The incorporation of polycarboxylate superplasticizers (PCEs) effectively improves the flowability of cement-based materials and reduces engineering construction costs. However, the widespread presence of montmorillonite (MMT) clay in low-quality aggregates inevitably induces competitive adsorption of PCE molecules, which severely weakens the water-reducing and dispersion performance of PCEs and deteriorates the working properties of concrete. Currently, the microscopic intercalation adsorption mechanisms governing interactions between structurally diverse PCEs and cement–clay mineral phases remain poorly understood. In this work, molecular dynamics simulations coupled with density functional theory calculations were implemented to unravel the competitive adsorption behaviors of PCEs with diverse side-chain lengths and acid-to-ether ratios. MMT was adopted as a typical clay substrate, while magnesium oxide (MgO) was used as a reliable cement particle analogue. The simulation results indicate that MgO exhibits considerably higher adsorption affinity for long-chain PCEs with high acid-to-ether ratios than MMT, and PCE2400 achieves the most stable and optimal adsorption performance on the MgO surface. Radius of gyration analysis further demonstrates that larger gyration values correspond to more fully stretched PCE molecular conformations, which are conducive to improving the dispersion efficiency and workability of cement paste. This study systematically clarifies the structure-dependent adsorption characteristics of PCEs on cement and clay substrates at the atomic level, providing a microscopic theoretical foundation for the structural optimization, targeted design, and practical application of high-performance PCE superplasticizers for clay-containing concrete systems.
Lewis acid catalysis plays a pivotal role in enhancing the efficiency and selectivity of polar Diels–Alder cycloadditions, yet the electronic factors governing its catalytic activity remain incompletely understood. In this work, the catalytic effects of Et₂AlCl and TiCl₄ on the polar (4 + 2) cycloaddition between isoprene and R-(−)-carvone were investigated within the framework of molecular electron density theory (MEDT). The results demonstrate that Lewis acid coordination substantially increases the electrophilic activation of the dienophile, promotes global electron density transfer, stabilizes the transition structures, and significantly lowers the activation barriers while preserving the one-step concerted but highly asynchronous reaction mechanism. Although the exo-transition states experience the greatest kinetic stabilization, the endo-cycloadducts remain thermodynamically favored, in excellent agreement with experimental observations. Electron localization function (ELF), quantum theory of atoms in molecules (QTAIM), and non-covalent interaction (NCI) analyses further reveal that Lewis acid activation accelerates the asynchronous formation of the two C–C σ bonds, strengthens covalent interactions within the transition structures, and enhances attractive non-covalent interactions responsible for transition-state stabilization. These findings provide a comprehensive electronic interpretation of Lewis acid catalysis and offer valuable mechanistic insights for the rational design of more efficient catalytic systems. All calculations were performed within the framework of molecular electron density theory (MEDT) using density functional theory (DFT) at the B3LYP/6-311++G(d,p) level, with the Stuttgart/Dresden (SDD) effective core potential and basis set employed for the Lewis acid catalysts. Complete potential energy surfaces were explored for both uncatalyzed and Lewis acid-catalyzed reaction pathways, and all stationary points were fully optimized and characterized by harmonic vibrational frequency calculations. Intrinsic reaction coordinate (IRC) calculations were carried out to confirm the connectivity of transition structures with the corresponding reactants and products. Global and local conceptual DFT descriptors together with frontier molecular orbital analyses were used to evaluate the electronic properties of the reactants. Bonding evolution along the reaction pathways was investigated using electron localization function (ELF), quantum theory of atoms in molecules (QTAIM), and non-covalent interaction (NCI) analyses. All quantum chemical calculations were performed using Gaussian 16, while topological analyses were carried out with Multiwfn and visualized using VMD.
Zr- and Hf-based metal–organic frameworks (MOFs) are excellent heterogeneous catalysts due to their chemical tunability, thermal stability, and well-defined active sites. Their use in organic synthesis continues to expand, as showcased in the N-alkylation of amines with alcohols, where UiO-66 and MOF-808 can efficiently catalyze the process to yield secondary amines. This reaction is proposed to occur via the borrowing hydrogen pathway, where the MOF catalyst first abstracts and then delivers hydrogen during the cycle, but the operating reaction mechanism remains unclear. To fill this gap in knowledge, we here report density functional theory simulations at M06-L level using cluster models of Zr6O8 nodes. Our computational results indicate that the borrowing hydrogen mechanism presents high Gibbs energy barriers and is not feasible under the reported experimental conditions. As an alternative route, we propose that traces of benzaldehyde, typically formed in benzyl alcohol solutions via aerobic oxidation, can indeed promote the reaction with reasonable Gibbs energy barriers. Additional kinetic experiments support this theoretical proposal.
Dermaseptins are peptides present in frog skin secretions and are recognized for their antimicrobial activity and potential as templates for new antibiotics. In this study, quantum-chemical descriptors were calculated for short fragments derived from four dermaseptins of the gliding tree frog Agalychnis spurrelli using density functional theory (DFT). At the B3LYP/6-311++G(d,p) level and within the adopted gas-phase fragmentation model, SP2-1 showed the highest HOMO energy, the smallest HOMO-LUMO gap, and the largest electrophilicity among the fragments analyzed. These results identify SP2–1 as the most electronically reactive fragment within this model. Because the calculations were performed on isolated short fragments, the results should be interpreted as a comparative analysis of local sequence regions rather than as a direct prediction of the activity of the complete peptides. The resulting ranking is qualitatively consistent with the biological behavior previously reported for these dermaseptins.
The electron density of delocalized bonds (EDDBs) is a nonlocal molecular density layer representing electrons that participate effectively in multicenter bond delocalization. Its numerical realization, the bond-orbital projection (BOP) method, has evolved over the past decade into an efficient framework for analyzing resonance and delocalization in large, topologically complex molecules. Here, we provide the first unified, implementation-level formulation of the current BOP algorithm and document its realization in the runEDDB program. Conceptually, BOP extends the familiar superposition principle from atomic orbitals to bond orbitals: neighboring localized bond orbitals are tested for their ability to form phase-coherent conjugating or anticonjugating combinations, and the efficiency of this superposition defines their contribution to bond delocalization. We derive the complete computational workflow from the projector algebra of an idempotent one-particle density matrix through two- and three-center bond-order-orbital projections to the final EDDB density. Robustness and performance are demonstrated for a 360-atom porphyrin nanoring. Across three parent basis sets, the full natural atomic orbital (NAO), natural minimal basis (NMB), and natural valence basis (NVB) representations recover essentially the same π -bond-delocalization pattern. For the largest density, NMB retains 97.5
