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

Due to wide variety of biological and pharmacological activities of benzimidazole derivatives, the terahertz fingerprint spectra of 2-(4-fluorophenyl)benzimidazole and 2-(4-chlorophenyl)benzimidazole are researched by employing terahertz time-domain spectroscopy and density functional theory systematically. Although the substituent at the para position on the benzene moiety are the same family elements (F, Cl) in the periodic table, both experimental and theoretical results show that there are substantial differences in their fingerprint spectra in the range of 0.2–2.5 THz, such as the amount, amplitude and frequency position of absorption peaks. The validity of these results was confirmed by isolated-molecule and solid-state calculations based on density functional theory. The possible reasons of these differences are different intramolecular hyperconjugative interactions, different elongation of the corresponding bond lengths, different HOMO-LUMO energy gaps, as well as different atomic motions within in the unit cell owing to the electron-withdrawing effects of different halogen atoms at the para position on the benzene moiety. These results indicate the importance of this spectral range as a conformational fingerprint region where even minor changes in the molecular configuration lead to major differences in its THz absorption.