Schiff bases are organic compounds defined by the azomethine group, a carbon-nitrogen double bond formed when a primary amine reacts with an aldehyde or ketone. This simple condensation reaction produces a family of molecules with a remarkable range of biological activities, including antiviral, antimicrobial, antifungal, antitumor, and antibacterial properties, which has made them a staple of medicinal chemistry. Aryl Schiff bases, in which the imine bond is attached to an aromatic ring, offer greater conjugation and stability than their aliphatic counterparts. Beyond biology, these versatile molecules serve as corrosion inhibitors, catalysts, metal-cation complexing ligands, optical switching materials, nonlinear optical materials, and sensors, making them one of the most broadly useful compound classes in modern chemistry.

The molecule at the heart of the new study, bis(2-nitrobenzylidene)hexane-1,6-diamine, contains two independent imine bonds, each of which can adopt either an E or Z configuration around the carbon-nitrogen double bond. In principle this yields four geometrical isomers, but because identical groups are attached to both imine bonds, only three distinct diastereomers are possible: ZZ, EZ, and EE. The team optimized all three structures at the B3LYP/6-311G(d,p) level of theory in the gas phase using the Gaussian 16 suite of programs. The energy ranking was unambiguous. The EE isomer, BSB-EE, lies 6.97 kilocalories per mole below the EZ isomer and 13.62 kilocalories per mole below the ZZ isomer, a direct consequence of steric hindrance in the Z-configurations where the bulky nitro-substituted phenyl rings crowd one another.

Symmetry also leaves a striking fingerprint on the dipole moments of the three isomers. BSB-EE, with its mirror-symmetric arrangement, has a calculated dipole moment of exactly zero, making it a genuinely non-polar molecule. By contrast, the EZ and ZZ diastereomers are strongly polar. Selected bond metrics remain remarkably consistent across all three forms, with carbon-nitrogen double bond lengths of 1.269 and 1.267 angstroms in the E and Z configurations respectively, and single carbon-nitrogen bonds of 1.453 and 1.457 angstroms. The dihedral angle around the imine linkage cleanly distinguishes the configurations, measuring close to 179 degrees for the E form and only 7 to 9 degrees for the Z form. These geometric signatures suggest that BSB-EE could serve as a robust scaffold where minimal intermolecular dipole interactions are desired, or where self-assembly is governed by shape and pi-conjugation rather than polarity.

Vibrational analysis provided a bridge between the computed structures and laboratory reality. The molecules, with the formula C20H22N4O4, exhibit 144 vibrational modes, and all calculated wavenumbers were scaled by a factor of 0.967 following the recommendations of the Computational Chemistry Comparison and Benchmark Data Base. The characteristic imine stretching vibration, the classic signature of Schiff base formation, appears in the 1600 to 1640 per centimeter region. For BSB-EE the calculated asymmetric C=N stretch falls at 1648 per centimeter, in close agreement with the experimentally measured value of 1633 per centimeter reported in earlier work. Aliphatic carbon-hydrogen stretches were computed at 2859 through 2969 per centimeter, aromatic stretches at 3072 through 3116 per centimeter, and the N=C-H stretch at 2982 per centimeter, all consistent with experimental benchmarks and confirming that the theoretical treatment faithfully reproduces the molecule’s vibrational behavior.

Optical properties revealed an equally clear stereochemical trend. Using time-dependent density functional theory, the team calculated the ultraviolet-visible absorption spectra of all three isomers. All three absorb in the near-ultraviolet to visible range, but BSB-EE absorbs at the shortest wavelength, 320 nanometers, while BSB-EZ absorbs at 340 nanometers and BSB-ZZ at 350 nanometers. More importantly, the oscillator strength, which measures the probability and intensity of the main electronic transition, decreases steadily from BSB-EE to BSB-EZ to BSB-ZZ. The dominant transitions are pi to pi-star promotions within the conjugated bis-Schiff base system, and the pronounced drop in oscillator strength indicates that the key transition is significantly more allowed in the EE geometry, likely because of superior orbital overlap. For applications requiring maximal visible absorption or fluorescence, BSB-EE emerges as the most promising candidate of the three.

Frontier molecular orbital analysis deepened the picture of electronic reactivity. The energy gap between the highest occupied and lowest unoccupied molecular orbitals governs kinetic stability, with large gaps indicating stability and small gaps indicating reactivity. BSB-EE has the largest gap at 4.561 electron volts, compared with 4.140 electron volts for BSB-EZ and 4.092 electron volts for BSB-ZZ, all falling in a moderate range of kinetic stability. Interestingly, BSB-EE is the strongest electron donor among the three isomers, with the most negative HOMO energy, while the EZ and ZZ forms are marginally better electron acceptors. Global reactivity descriptors including electronegativity, chemical hardness, ionization potential, and electron affinity differ only modestly across the isomers, reinforcing the central lesson that small geometric changes can meaningfully shift electronic distribution and reactivity without wholesale transformation of the molecule’s chemical character.

Perhaps the most surprising results came from the nonlinear optical calculations. Nonlinear optical materials respond to intense electromagnetic fields such as laser irradiation, producing phenomena like second-harmonic generation, third-harmonic generation, and two-photon absorption, with applications in optoelectronics, telecommunications, and photonics. Schiff bases typically combine electron-donating and electron-accepting groups linked by conjugated pi-systems, a push-pull architecture that enhances intramolecular charge transfer and boosts nonlinear response. When compared against urea, the standard reference material, the bis-Schiff base derivatives showed strongly enhanced polarizability. BSB-EZ displayed the strongest second-order response, while BSB-EE showed no measurable second-order activity, a consequence of its centrosymmetry. Yet BSB-EE posted a large positive second hyperpolarizability of 16.48 times ten to the minus forty electrostatic units, indicating powerful third-order nonlinearity, whereas the EZ and ZZ isomers showed negative gamma values of smaller magnitude, implying distinctly different third-order response characteristics relevant to Kerr effects and two-photon processes.

The team then turned to biology, docking the optimized BSB-EE structure into two protein targets from the RCSB Protein Data Bank: 6LU7, the main protease of SARS-CoV-2, a critical enzyme in viral replication, and 1HSG, the crystal structure of HIV protease complexed with an inhibitor. Using AutoDock 4.2, the ligand achieved favorable binding energies at both active sites. In 6LU7, hydrogen bonds form between the imine nitrogen atoms and the residues ASP30 and TYR32 at distances of 2.37 and 2.29 angstroms, while in 1HSG a hydrogen bond forms with THR216 at 2.03 angstroms. The phenyl rings of the ligand settle completely into hydrophobic pockets of the active sites, supplemented by van der Waals, pi-alkyl, and pi-anion electrostatic interactions with surrounding amino acid side chains. Benchmarking against doxorubicin, a known inhibitor with a binding affinity of minus 6.98 kilocalories per mole, revealed superior predicted inhibitory potential for BSB-EE against 6LU7, underscoring its promise as a lead compound.

The authors are careful to note the limitations of their approach. Gas-phase calculations neglect solvation, specific solute-solvent interactions, and crystal packing, and docking scores are only predictions that require experimental validation through enzymatic assays and binding studies, followed by structure-activity optimization and thermodynamic and kinetic analyses. Future work will include solvent-embedded infrared and nonlinear optical analyses and molecular dynamics-assisted docking to solidify structure-property-bioactivity correlations under physiologically relevant conditions. Natural bond orbital analysis already points to intramolecular charge transfer between electron-rich carbon and oxygen atoms and electron-deficient, positively polarized nitrogen sites, consistent with the push-pull delocalization that underpins the molecule’s nonlinear optical promise. Taken together, the study delivers a systematic rationale for treating BSB-EE as a versatile lead, one molecule whose shape alone tunes its stability, its optics, and its potential as a weapon against viral proteases.

Subject of Research: Computational analysis of the physicochemical properties and protein-binding potential of bis-Schiff base isomers derived from hexamethylenediamine and o-nitrobenzaldehyde

Article Title: Physicochemical properties (IR, FMOs, NLO, and molecular docking) of bis-Schiff bases derived from hexamethylenediamine and o-nitrobenzaldehyde

Article References: Rezvan, V. H., & Rajendran, S. (2026). Physicochemical properties (IR, FMOs, NLO, and molecular docking) of bis-Schiff bases derived from hexamethylenediamine and o-nitrobenzaldehyde. Discover Chemistry, 3(1), Article 557. https://doi.org/10.1007/s44371-026-00992-z

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00992-z

Keywords: bis-Schiff base, DFT, B3LYP, frontier molecular orbitals, nonlinear optical properties, molecular docking, SARS-CoV-2 main protease, HIV protease, geometrical isomers, UV-Vis spectroscopy, FTIR, medicinal chemistry