The team began with geometry. After fully optimizing the molecule without symmetry constraints and confirming through frequency calculations that the structure represented a true minimum on the potential energy surface, they obtained bond lengths and angles that tell a subtle electronic story. The sulfonyl S-O bonds came out at 1.441 and 1.449 angstroms, slightly shorter than the corresponding values in the antibiotic sulfadiazine, while the S-N bond of 1.699 angstroms sat between typical single- and double-bond lengths, a signature of resonance delocalization through the sulfonamide bridge. The authors attribute this enhanced orbital overlap to the electron-withdrawing fluorinated substituents and the fused heterocyclic ring, which pull electron density away from the sulfur center. Carbon-fluorine bonds ranged from 1.335 to 1.376 angstroms, reflecting fluorine’s powerful inductive effect, and aromatic carbon-carbon distances fell in the expected 1.37 to 1.42 angstrom window consistent with effective pi-conjugation.

Vibrational analysis reinforced the structural picture. The simulated infrared spectrum, scaled by the standard factor of 0.967, reproduced the characteristic fingerprint of an N-substituted sulfonamide: an intense N-H stretch at 3451 wavenumbers with an infrared intensity of 93.63 kilometers per mole, aromatic and aliphatic C-H stretches between 3104 and 2931 wavenumbers, coupled C-C and C-N ring vibrations near 1593 and 1542 wavenumbers, and the diagnostic asymmetric SO2 stretching band at 1328 wavenumbers. Each assignment, supported by potential energy distribution analysis, matched previously reported values for related heteroaromatic sulfonamides, validating both the optimized geometry and the computational protocol.

Nuclear magnetic resonance calculations added another layer of confirmation. Using the gauge-including atomic orbital method, the team computed carbon-13 shifts spanning 56.15 to 167.39 parts per million and proton shifts from 3.20 to 7.72 parts per million, then plotted them against empirical ChemDraw predictions. Most points clustered tightly around the line of perfect agreement, with modest deviations for carbons above 150 parts per million, where the quantum treatment explicitly accounts for geometry, electron density, and substituent-induced polarization. The most deshielded carbon, C19 at 167.39 parts per million, sits flanked by oxygen and nitrogen atoms within the triazolopyrimidine framework, while the most shielded, C32 at 56.15 parts per million, belongs to an electron-rich methoxy group.

The electronic heart of the study lies in the frontier molecular orbital analysis. Time-dependent DFT calculations across seven solvent environments, from gas phase to water, revealed three ultraviolet absorption bands at roughly 262, 277, and 298 nanometers, dominated by pi to pi-star transitions. The HOMO-LUMO gap ranged from 4.61 to 4.89 electron volts, with the largest value in water, where the high dielectric constant stabilizes the charge distribution through the polarizable continuum model. The orbital maps show electron density concentrated on the aromatic and sulfonamide donor regions in the HOMO and redistributed toward the electron-deficient triazine and fluorinated acceptor units in the LUMO, an architecture that drives efficient intramolecular charge transfer. Natural bond orbital analysis quantified this delocalization, with pi to pi-star stabilization energies reaching 51.49 kilojoules per mole and lone-pair donations from oxygen and nitrogen adding further stabilization.

Where the study turns genuinely eye-catching is in its nonlinear optical results. DTPB’s first-order hyperpolarizability came out at 1.5 times ten to the minus thirty electrostatic units, roughly eleven times higher than urea, the classic benchmark NLO material. Combined with a substantial total dipole moment of 8.5477 debye and a highly polarizable electron cloud, the molecule’s donor-acceptor architecture appears tailor-made for applications in optical switching, frequency conversion, photonic signal processing, and potentially LEDs, optical sensors, and solar energy technologies. The authors suggest that the same charge-transfer pathway that stabilizes the molecule electronically also amplifies its response to applied electromagnetic fields.

The team also brought mathematical chemistry to bear, converting DTPB into a molecular graph of 32 vertices and 34 edges and computing degree-based topological indices. First and Second Zagreb indices of 170 and 201, a Hyper Zagreb index of 878, and a Forgotten index of 476 all substantially exceed the values reported for the comparison molecule CMIO, confirming that the fluorinated substituents, sulfonamide group, and fused heterocycles create an unusually branched and interconnected topology. Such descriptors feed directly into quantitative structure-activity models that link molecular shape to biological and physicochemical behavior.

On the biological side, the picture is more sobering but scientifically coherent. Electrostatic potential maps pinpointed the sulfonyl and methoxy oxygen atoms as the primary electron-rich, hydrogen-bond-accepting sites, while the sulfonamide N-H proton emerged as the key electron-poor donor. Electron localization function and localized orbital locator maps confirmed the same heteroatoms as the principal reactive centers, and non-covalent interaction analysis showed that van der Waals contacts, not steric strain, dominate the molecule’s conformational stability. Docking simulations with AutoDock Vina against three protein targets produced favorable but moderate binding energies of minus 2.73, minus 3.26, and minus 4.17 kilocalories per mole, with the strongest interaction at the acetohydroxyacid synthase active site of protein 1YHY, each complex anchored by one hydrogen bond and a network of hydrophobic contacts.

Pharmacokinetic screening, however, tempered any drug-design enthusiasm. The Boiled-Egg model placed DTPB outside both the intestinal absorption and blood-brain barrier regions, consistent with its high topological polar surface area of about 125 square angstroms and moderate lipophilicity. The bioavailability radar showed acceptable size, flexibility, and saturation but flagged polarity and solubility as limiting factors, leading the authors to conclude that the molecule is better suited to peripheral rather than central nervous system applications, and that reducing polarity through structural modification could improve its profile.

Taken together, the study delivers something rarer than a single headline result: a complete, internally consistent digital twin of a complex fluorinated molecule, from bond lengths to binding poses. By showing that one computational framework can simultaneously predict spectra, quantify charge transfer, rank optical performance, and map biological recognition, the work offers medicinal and materials chemists a template for screening candidates before a single synthesis is attempted. The authors position their findings as a theoretical foundation for future experimental validation, molecular dynamics simulations, and the rational design of triazolopyrimidine-sulfonamide derivatives, whether the goal is a better crop-protection chemistry, a new photonic material, or simply a deeper understanding of how fluorine reshapes molecular electronics.

Subject of Research: Density functional theory investigation of the electronic structure, nonlinear optical properties, and bioactivity of the fluorinated triazolopyrimidine sulfonamide DTPB

Article Title: DFT investigation of 2-(2,2-difluoroethoxy)-N-(5,8-dimethoxy[1,2,4]triazolo[1,5- c ]pyrimidin-2-yl)-6-(trifluoromethyl)benzenesulfonamide (DTPB): Electronic structure, nonlinear optical response and bioactivity insights

Article References: Sasikala, L., Avinash, P., Suganthi, M., Suvitha, A., Britto, S., Vimal, S., & Prabhaharan, M. (2026). DFT investigation of 2-(2,2-difluoroethoxy)-N-(5,8-dimethoxy[1,2,4]triazolo[1,5-c]pyrimidin-2-yl)-6-(trifluoromethyl)benzenesulfonamide (DTPB): Electronic structure, nonlinear optical response and bioactivity insights. Results in Chemistry, 31, Article 103949. https://doi.org/10.1016/j.rechem.2026.103949

Image Credits: AI Generated

DOI: Not provided

Keywords: DFT, B3LYP, triazolopyrimidine, sulfonamide, fluorinated compounds, nonlinear optics, HOMO-LUMO gap, molecular docking, NBO analysis, molecular electrostatic potential, topological indices, pharmacokinetics

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