Experimental and theoretical study of fingerprint spectra of 2-(4-fluorophenyl)benzimidazole and 2-(4-chlorophenyl)benzimidazole in terahertz range

Characterizing hydrogen bonds in crystalline form of guanidinium salicylate in the terahertz range

For pharmaceutical compounds with poor solubility, there is an effective method to address this dilemma without tampering their intrinsic chemical properties by forming weak hydrogen bonds. Guanidinium salicylate, which is a typical pharmaceutical salt with a complex crystal structure, was systematically investigated by terahertz time-domain spectroscopy combined with density functional theory in order to obtain the complete information of weak hydrogen bonds. As a result of the influence of weak hydrogen bonds, there are substantial differences between guanidinium salicylate and its parent molecule (salicylic acid) in the experimental fingerprint spectra in the range of 0.2-2.5 THz, such as the number, amplitude and frequency positions of absorption peaks. With the help of isolated molecule density functional theory calculations, the possible sites of weak hydrogen bonds were determined by natural bond orbital analysis. It can be concluded that there is an intricate hydrogen bond network due to the polar distribution of molecular electrostatic potential. Furthermore, all THz absorption peaks were assigned to their corresponding vibrational modes and the complete information of the related hydrogen bonds (including type, role, angle, and bond length) was determined by using dispersion-corrected density functional theory. The results laid a good foundation for further study on the enhancement of solubility of pharmaceutical salts by forming weak hydrogen bonds.

Vibrational spectra of pyrazinamide and isoniazid studied by terahertz spectroscopy and density functional theory

Pyrazinamide and isoniazid, as two first-line anti-tuberculosis drugs, are investigated by terahertz time-domain spectroscopy (THz-TDS). Both pyrazinamide and isoniazid have three absorption peaks, at 0.50, 0.71, 1.42 THz and 1.16, 1.46, 1.56 THz, respectively, which can be used as the basis for qualitative identification of these two drugs. In order to gain insight into the origin of the characteristic absorption peaks, density functional theory (DFT) based on single molecular, dimer, and crystalline structures of pyrazinamide and isoniazid are performed. The purpose of the calculation based on the single molecular structure is to understand the intramolecular interaction, while those based on the dimer and crystalline structures are to investigate the intermolecular interactions in PNZ and INZ. Comparing the theoretical results of the dimer and crystalline based structures reveals that the crystalline structure leads to vibrational spectra that are closer to the experimental values in terms of the number of absorption peaks and the positions of the absorption peaks. Vibrational mode assignments can be summarized as that the characteristic absorption peaks of pyrazinamide mainly come from intermolecular interaction, and the characteristic absorption peaks of isoniazid originate from both the intramolecular and intermolecular interactions. Our experimental and theoretical results indicate that the combination of THz-TDS with DFT is an effective approach for identification of molecules with pharmaceutical significance.