Spectral investigation of the intramolecular charge-transfer in some aminotriazole Schiff bases

3-Amino-1,2,4-triazole Schiff bases were reported to contain intramolecular charge-transfer. The enhancing and depressing effects were remarkable as the substituent was changed from electron-donating to electron-withdrawing groups. The path of the resonating delocalization was reversed in the case of the p-NO2 group. To validate these results we effectively used Weinhold et al's natural bond orbital analysis to assess the UV and FT-IR spectrophotometric monitoring of the change reflected in this phenomenon when the substituent in the benzene ring is altered. The NBO analysis was simulated by ab inito computations at the HF/6-31G(d) level of theory, in order to properly detect any possible presence of a hydrogen bond association. The changes occurring in electron occupancies of double-centered bonds, antibonding orbitals and in lone-pair orbitals appraised the results, as did the s and p character listings of the two-centered bonds and the simultaneous changes occurring in the geometric parameters of the molecules in question. Contrary to its normal preference, in these molecules the nitrogen used sp2 hybrid orbitals for its interaction, housing its electron lone-pair in the third p hybrid orbital. Furthermore, NBO analysis reflected the presence of a very soft intramolecular hydrogen association (C–H⋯π), labelled by UV and FT-IR assignments, between the benzene and triazole rings in all Schiff bases but p-N(Me)2. The n–π* stabilization energy decreased in the order: p-OH > p-OCH3 > p-Cl > p-CH3 > H > p-NO2 > o-OH. The relation between the band position and Hammett substitution constant is interpreted in relation to the molecular structure.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Charge-transfer stabilizes molecules by influencing their geometric orientation. ► Different substituents in the phenyl ring affect this property remarkably. ► Follow-up of this property by spectral investigations was assessed by ab initio theoretical calculations. ► Resonating delocalization reversed its path in the case of the p-NO2 substituent. ► A soft intramolecular hydrogen association (C–H⋯π) exists between the benzene and triazole rings.