=O stretching vibration (from tetrahydropyrido-pyrimidinone ring) characteristic band from 1648 cm−<sup>1</sup> in the drug substance spectrum is displaced to 1649 cm−<sup>1</sup> and 1642 cm−<sup>1</sup> in the PM and KP spectra and is markedly reduced in intensity in both binary compounds. In addition, the spectral band attributed to aliphatic C-H stretching vibration from 2936 cm−<sup>1</sup> in RSP shifted to 2929 and 2924 cm−<sup>1</sup> in the spectral pattern of PM and KP, respectively. In the spectral region of 1050–1000 cm−<sup>1</sup> the parent substances, RSP and RM-β-CD, exhibit bands at 1028 and 1027 cm−<sup>1</sup> , while in the spectral pattern of binary products, two bands can be observed, at 1009 and 1035 cm−<sup>1</sup> in PM and at 1006 and 1037 cm−<sup>1</sup> in KP, respectively. A marked reduction in intensity and a shift to different wavenumbers (1535 cm−<sup>1</sup> ) in the PM and KP spectra is also noticed for the band assigned to the C=C stretching vibration of the aromatic ring from 1534 cm−<sup>1</sup> in the RSP spectrum. Furthermore, the bands from 2812, 2759, and 1130 cm−<sup>1</sup> in the RSP spectrum disappeared in binary KP and PM spectra.

The UATR-FTIR spectroscopy pointed out a decreasing in the intensity of RSP characteristic bands along with the shifting to different wavenumbers and the disappearance

of several peaks in the spectral pattern of PM and KP. These data give evidence about the interaction between the antipsychotic drug substance and RM-β-CD.
