The experimental side of the study relied on laser desorption/ionization time-of-flight mass spectrometry, a technique in which a pulsed ultraviolet nitrogen laser at 337 nanometers strikes a sample and ejects ions that can then be sorted by mass. The researchers deposited tiny droplets of an aqueous chloroauric acid solution, HAuCl4, onto a stainless-steel target plate with no organic matrix, and fired 300 laser shots per spectrum. Earlier work had already revealed that clusters of the general formula Au(n)Cl(n+m)−, with m equal to 1, 3, 5, or 7, appear in such spectra, but the chlorine-richest members, the Au(n)Cl(n+5)− series, had never been systematically characterized. The new measurements show that these hyperchlorinated anions are not random debris. At an optimal laser intensity of 2,300 arbitrary units, the Au3Cl8− anion emerges as the most abundant species in the entire spectrum, with Au2Cl7− and Au4Cl9− also producing strong signals, while Au5Cl10− and Au6Cl11− appear with considerably lower intensities.

The most striking result of the intensity analysis is that cranking up the laser power reverses the abundance hierarchy of the clusters. At a low laser intensity of 1,300 units, the relative peak intensities for the two-gold-atom series follow the order Au2Cl3− greater than Au2Cl5− greater than Au2Cl7−, meaning the chlorine-poor species dominate. At 2,300 units, that order flips completely, and the chlorine-richest anion becomes the most abundant. The same reversal appears for the three- and four-gold-atom series, where Au3Cl8− and Au4Cl9− rise from minor players to dominant species as the laser fluence increases. The interpretation offered by the team is thermodynamic: higher laser intensity rapidly raises the concentration of free chlorine atoms in the plume, so reactions that add two chlorine atoms at a time to a growing cluster become thermodynamically favored, steering the population toward the chlorine-rich end of the family.

To understand what these ions actually look like, the researchers turned to density functional theory. Global minimum searches were conducted with a basin-hopping-style algorithm supplemented by manually constructed isomers, and the resulting candidate structures were optimized in Gaussian 09 at the TPSSh level of theory. For gold, the calculations employed the ECP60MDF relativistic pseudopotential with its associated MDF60 basis set, a choice that captures the relativistic effects known to be indispensable in gold chemistry, while chlorine atoms were described with the aug-cc-pVTZ basis set. Frequency analyses confirmed every optimized structure as a true minimum on its potential energy surface, and zero-point energy corrections were included in all reported binding energies. The team notes that no counterpoise correction for basis-set superposition error was applied, though the large augmented triple-zeta basis substantially mitigates that error, and full corrected calculations are left for future higher-level work.

The computed global minima tell a remarkably consistent structural story. Except for the smallest member, Au2Cl7−, every cluster in the series adopts a nearly planar zigzag geometry, echoing the motifs previously established for the Au(n)Cl(n+1)− and Au(n)Cl(n+3)− series. In these frameworks, the two terminal gold atoms sit in a tetracoordinate Au(III) state, each bonded to four chlorine ligands, while the internal gold atoms carry a lower-coordinated Au(I) character. The zigzag backbone is invariably flanked by AuCl4− units at both ends, and additional chlorine atoms attach at positions vertical to the main chain. Isomers that relocate a single tetracoordinate Au(III) center from a terminal to an internal position pay an energetic penalty of roughly 6 to 11 kilocalories per mole, underscoring how strongly the terminal placement is preferred. Notably, low-lying isomers containing explicit Au-Au covalent bonds, which appear in the lower-chloride series, are absent among the most stable hyperchlorinated structures, likely because the pronounced excess of chlorine over gold atoms discourages such motifs.

The aurophilic analysis hinges on a simple distance criterion. The sum of the van der Waals radii for two gold atoms is 3.32 angstroms, while a typical covalent Au-Au bond measures about 2.72 angstroms, so aurophilic contacts are expected to fall between those two values. In the hyperchlorinated series, all terminal Au-Au distances, meaning the separations between a terminal Au(III) center and its neighbor, measure between 3.82 and 3.90 angstroms, well beyond the threshold and therefore showing no aurophilic attraction whatsoever. In stark contrast, every non-terminal distance, the spacing between two adjacent internal Au(I) centers, falls within a narrow window of 2.90 to 2.92 angstroms, confirming robust aurophilic bonding. Remarkably, these internal distances remain virtually invariant as the chain grows longer, and they are even marginally shorter than the corresponding distances reported for the lower-chloride series, suggesting that the extra chlorine atoms slightly enhance the aurophilic pull between the internal Au(I) centers along the backbone.

Two complementary electronic analyses confirm the origin of the interaction. Electron localization function maps show that the electron-pair density between terminal Au(III) and adjacent atoms is virtually nil, while a distinct localization domain emerges between internal Au(I) pairs, growing in area and intensity with cluster size until it reaches a plateau for clusters with four or more gold atoms. Natural bond orbital calculations reveal that the terminal Au(III) centers carry substantially higher positive charges, around +0.47 elementary charges, compared with roughly +0.34 for the internal Au(I) centers. The higher coordination number and oxidation state of Au(III) deplete its 5d electrons, reduce 6s occupancy, and increase 6p participation. Crucially, significant 5d/6s orbital delocalization occurs exclusively between adjacent internal Au(I) centers, identifying the electronic fingerprint of the aurophilic attraction, while negligible orbital mixing appears between terminal Au(III) and neighboring Au(I) centers. The aurophilic interaction first emerges at Au4Cl9− and rapidly stabilizes for larger clusters, a size-dependent saturation that mirrors the plateau seen in the localization maps.

The thermodynamic analysis completes the picture. The binding energy for the series is defined as the energy change for the incremental growth reaction in which an Au(n−1)Cl(n+4)− cluster captures an AuCl unit. All computed binding energies are negative, ranging from −188 to −176 kilojoules per mole, confirming that cluster growth is exothermic and that these anions are thermodynamically viable. With increasing nuclearity, the absolute binding energy decreases monotonically before converging asymptotically to approximately −176 kilojoules per mole, a convergence that parallels the trends in Au-Au bond lengths and electron localization. Comparison with the previously reported Au(n)Cl(n+1)− and Au(n)Cl(n+3)− series shows that this core thermodynamic behavior, the gradual convergence of binding energy and aurophilic strength, is largely invariant with respect to chloride content, although higher chloride substitution accelerates the size convergence.

The reaction-energy framework also explains the puzzling intensity patterns observed at low laser power. Certain ions, such as Au3Cl6− and Au4Cl7−, appear weaker than their neighbors under low fluence because their formation and depletion pathways release different amounts of energy. For Au3Cl6−, four reaction pathways were analyzed, releasing −187, −362, −183, and −357 kilojoules per mole respectively, and the balance among these competing routes determines the observed signal. Adding excess chlorine atoms to form Au3Cl10− introduces significant steric repulsion within the cluster, rendering that reaction far less thermodynamically favored, which is verified by the weak experimental signal of Au3Cl10−. As laser intensity rises, the chlorine atom concentration climbs, the two-chlorine-addition routes dominate, and the chlorine-rich anions prevail. Because the proposed reactions involve strong structural inheritance and do not require breaking chemical bonds, making them nearly barrierless, the assumption of thermodynamic rather than kinetic control is reasonable, the authors note.

The study closes with a provocative outlook. It is reasonable to expect that clusters with even higher chlorine-to-gold ratios, beyond the n+5 stoichiometry, could also exist, retaining similar yet extended zigzag backbones. Because the chlorine species concentration can potentially be modulated during synthesis, the structural flexibility and compositional diversity of these one-dimensional gold-halogen chains may open new avenues for catalytic activity in relevant reactions. The work demonstrates that aurophilicity, born from relativistic effects that make gold unlike any of its lighter congeners, is not merely a curiosity of coordination chemistry but a genuine structure-directing principle, one that sculpts matter at the nanoscale into precise one-dimensional architectures even under the{

Subject of Research: Aurophilicity as a structure-directing principle in hyperchlorinated AunCln+5- (2 ≤ n ≤ 7) clusters

Article Title: Aurophilicity as a structure-directing principle in hyperchlorinated AunCln+5- (2 ≤ n ≤ 7) clusters

Article References: Chen, Y., Xu, S., Wang, S., Li, S., Veljković, F., Veličković, S., & Kong, X. (2026). Aurophilicity as a structure-directing principle in hyperchlorinated AunCln+5- (2 ≤ n ≤ 7) clusters. Frontiers in Chemistry, 14, Article 1986508. https://doi.org/10.3389/fchem.2026.1986508

Image Credits: AI Generated

DOI: 10.3389/fchem.2026.1986508

Keywords: aurophilicity, gold clusters, gold chloride, mass spectrometry, density functional theory, Au(I), Au(III), relativistic effects, zigzag geometry, binding energy, NBO analysis, electron localization function

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. V., 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: 10.1016/j.rechem.2026.103949

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

The team built a finite, hydrogen-terminated fragment of an armchair (5,5) single-walled carbon nanotube roughly 7 angstroms long, with a diameter of about 6.8 angstroms that makes the surface highly curved, and decorated one end with a carboxyl group. Using density functional theory with the M06-2X hybrid meta-GGA functional and the 6-311+G(d,p) basis set, a combination well benchmarked for hydrogen bonding and dispersion, they optimized the bare nanotube, the free isoniazid molecule, and a series of candidate adsorption complexes. Guided by the molecular electrostatic potential of isoniazid, which maps electron-rich and electron-poor regions of the drug, the researchers generated several chemically plausible starting orientations and let the optimizer decide which one nature would prefer.

The verdict was clear. Among six representative configurations, the lowest-energy arrangement bound isoniazid with an adsorption energy of −18.65 kcal per mole, or about −0.81 electron volts. That is dramatically stronger than the roughly 2 kcal per mole reported for isoniazid on pristine nanotubes, and it even edges out the 14 to 16 kcal per mole achieved with boron-doped tubes. The interaction still sits in the range associated with noncovalent adsorption, which is exactly what a drug carrier wants: strong enough to hold the payload, weak enough to release it inside a cell. Crucially, when the team repeated the optimization inside a polarizable continuum model of water, the adsorption energy softened to −13.38 kcal per mole but remained negative, meaning the complex stays thermodynamically favorable in an aqueous, body-like environment.

Thermodynamics told the same story from a different angle. At 298.15 kelvin and one atmosphere, the adsorption enthalpy was −16.65 kcal per mole in the gas phase and −12.67 kcal per mole in the implicit solvent, while the Gibbs free energy change was −7.40 and −3.39 kcal per mole respectively. Both negative enthalpies confirm the process is exothermic, and both negative free energies confirm it is spontaneous. The entropy change was negative in both environments, around −31 calories per mole kelvin, which simply reflects the loss of translational and rotational freedom when a free drug molecule locks onto a surface. Solvation barely touched the entropy term, indicating that water weakens the energetic driving force mainly by stabilizing the separated drug and nanotube rather than by changing how the complex forms.

To find out what physically holds the pair together, the researchers turned to the Quantum Theory of Atoms in Molecules, which examines the topology of the electron density between atoms. The analysis revealed two intermolecular bond paths in both the gas phase and water: one between a carboxyl oxygen on the nanotube and a hydrogen on the drug, and another between an N–H group of isoniazid and an oxygen of the nanotube’s carboxyl group. At each bond critical point, the electron density Laplacian was positive while the local energy density was negative, a signature of hydrogen bonds with partial covalent character. In other words, the drug is not just loosely clinging to the tube through vague dispersion forces; it is held by two specific, directional hydrogen bonds.

Natural Bond Orbital analysis quantified the electronic handshake behind those bonds. The dominant donor–acceptor interaction funnels electron density from the lone pair of the nanotube’s carboxyl oxygen into the antibonding σ* orbital of the drug’s N–H bond, with second-order stabilization energies of 12.78 and 7.21 kcal per mole for the two oxygen lone pairs in the gas phase. In water those values dipped to 10.50 and 6.31 kcal per mole, mirroring the weaker overall adsorption, but the same charge-transfer pathway survived. This is the classic electronic fingerprint of a hydrogen bond, and it dovetails perfectly with the QTAIM picture: adsorption proceeds by electron delocalization from the functionalized nanotube into the drug molecule.

Frontier molecular orbital analysis added a systems-level perspective. Isolated isoniazid has a generous HOMO–LUMO gap of 4.76 electron volts, but forming the complex collapses it to 1.74 electron volts in the gas phase and 1.51 electron volts in water, signaling substantially increased electronic delocalization across the drug–carrier interface. Meanwhile, the global hardness of the system rises from 0.40 electron volts for the bare nanotube to 0.87 electron volts for the complex, and electrophilicity falls from 14.54 to 7.05 electron volts, indicating that the assembled complex is electronically more stable and less eager to accept additional charge than the nanotube alone. Solvent polarization nudges these descriptors slightly, softening the complex and modestly raising its electrophilicity, but the overall electronic character established in vacuum persists in water.

Spectroscopy provided the final layer of confirmation. Gauge-including atomic orbital NMR calculations showed that the carbonyl group at the heart of the interface undergoes the largest electronic rearrangement: the isotropic shielding of the carbonyl carbon jumps from 9.77 to 24.87 parts per million upon adsorption, while the carbonyl oxygen’s shielding swings from −141.83 to 8.14 parts per million and its anisotropy plummets from 655.81 to 119.95 parts per million. The N–H group involved in the hydrogen bond also shifts, with its hydrogen’s isotropic shielding dropping from 26.39 to 21.62 parts per million. Calculated infrared spectra showed corresponding changes in C=O and N–H stretching frequencies, all consistent with a hydrogen-bonding environment reshaped by adsorption. Together, the energetic, topological, orbital, and spectroscopic lines of evidence converge on a single, coherent mechanism.

The authors are candid about the model’s limits. The calculations used a short, highly curved nanotube fragment with a single carboxyl group, so the reported energies are specific to this finite cluster rather than universal values for longer or multi-walled tubes. Chirality was not systematically varied, the conformational search was guided rather than exhaustive, and the implicit solvent cannot capture individual water molecules competing for hydrogen-bonding sites. The study is also entirely computational, awaiting experimental measurements of loading capacity, release behavior, and spectroscopic signatures. Still, the message is compelling: a simple carboxyl decoration transforms a weakly interacting carbon nanotube into a substantially stronger, thermodynamically spontaneous, water-stable carrier for isoniazid. If subsequent explicit-solvent simulations and laboratory tests bear this out, functionalized nanotubes could one day ferry higher, sustained doses of tuberculosis therapy directly to infected cells, easing the resistance problem one hydrogen bond at a time.

Subject of Research: Density functional theory investigation of isoniazid adsorption on carboxyl-functionalized (5,5) single-walled carbon nanotubes for drug delivery

Article Title: DFT investigation of the electronic structure, NMR parameters, and adsorption mechanism of isoniazid on carboxyl-functionalized (5,5) SWCNT

Article References: Hamedani, S., Shadi, M., & Moradi, M. (2026). DFT investigation of the electronic structure, NMR parameters, and adsorption mechanism of isoniazid on carboxyl-functionalized (5,5) SWCNT. Results in Chemistry, 31, Article 103916. https://doi.org/10.1016/j.rechem.2026.103916

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103916

Keywords: carbon nanotubes, isoniazid, drug delivery, DFT, hydrogen bonding, QTAIM, NBO analysis, NMR shielding, tuberculosis, nanomedicine, functionalization, computational chemistry

5-fluorouracil has been a cornerstone of cancer treatment for decades, particularly against colorectal, breast, and gastrointestinal tumors. The drug works by inhibiting thymidylate synthase, an enzyme essential for DNA replication, thereby triggering apoptosis in rapidly dividing cancer cells. But the same biological potency that makes it effective in the clinic makes it hazardous once it escapes into the environment. Hospital effluents and surface waters are now routinely found to contain measurable concentrations of the compound, and conventional wastewater treatment systems are often unable to eliminate it completely. Because 5-fluorouracil is chemically stable and biologically active even at low concentrations, it poses risks to aquatic life and may contribute to the spread of drug-resistant microorganisms, creating an urgent need for removal strategies that actually work.

The materials at the heart of the new study are carbon coordination frameworks, conical carbon architectures with extended pi-conjugation that can be functionalized with remarkable precision. At the apex of each cone, the researchers embedded a porphyrin-like ring containing four nitrogen atoms and twenty carbon atoms, creating a cavity perfectly suited to host a single metal ion. This design prevents metal aggregation and ensures atomic dispersion, meaning every metal center remains individually accessible to incoming molecules. The team selected two representative metals for comparison: iron in its +2 oxidation state, a transition metal cation, and magnesium in its +2 state, a group IIA cation. Both were positioned at the central cavity of the nitrogen site to form the M-N4 carbon coordination frameworks that served as the computational testbeds.

Using the M06-2X exchange-correlation functional with the 6-31G(d) basis set in the Gaussian 16 package, the researchers systematically explored how 5-fluorouracil orients itself on these frameworks. Two stable adsorption configurations emerged for each metal, distinguished by which of the drug’s oxygen atoms coordinates to the metal center. The results were striking. The iron-based complexes bound the drug with adsorption energies of -30.89 and -28.90 kilocalories per mole for the two configurations, while the magnesium analogues managed only -25.23 and -22.19 kilocalories per mole. All values fell within the chemisorption regime, indicating the formation of moderately strong chemical bonds rather than weak physical attraction. The contact distances told the same story: the iron-oxygen separations measured approximately 1.99 angstroms, short enough to signal strong metal-ligand bonding, whereas the corresponding magnesium-oxygen distances were noticeably longer.

To understand why iron grips the drug so much more effectively, the team deployed a battery of electronic structure analyses. Atoms-in-Molecules theory examines the electron density at so-called bond critical points, the locations where electron density accumulates between interacting atoms. For the iron-oxygen contacts, the analysis revealed low electron density but a positive Laplacian paired with negative total energy density, a signature of partial covalency. The magnesium-oxygen contacts, by contrast, showed positive energy densities consistent with weak van der Waals interactions. Hydrogen bonds between a hydrogen atom of the drug and a nitrogen site of the framework provided additional stabilization in all complexes, with the strongest contact in the iron-based system reaching an estimated energy of -35.49 kilojoules per mole. Reduced density gradient analysis visualized these interactions directly, showing green iso-surfaces for the dispersive metal-oxygen contacts and blue regions marking the stabilizing hydrogen bonds.

Natural Bond Orbital analysis added another layer of mechanistic insight by quantifying the donor-acceptor interactions that stabilize the adsorbed complex. The dominant contribution came from charge donation from the oxygen lone-pair orbitals of the drug into antibonding orbitals of the iron center, while the interaction involving nitrogen lone pairs exhibited the largest second-order perturbation energy, indicating pronounced electron delocalization toward the metal. The charge-transfer parameter derived from conceptual density functional theory reached 0.0217 for the iron complex versus 0.0175 for the magnesium one, confirming that the iron framework accepts more electron density from the drug. Electrostatic potential maps made this exchange visible: the electron-rich oxygen atoms of the drug carry negative potential while the electron-deficient iron center is positive, and after adsorption the framework fragment gains negative character, direct evidence of charge flowing from drug to material.

Interestingly, the study also ruled out some potential applications. Adsorption of 5-fluorouracil induced only minor changes in the band gap of the frameworks, which measured 3.204 electron volts for the pristine iron system, confirming its semiconducting nature. The work function and the simulated ultraviolet-visible spectra likewise showed only slight variations upon drug binding. These findings imply that the frameworks would not function effectively as conductivity-based or optical sensors for detecting the drug, even though they bind it strongly. The metal-nitrogen bond lengths, however, did elongate measurably upon adsorption, suggesting that structural markers rather than electronic ones might serve as sensitive indicators of capture events in future experimental designs.

The research also looks toward the era of artificial intelligence. The authors benchmarked several graph neural network architectures, including GCN, GAT, MPNN, and GIN, for predicting band gaps within the QM9 chemical space, finding that the GIN architecture achieved the lowest prediction error at 0.082 electron volts mean absolute error. They emphasize that no machine-learning model was trained on their adsorption data, and the discussion is presented as a forward-looking perspective rather than a validated predictive framework. Still, the DFT-derived descriptors of adsorption energy, charge transfer, and electronic structure constitute a structured reference dataset that could seed future high-throughput screening campaigns, potentially accelerating the rational design of nano adsorbents without requiring exhaustive quantum calculations for every candidate material.

The practical takeaway is clear: iron-centered nitrogen-doped carbon coordination frameworks are markedly more promising than magnesium analogues for capturing 5-fluorouracil from contaminated water, thanks to shorter metal-oxygen contacts, partially covalent bonding, and superior charge-transfer capability. While the calculations were performed in the gas phase, supplementary polarizable continuum model calculations in water confirmed that the two adsorption configurations remain nearly isoenergetic and that the overall mechanism persists under aqueous conditions. As pharmaceutical pollution intensifies worldwide, studies like this one demonstrate how computational chemistry can shortlist the most effective materials before a single laboratory experiment is run, turning the abstract mathematics of electron density into a concrete blueprint for cleaner water.

Subject of Research: Density functional theory investigation of 5-fluorouracil adsorption on Fe(II)- and Mg(II)-centered N4 carbon coordination frameworks for pharmaceutical pollutant removal

Article Title: DFT investigation of 5-fluorouracil adsorption on Fe/Mg–N₄ carbon coordination frameworks: mechanistic insights

Article References: Radhi, I. J., Amir, I. Q. A., Shaker, Z. M., Soleimani-Amiri, S., Imanov, H. A., Khanmohammadi, A., & Vessally, E. (2026). DFT investigation of 5-fluorouracil adsorption on Fe/Mg–N₄ carbon coordination frameworks: mechanistic insights. Journal of Saudi Chemical Society, 30(3), Article 36. https://doi.org/10.1007/s44442-026-00086-6

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00086-6

Keywords: 5-fluorouracil, DFT, carbon coordination frameworks, Fe-N4, Mg-N4, adsorption, chemisorption, AIM analysis, NBO analysis, water purification, pharmaceutical pollution, graph neural networks