Quantum-Chemical Studies to Approach the Antioxidant Mechanism of Nonphenolic Hydrazone Schiff Base Analogs: Synthesis, Molecular Structure, Hirshfeld and Density Functional Theory Analyses

Abstract

In the present study, five nonphenolic (E)-N′-benzylidenebenzohydrazides including three new compounds were synthesized and evaluated for their free radical scavenging activities using 2,2-diphenyl-1-picrylhydrazyl (DPPH). X-Ray analysis of a single crystal of (E)-N′-(4-chlorobenzylidene)benzohydrazide (3c) revealed a triclinic, space group P-1 structure with a trans configuration around the azomethine (-C2=N2-) double bond. The three-dimensional Hirshfeld surfaces and the related two-dimensional fingerprint plots were also drawn to study the plausible intermolecular interactions. Density functional calculations of structures, electronic densities, frontier molecular orbitals modeling, and Mulliken charge analysis of all compounds were performed at the B3LYP/6-311G level of theory. Theoretical vibrational frequencies were predicted and compared with experimental values, and results supported the validity of optimized geometry of noncrystalline compounds. All synthesized compounds showed significant DPPH radical scavenging activity, although compound 3d exhibited greatest antioxidant activity with an IC50 value of 11 μM. The results of DFT analysis were used to explain the proposed antioxidant mechanism of (E)-N′-benzylidenebenzohydrazide analogs. This analysis revealed that protons attached to N, O, and C atoms possessing high negative charge are involved in the production of free radicals that scavenge DPPH. Moreover, the antioxidant activities of (E)-N′-benzylidenebenzohydrazide analogs correlated well with HOMO-LUMO energy difference of molecules.