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[Display omitted] Reaction of 3-hydroxy-2,5-di- tert -butyl-1,4-benzoquinone with phenylhydrazine hydrochloride mainly leads to the corresponding hydrazone at the position 1 existing in tautomeric equilibrium with 3,6-di- tert -butyl-4-(phenyl- diazenyl)catechol. Thermodynamic parameters of the tautomerization were evaluated by NMR spectroscopy. The hydrazone structure was confirmed by XRD study. [ABSTRACT FROM AUTHOR]

Bifurcating electron transferring flavoproteins (Bf-ETFs) tune chemically identical flavins to two contrasting roles. To understand how, we used hybrid quantum mechanical molecular mechanical calculations to characterize noncovalent interactions applied to each flavin by the protein. Our computations replicated the differences between the reactivities of the flavins: the electron transferring flavin (ETflavin) was calculated to stabilize anionic semiquinone (ASQ) as needed to execute its single-electron transfers, whereas the Bf flavin (Bfflavin) was found to disfavor the ASQ state more than does free flavin and to be less susceptible to reduction. The stability of ETflavin ASQ was attributed in part to H-bond donation to the flavin O2 from a nearby His side chain, via comparison of models employing different tautomers of His. This H-bond between O2 and the ET site was uniquely strong in the ASQ state, whereas reduction of ETflavin to the anionic hydroquinone (AHQ) was associated with side chain reorientation, backbone displacement, and reorganization of its H-bond network including a Tyr from the other domain and subunit of the ETF. The Bf site was less responsive overall, but formation of the Bfflavin AHQ allowed a nearby Arg side chain to adopt an alternative rotamer that can H-bond to the Bfflavin O4. This would stabilize the anionic Bfflavin and rationalize effects of mutation at this position. Thus, our computations provide insights on states and conformations that have not been possible to characterize experimentally, offering explanations for observed residue conservation and raising possibilities that can now be tested. [ABSTRACT FROM AUTHOR]

• Comprehensive analysis delves into molecular structure & IHB potency via varied methods. • The trend in IHB strength between the compared molecules is: BA > DFBA > TFBA. • Full assignments of NMR, IR, and UV spectra of DFBA were performed. • The vibrational and UV results showed existence of two stable tautomers. The molecular structure, tautomerism, spectroscopy properties, and intramolecular hydrogen bonding of 4,4-difluoro-1-phenyl-1,3-butanedione, also known as difluorobenzoylacetone (DFBA), have all been analyzed using density functional theory (DFT) calculations and spectroscopic techniques (IR, UV, and NMR). The results were compared with previously reported values for trifluorobenzoylacetone (TFBA) and benzoylacetone (BA). The energy difference between the two stable cis-enol forms,calculated at the B3LYP/6–311++ G ** level, is 0.96 kcal/mol.Therefore, the coexistence of both forms in comparable ratios is possible, as revealed by comparing the calculated and experimental vibrational band frequencies and their intensities. Furthermore,various computational methods were employed to examine the intramolecular hydrogen bond (IHB) strength and molecular stability,including geometry, atoms in molecules (AIM), and natural bond orbitals (NBO) theories. The hydrogen bond strength of DFBA falls between that of BA and TFBA, beinggreater than TFBA but lower than BA. Transition state study show that tautomerism between both forms occurs via the proton transfer. [Display omitted] [ABSTRACT FROM AUTHOR]

• NMR and theoretical calculations are used to study tautomerism of three herbicides. • Classical comparison parameters MAE and R2 give uncertain assignation. • DP4+ method permits to assign tautomeric forms with high level of confidence. • Metribuzin and diclosulam exist, each one, as one major tautomer in DMSO. • Clethodim appears as a mixture of E- and Z -ketoenolimine forms in DMSO. Pesticides are widely used in agriculture, and they can cause serious damage to human health and to the environment. In this work, three herbicides are studied regarding possible tautomeric equilibria: clethodim, diclosulam and metribuzin. Tautomerism is a key reaction that is directly related to action, disposition and toxicity of herbicides, and here lies the importance of this study. The specific aim of this work is to compare classical parameters (MAE and R2) with DP4+ method as a tool for obtaining relevant information about tautomeric equilibria in the three compounds, contrasting experimental NMR data with theoretical calculations carried out using DFT methods. 1H and 13C NMR spectra of the three herbicides were recorded in DMSO‑ d 6. Additionally, HSQC and COSY spectra were obtained for clethodim. Theoretical calculations were carried out in order to obtain optimized geometries in vacuum (using B3LYP method at 6–31G(d,p) level of theory); and chemical shifts of nuclei corresponding to all possible tautomers of each herbicide were obtained by means of B3LYP-GIAO method using 6–311+ G (2d,p) basis set. In the three cases, classical comparison parameters MAE and R2 fail in the assignation of tautomers since they gave doubtful or even inconsistent results. In the case of metribuzin, MAE values obtained from 1H and 13C point towards different structures. R2 and MAE values obtained from 1H and 13C are very close when comparing the two tautomers, and the assignation is not certain. The case of clethodim is more confusing, since the R2 and MAE values suggest that all signals correspond to the same structure. DP4+ method, on the other hand, gave more reliable results (within the set of proposed structures). It is important to remark that DP4+ method has been used before for tautomer assignation in only one case (to the best of our knowledge), and it has also proven successful in this context of use. We concluded that metribuzin and diclosulam exist in solution in one major tautomeric form (aminotriazinone and sulfonamide, respectively), while two tautomers of similar energy (E - and Z -ketoenolimine complexed with DMSO) are proposed for clethodim. [Display omitted] [ABSTRACT FROM AUTHOR]

• (E)-N'-(2-hydroxy-5-(thiophen-3-yl)benzylidene)benzohydrazide (TBH) was successfully synthesized and characterized. • Optimization of molecular structure was performed by Fuzzy Logic Approach (FLA) and Density Functional Theory (DFT). • Structural and spectroscopic analysis of TBH molecule were executed with DFT/B3LYP/6–311(d,p) calculations. • The tautomeric behavior, chemical reactivity, charge transfer and hyperconjugative properties of the molecule were explained theoretically using DFT. The spectral properties, including 1H–13C NMR, FTIR, and UV–Vis, of the newly synthesized compound, (E)-N'-(2-hydroxy-5-(thiophen-3-yl)benzylidene)benzohydrazide (TBH) , were calculated using DFT/B3LYP/6–311G(d,p) and compared with experimental spectra. Experimental FTIR, UV–Vis, NMR and DFT calculations showed that the TBH was in the form of keto-amine/phenol-imine tautomer stabilized by intramolecular hydrogen bonding in solid state and solution. Theoretical calculations were also accomplished for the HOMO-LUMO energies, MEP surface, net atomic charges, and NBO analysis. The experimental and theoretical band gap (ΔE) values examined in different organic solvents for TBH were found to be in the range of 3.87–3.90 eV and 3.84–3.93 eV, respectively. The theoretical results revealed the molecule's high intramolecular charge transfer, realistic molecular stability, and high chemical reactivity. A good agreement was observed between the experimental data and the DFT-based calculations for the TBH molecule. Additionally, the DFT calculations were integrated with the fuzzy logic approach (FLA), and the Mamdani fuzzy logic system was used to pattern the potential energy of the molecule based on its two torsion angles. [Display omitted] [ABSTRACT FROM AUTHOR]

This comprehensive review focuses on spontaneous mutations that may occur during DNA replication, the fundamental process responsible for transferring genetic information. In 1963, Löwdin postulated that these mutations are primarily a result of proton transfer reactions within the hydrogen-bonded DNA base pairs. The single and double proton transfer reactions within the base pairs in DNA result in zwitterions and rare tautomers, respectively. For persistent mutations, these products must be generated at high rates and should be thermodynamically stable. This review covers the proton transfer reactions studied experimentally and computationally. The review also examines the influence of externally applied electric fields on the thermodynamics and kinetics of proton transfer reactions within DNA base pairs, and their biological implications. [ABSTRACT FROM AUTHOR]

• This study presents a new technique for computationally profiling thymol and geraniol as natural alternative repellents against mosquitoes. • Toxicity predictions using Protox-II, T.E.S.T, and SwissADME indicated that thymol and geraniol belong to toxicity class 4 and 5, respectively. • Off-target predictions via SwissTarget Predictions, LigTMap, Pharmapper, and SuperPred showed that these molecules can bind to 614 human proteins. • The degradation of thymol and geraniol were performed using xTB metadynamics and the outcomes showed that the degradants for both compounds were stable and had lower toxicity profile. • Nine tautomers were generated via quantum mechanics for thymol and four for geraniol, with RMSD ranging from 3.8 to 6.3 Å for thymol and 3.6 to 4 Å for geraniol after superimpositions. • DFT studies found that HOMO-LUMO values and electronegativity parameters of thymol and geraniol did not differ significantly from their isomers. • Binding affinity studies against 614 proteins, analysed via PCA and violin plots, highlighted the probable range of binding. With growing interest in natural compounds as alternative mosquito repellents, assessing the toxicity and structure of potential repellent naturals like thymol (monoterpene phenol) and geraniol (monoterpene alcohol) is vital for understanding their stability and human impact. This study aimed to determine the structural, toxicity, and binding profiles of thymol and geraniol using computational predictions, xTB metadynamics, quantum mechanics, and principal component analysis. Toxicity studies using Protox-II, T.E.S.T, and SwissADME indicated that thymol and geraniol belong to toxicity class 4 and 5, respectively, with low toxicity predictions in other endpoints. Overall pharmacokinetic profile was generated via pkCSM. Off-target predictions via SwissTarget Predictions, LigTMap, Pharmapper, and SuperPred showed that these molecules can bind to 614 human proteins. The degradation of thymol and geraniol were performed using xTB metadynamics and the outcomes showed that the degradants for both compounds were stable and had lower toxicity profile. Nine tautomers were generated via quantum mechanics for thymol and four for geraniol, with RMSD ranging from 3.8 to 6.3 Å for thymol and 3.6 to 4 Å for geraniol after superimpositions. DFT studies found that HOMO-LUMO values and electronegativity parameters of thymol and geraniol did not differ significantly from their isomers. Binding affinity studies against 614 proteins, analysed via PCA and violin plots, highlighted the probable range of binding. These multifaceted in-silico findings corroborate the stability and potential utility of thymol and geraniol as safer alternatives in repellent applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • The object reaction occurs via two subsequent elementary electrophilic attacks. • Each attack consists of two intermediates and a transition state. • The reaction is tautomero-selective. • Electronic structure and topological analysis indicate the product as a pseudocycle. The molecular mechanism of the phosphorylation reaction of 5,7-Dichloro-4,6-DinitroBenzofuroxane (DCDNBFX) by tertiary phosphines is clarified by means of a state-of-the-art computational investigation. The reaction with trimethylphosphine is used as a study case. The possible electrophilic attack sites are analysed and the most probable pathway is established. Structural, electrostatic, electronic and topological properties of the product are presented along with the analysis of the vibrational spectra. The results are corroborated by experimental evidences from the literature. [ABSTRACT FROM AUTHOR]

• Ni/Cu(II) complexes were derived from (E)-2-(((2-ethylphenyl)imino)(phenyl)methyl)phenol. • Molecular structures were established by single crystal X-ray diffraction. • Cu(II) complexes exhibited the polymorphic property. • Supramolecular and Hirshfeld surfaces analyses were discussed. • Antibacterial, antioxidant and electrochemical properties were examined. A new Schiff base proligand, (E)-2-(((2-ethylphenyl)imino)(phenyl)methyl)phenol (H L), was synthesized from 2-hydroxybenzophenone and 2-ethylaniline and its coordination chemistry with nickel(II) and copper(II) acetate explored. In solution, the proligand H L undergoes ketoimine-enolimine tautomerism which settles exclusively into an enolimine form in the solid-state. Interestingly, while a single crystalline habit was obtained for the nickel complex 1 (block-shaped single crystals), the copper species 2 was found to solidify in two detectable polymorphs: block-shaped crystals (2a , monoclinic space group P 2 1 / c) and rhombic crystals (2b , triclinic space group P -1). Single-crystal X-ray diffraction studies reveal that the N,O Schiff base ligand chelates the divalent metal ions with square-planar geometry in the isostructural 1 and 2a and in a distorted square-planar geometry with a dihedral chelate-plane angle of 27.9° in 2b. The copper complex 2 exhibits significant antibacterial activity against the E. coli and medium activity against S. aureus bacteria. The physical properties of the complexes were further probed using thermal techniques, while their electronic structure was investigated by electrochemistry and electronic absorption spectroscopy. [Display omitted] [ABSTRACT FROM AUTHOR]

• 4-hydroxyquinolin-2(1H)-one was investigated by IR spectroscopy & DFT calculations. • Two low-energy tautomers were identified in cryogenic matrices: cis-4HQ2O & 2,4-QO. • Tautomers differ slightly in energy; 2,4-QO is 4.8 kJ/mol above cis-4HQ2O. • UV irradiation of argon matrix induced formation of cis-2HQ4O & fragmentation. • Study enhances understanding of 4HQ2O structure & photochemistry. 4-Hydroxyquinolin-2(1 H)-one (4HQ2O) was synthesized, isolated in cryogenic matrices (argon and xenon), and studied by infrared spectroscopy. Quantum chemical calculations carried out at the DFT(B3LYP)/6–311++ G (3df,3pd) level of theory were used to determine the conformational and tautomeric properties of the molecule. Two tautomeric forms were identified in the as-deposited matrices with the help of the theoretical data. To investigate the photochemistry of the compound, in situ broadband ultraviolet (λ > 283 nm) irradiation of the as-deposited argon matrix was performed. This irradiation led to the generation of an additional tautomer, together with the products of fragmentation of the heterocyclic ring of the molecule, specifically isocyanic acid and carbon monoxide. Photoproducts such as 1,3-dihydro-2 H -indol-2-one and cyclohepta-1,2,4,6-tetraene were also observed in the photolyzed argon matrix. A comprehensive assignment of the infrared spectra of all the species observed experimentally is presented. [Display omitted] [ABSTRACT FROM AUTHOR]

• Electric fields impact guanine conformations: Guanine dynamically switches between keto and enol forms, affecting its reactivity. This study reveals distinct responses of these forms to electric fields, suggesting potential control over guanine's behavior. • Enhanced reactivity of keto form: The keto form shows a significant decrease in energy gap, especially in water, under electric field influence, indicating increased reactivity and potential for applications requiring active molecules. • Stabilizing effect on enol form: Conversely, the enol form exhibits a stabilizing effect with increasing electric field strength, suggesting potential use in scenarios requiring stable configurations. • Paving the way for guanine-based nanoelectronics: I-V curve analysis indicates promise for guanine, particularly the keto form in water, for nanoelectronic applications. The study also offers insights into solute–solvent interactions and molecular stability, crucial for material development. • Unveiling hidden potential: By understanding guanine's response to electric fields, this research opens doors for novel materials with specific properties tailored for various applications, expanding the potential of this key DNA/RNA building block. Guanines, key players in DNA and RNA, exist as dynamic keto and enol forms, influencing reactivity and potential applications. This study explores the impact of external electric fields on these forms using computational methods in gas and solvent (water) phases. Analyzing the energy gap, dipole moment, enthalpy, and Gibbs free energy revealed distinct responses. The keto form showed a significant decrease in the energy gap in the presence of an electric field, particularly in water, indicating enhanced reactivity. Conversely, the enol form exhibited a stabilizing effect with increasing electric field strength. Thermodynamic properties highlighted solvent-dependent interactions and electric field-induced changes in electron density distribution. The study further investigated guanine's potential for nanoelectronics by calculating I-V curves, revealing promising characteristics, especially for the keto form in the aqueous phase. Additionally, the analysis of solvation and cohesive energies provided insights into solute–solvent interactions and intrinsic molecular stability. Overall, this research contributes to understanding how guanine responds to electric fields, paving the way for novel guanine-based materials with potential applications in nanoelectronics. [ABSTRACT FROM AUTHOR]

Aggregation of profilin-1 (PFN1) causes a fatal neurodegenerative disease, familial amyotrophic lateral sclerosis (fALS). Histidine (His) tautomerism has been linked to the formation of fibril aggregation causing neurodegenerative disease. Characterization of intermediate species that form during aggregation is crucial, however, this has proven very challenging for experimentalists due to their transient nature. Hence, molecular dynamics (MD) simulations have been performed on the His tautomeric isomers εε, εδ, δε, and δδ of PFN1 to explain the structural changes and to correlate them with its aggregation propensity. MD simulations show that His133 presumably plays a major role in the aggregation of PFN1 upon His tautomerism compared to His119. Further, the formation of a new 3 10 -helix is observed in εε and δε but 3 10 -helix is not observed in δδ and εδ isomers. In addition, our findings unveil that β-sheet dominating conformations are observed in His119(δ)-His133(δ) δδ isomer of PFN1 with significant antiparallel β-sheets between residues T15-G23, S29-A33, L63-L65, Q68-S76, F83-T89, T97-T105, and K107-K115, suggesting a novel aggregation mechanism possibly occur for the formation of PFN1 aggregates. Overall, these results propose that MD simulations of PFN1 His tautomers can provide a detailed microscopic understanding of the aggregation mechanisms which are hard to probe through experiments. [ABSTRACT FROM AUTHOR]

• Energy profiles indicate that proton transfer is the rate-determining step in thiazaphospholidine synthesis. • The proposed mechanism involves tautomers and a multi-step approach, challenging simplistic views. • DFT modeling detailed the entire reaction process, including intermediates and energy barriers. • Predictions on isomer formation from asymmetrical reagents enhance experimental efficiency. • Reduced density gradient (RDG) analysis shows significant intermolecular interactions during the reaction. Phosphorylated heterocycles constitute an interesting class of molecules featuring complex, often unproven reaction mechanisms. Their synthesis is approached through experimental trial-and-error methods. Here we present a theoretical study of the reaction mechanism leading to the formation of thiazaphospholidine, as an example of such compounds. Our results reveal that proton rearrangement, rather than nucleophilic attack, plays a significant role in the rate-determining step of the reaction. This insight allows the prediction of which isomer will form from unsymmetrical reagents, potentially leading to more efficient experimental procedures for producing tailor-made structures that exhibit desired properties. We demonstrate that seemingly simple reactions, such as the nucleophilic substitution of an amino group on trivalent phosphorus, may require a more intricate explanation involving less favored tautomeric structures and a multi-step approach and are therefore not completely intuitive. [ABSTRACT FROM AUTHOR]

• Structures of reaction products between sulfolane cis - or trans -amino alcohols and aromatic aldehydes were defined • New sulfolane hydroxyimines were synthesized and characterized by 1D/2D NMR and X-ray techniques • cis -Imines in the CDCl 3 solution exist in equilibrium with two cyclic tautomers, while the corresponding trans -imines are not able to similar tautomeric transformations. The introduction of electron withdrawing substituents in the benzene ring, contributes to the increase of the content of cyclic (1,3-oxazolidine) forms • It was shown that in DMSO- d 6 solution equilibrium hydroxyimine-oxazolidine shifted towards hydroxyimine in time • Pharmacological profile of the products was evaluated in silico using ADMETlab 2.0 and SwissADME software Cyclic sulfones and their derivatives have vast applications in biological, pharmaceutical, medicinal and in many other fields. Herein, we report synthesis of ten new hydroxyimines bearing sulfolane moiety by reaction of cis - and trans -amino alcohols with aromatic aldehydes. An extensive spectroscopic characterization of the products, using 1D and 2D NMR techniques (1H, 13C, NOE, NOESY, COSY, HSQC, HMBC) showed ring-chain tautomerism in deuterochloroform solution for cis -hydroxyimines. The content of the cyclic (1,3-oxazolidine) form increases with increasing electron-withdrawing strength of the substituent in the para position of the benzene ring of the imine. The structure of cis -(3 S ,4 R)-3-hydroxy-4-((E)-(2-hydroxybenzylidene)amino)tetrahydrothiophene 1,1-dioxide was confirmed using XRD analysis. In addition, ADMET properties of products were evaluated in silico and showed high drug similarity. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Proton carriers have a critical role in enhancing proton transport on the proton exchange membrane, which improves fuel cell efficiency. • The triazole molecule is a molecule with high energy stability that is formed of three N-atoms as a proton donor and acceptor. • The pairs of 1,2,4-triazole and protonated 1,2,4-triazole molecule consist of symmetric and asymmetric pair configurations. • Proton transfer in symmetric and asymmetric pairs depends on the distance between the proton donor and acceptor to generate a double well potential. • The solvents influence proton transfer in 1,2,4-triazole molecules through changes in the proton donor–acceptor distance. Triazole can be used as an effective proton carrier because it has one proton donor and two proton acceptors. Density functional theory calculations were performed to explore the proton transfer mechanism of the triazole molecules in gas and solvents. We employed implicit solvent method to analyze the increase of proton donor–acceptor distance (PDAD) in proton transfer mechanism. The solvents reduce the relative energy in tautomers of 1,2,4-triazole molecules, and their protonated form. Proton transfer occurs only at symmetric pairs in the gas, small PDAD, whereas proton transfer in an asymmetric pair is possible in solvents as the PDAD increases. [ABSTRACT FROM AUTHOR]

• Defect–free, ordered COF composite membranes was prepared by the improved IP process. • Monomers synthesis process and self–assembly of the oligomers were decoupled. • Self-assembly of the oligomers occurs on the support surface without being disturbed. • The TpPa–II/PMIA composite membrane exhibited excellent dye separation performance. • It maintains both its performance and structural stability in long-term filtration. Covalent organic frameworks (COFs) are crystalline porous materials composed of organic building blocks linked by covalent bonds, which have attracted great attention for membrane–based separation applications. However, fabrication of COF membranes with high quality and performance remains a challenge, especially for the β –ketoenamine linked COFs. In the conventional IP methods, the synthesis process (the Schiff–base reaction of the monomers and the tautomerism of the oligomers) and the crystallization process (the self–assembly of the oligomers) of the COFs materials occur simultaneously on the surface of the support membrane, which can result in low crystallinity, high defect density, and poor separation performance of the COF layer. In this study, a novel strategy for fabricating large-area, defect–free COFs composite membranes by interfacial polymerization (IP) was presented to overcome this problem. By incorporating both the amine and aldehyde monomers in one organic phase, and dissolving organic acid catalyst in another phase (the water), the synthesis process mostly take place in the bulk of the organic phase, while the crystallization occurs on the surface of the support membrane under the influence of the organic acid catalyst. This strategy effectively decoupled the synthesis process with the crystallization, providing a stable growth region for crystalline layer, ultimately leading to the formation of a thin, defect–free and highly ordered COF layer on the support membrane. The strategy was demonstrate by synthesis of triformylphloroglucinol phenylenediamine (TpPa)–Ⅱ/Poly(m–phenylene isophthalaminde) (PMIA) composite membrane. We characterize the structure and morphology of the TpPa–Ⅱ/PMIA composite membrane by various analytical techniques to verify our hypothesis, and evaluate its separation performance using pure water permeance and dyes retention rate as indicators. The results show that the TpPa–Ⅱ/PMIA composite membrane has an 80 nm thick top-layer with regular COF structure and orderly pores. It exhibits an excellent dye separation performance, with a (pure water permeance) PWP of 105.6 L·m−2·h−1·bar−1 and CR rejection of 99.2 %, which surpasses most of the TpPa–type COFs composite membranes reported in literature, as well as the reference (TpPa–I/PMIA membrane) in this work. Moreover, the TpPa–Ⅱ/PMIA composite membrane maintains both its performance and structural stability during a 240–hour cross–flow filtration of the dye solution, demonstrating its excellent potential in the dye wastewater treatment. [ABSTRACT FROM AUTHOR]

[Display omitted] • Spectroscopic studies of hyaluronan-curcumin conjugate prepared with a green procedure. • Increased stability for the curcumin bound to the hyaluronan under slightly basic solution. • Self-assembling behavior of the developed hyaluronan-curcumin conjugate. • Micellar self-aggregation of HA-CUR is able to protect the curcumin. A hyaluronan-curcumin conjugate was synthesized by employing a green procedure, the final aim was to improve the aqueous solubility and the bioavailability of the curcumin by reducing its hydrolytic degradation. A comprehensive study on the obtained macromolecular system was carried out in order to investigate the integrity and the stability of the curcumin bound to the polysaccharide. These findings may help for the evaluation of the therapeutic potential of the developed curcumin vehicle. In particular, comparative spectroscopic studies were performed in different system solvents for both, pure curcumin and curcumin conjugated to hyaluronic acid, at different temperature and different pH values. An increased stability for the curcumin bound to the hyaluronic acid was found under slightly basic aqueous solution, while the Uv–vis as well as the fluorescence spectral features ensures that the chemical structure of the curcumin is fully retained. The self-assembling behavior of the developed hyaluronan-curcumin conjugate (HA-CUR) was also investigated. It is likely that the micellar self-aggregation is able to protect the curcumin, embedded in the inner core, from the hydrolitic decomposition. [ABSTRACT FROM AUTHOR]

• Tautomeric equilibrium in solution of kynurenic acid derivatives. • Use of NMR to study the behavior of kynurenic acid derivatives. • Correlation between experimental and computationally calculated NMR d values. The 4-quinolinone derivatives present a keto-enol tautomerism, which can be studied using different analytical techniques, including NMR. In this work, the spectroscopic study in solution of numerous derivatives with different substitution patterns in positions 1–4 of the quinolone rings, and their correlation with simulated spectra by density functional theory-gauge independent atomic orbitals method (DFT-GIAO) approximation is presented. Polyfunctionalized derivatives where the tautomeric equilibrium is blocked by N - or O -substitution allow us to observe the different behavior in chemical shifts by NMR. The study of some derivatives in the solid state by ss-NMR and X-ray Crystallography is also included. [Display omitted] [ABSTRACT FROM AUTHOR]

• The tautomeric interconversions of pyrrole-based Schiff-base ligands are discussed. • Metal ion- and anion-coordination-driven tautomeric changes are presented. • Tautomerism alters the ligating behaviour in primary & secondary coordination spheres. • Two tautomers: imino-pyrrole & amine-azafulvene are described in solid and solution. • The detailed spectroscopic and crystallographic signatures are highlighted. Pyrrole-based Schiff-base ligands normally exist in two self-adaptable tautomeric forms: imino-pyrrole and amine-azafulvene. Upon metal ion or anion coordination, the imino-pyrrolic forms of the ligands sometimes undergo structural transformations to adopt amine-azafulvene forms and vice versa. The tautomerization alters the ligating behaviour of the molecules in both primary (anionic coordination versus dative coordination) and secondary (H-bond acceptor versus H-bond donor) coordination spheres, and helps in stabilizing a variety of metal ions with different oxidation states. This property of the ligands guides to understand the mechanisms of several enzymatic functions at the active sites. This review article highlights the coordination-driven tautomeric changes in various mono-, bis-, and tris-pyrrolic Schiff-base ligands. The detailed spectroscopic (1H NMR, IR) and crystallographic (single crystal XRD) signatures are also described in identifying the two tautomeric forms both in the solid and solution states. The coordination-induced structural transformations of two tautomeric forms (imino-pyrrole and amine-azafulvene) of pyrrole-based Schiff-base ligands which offer variable primary and secondary coordination spheres are discussed along with their spectroscopic and crystallographic signatures. [Display omitted] [ABSTRACT FROM AUTHOR]

• It is shown that the tautomeric state plays a crucial role for the optical properties of azo dyes. • Currently there is no suitable DFT approach to predict the optical spectra of tautomeric azo dyes. • Do not neglect the tautomeric state. In a paper of Zhu and co-workers ("Study of the electronic effect and quantitative spectra predictions of o-methoxyanilineterminated monoazonaphthols: a combined experimental and DFT study" by Guoxun Zhu, Yan Lin, Wenxian Zhou, Huacan Song and Zhengquan Li, RSC Advances, 2023, 13 , 33736), the authors derived a protocol for the absorption spectra prediction is sulphonated azo naphthols assuming that they exist as single azo tautomers. However, as it is well known before and as shown by the available experimental data and the calculations, performed in this note, the studied dyes exist almost solely in hydrazone tautomeric form, which unavoidably affects the conclusions and linear relations made in the original paper. A detailed benchmarking on the spectra prediction, involving 39 density functionals (33 of them hybrid and 6 – pure) indicates that there is no a functional correctly predicting the absorption spectra of the tautomers in azo naphthols. [ABSTRACT FROM AUTHOR]

The porphycene with different tautomerization configurations has attractive applications in molecular switch. However, the unambiguous identification of porphycene tautomers is difficult owing to the transient transformation processes. Here, we theoretically demonstrate that the ultrafast non-resonant Raman imaging can effectively identify and distinguish different tautomers, even the deuterated counterparts, providing both structural and chemical information of vibrational modes for a single porphycene molecule. Besides, the intramolecular hydrogen bonding in porphycene could affect the details of images, which in return offers a visualization of hydrogen bonding effects. These findings serve as good references for Raman images on proving single molecular transformation. [Display omitted] • Non-resonant Raman images can identify different tautomers of deuterated porphycene. • The effects of isotope and intramolecular hydrogen bonds can be visualized. • Raman images provide both structural and chemical information of vibrational modes. [ABSTRACT FROM AUTHOR]

• A 0.5-Å atomic displacement can lead to a dramatic change in desalination behavior. • TpHZ-OH show weaker adhesion to water molecules than TpHZ-NH does. • TpHZ-OH has smoother channel paths for water molecules to transport. • TpHZ-OH multilayer can provide 100% rejection to NaCl. Covalent organic frameworks (COFs) have gained increasing interest as promising building blocks for desalination membranes due to their inherent porosity. Efforts have been made to prepare COF membranes and explore their transport mechanisms, however the effect of tautomerization has not been reported. Using non-equilibrium molecular dynamics, we discover here that a mere 0.5-Å atomic displacement between two tautomers of an imine-based COF (TpHZ) can surprisingly lead to dramatic changes in the desalination behavior of their multilayers. The tautomeric transformation (from TpHZ-NH to TpHZ-OH) improves the water permeability by about 1.6–3 times. The interior resistance rather than the entrance resistance is found to dominate the water transport. The weaker adhesion and the smoother channel path of TpHZ-OH result in a much lower interior resistance. Furthermore, NaCl rejection increases from ∼ 89 % to 100 % due to the stronger sieving effect resulting from the reduced compensatory effect of TpHZ-OH for ion hydrations. [ABSTRACT FROM AUTHOR]

[Display omitted] N -Benzyl aldimines react with arylacetylenes in the presence of ButOK/DMSO superbase system to afford 2,3,5-triaryl-1-pyrrolines as two tautomers with 1,2- and 1,5-location of the double bond, both being the trans -diastereomers. This version of the C=N bond ethynylation differs from the previous one with N- benzyl ketimines. The oxidation of the pyrroline tautomeric mixtures without their isolation gives 2,3,5-triaryl-1 H -pyrroles. [ABSTRACT FROM AUTHOR]

Aggregated amyloid β-peptide (Aβ) in small oligomeric forms inside the brain causes synaptic function disruption and the development of Alzheimer's disease (AD). Histidine is an important amino acid that may lead to structural changes. Aβ42 monomer chain includes 3 histidine residues that considering two ε and δ tautomers 8 isomers, including (εεε) and (εδδ) could be formed. Molecular dynamics simulation on homodimerization of (εεε) (the most common type of tautomers) and (εδδ) tautomers with different initial configurations using monomer chains from our previous work were performed to uncover the tautomeric behavior of histidine on Aβ42 aggregation in a physiological pH which is still largely unknown and impossible to observe experimentally. We found a higher propensity of forming β-sheet in (εδδ) homodimers and specifically in a greater amount from Aβ42 than from Aβ40. A smaller amount of β-sheet formation was observed for (εεε) homodimers compared with (εδδ). Additionally, interactions in (εδδ) homodimers may indicate the importance of the hydrophobic core and C-/N-terminals during oligomerization. Our findings indicate the important role of the tautomeric effect of histidine and (εδδ) homodimers at the early stage of Aβ aggregation. • (εδδ) tautomers show a high propensity to generate β-sheets during homodimerization. • (εδδ) homodimers may play an important role at the early stage of Aβ aggregation. • Histidine tautomerism can affect the structural properties of the amyloid peptides. [ABSTRACT FROM AUTHOR]

[Display omitted] • Thiol tautomers were photogenerated from the thione forms of N -methylthiourea, thiobenzamide and 2-cyanothioacetamide. • The thiol tautomers of N-methylthiourea and thiobenzamide were transformed into the thione forms by hydrogen-atom tunneling. • For 2-cyanothioacetamide, one of the photogenerated S-H rotamers was converted into another by tunneling of hydrogen atom. The most stable thione tautomeric forms of N -methylthiourea, thiobenzamide and 2-cyanothioacetamide were isolated in low-temperature argon matrices. The higher-energy thiol tautomers of these compounds were generated upon irradiation of matrix-isolated monomers with UV (λ > 270 nm) light. For N -methylthiourea and thiobenzamide, kept in the dark at 3.5 K for a long period of time, a spontaneous thiol → thione hydrogen atom tunneling transformation occurred. Only the thiol isomers with the favorably oriented hydrogen atom of the imino group underwent these hydrogen-atom tunneling processes. The other thiol isomers, with the hydrogen atom of the imino group oriented towards the sulfur atom, did not undergo the thiol → thione conversion. For the photogenerated thiol forms of 2-cyanothioacetamide, no spontaneous thiol → thione tautomeric transformation was detected. Instead, only the spontaneous conformational change of one S-H rotamer of the thiol 2-cyanothioacetamide tautomer into the other S-H rotamer was observed. [ABSTRACT FROM AUTHOR]

The formation of an ion-association complex (IA) between sulfonephthalein dye and basic nitrogen-containing compound in an organic solvent medium has been for the first time used to develop an automated SIA method. In highly polar aprotic solvents, the tautomeric equilibrium for such dyes is strongly shifted towards the colorless lactonic form. The addition of a basic nitrogen-containing substance leads to the formation of IA with a highly colored quinonoid form, which is accompanied by an increase in the absorbance of the dye band at approximately 400 nm. Protonation of pyridine nitrogen in loratadine, structure and binding places of IA were shown using quantum-chemical calculations. The very simple, direct and non-extraction spectrophotometric SIA method with high throughput of 43 h−1 was developed based on the formation of IA between loratadine and bromocresol purple in the medium of acetonitrile used both as solvent and carrier. The calibration graph was linear in the concentration range from 1.0 to 20 mg L−1 with correlation coefficient of 0.9992. The developed method was successfully applied to the analysis of pharmaceutical formulations. [Display omitted] • Interaction of sulfonephthalein dyes and nitrogen-containing compounds. • Shift of tautomeric equilibrium in an organic solvent medium. • Non-extraction spectrophotometric SIA method for loratadine determination. • Loratadine was determined using its ion associate with bromocresol purple. [ABSTRACT FROM AUTHOR]

Tautomers are widespread in organic chemistry, and their unique reactivity allows diverse transformations. In this work, the incorporation of nearby amide ring-fusion site offered a versatile platform for controlling dual reactivity based dynamic covalent chemistry (DCC) of aldehyde ring-chain tautomers. A suite of 2-formylbenzosulfonamide derived tautomers were prepared, and the intramolecular ring-chain equilibrium and intermolecular dynamic covalent reactions along the underlying mechanistic foundation were investigated. By regulating the substituent electronic effect and solvent effect, ring-chain tautomerization equilibrium was readily regulated in solution. The adjacent amide NH participates in intramolecular hydrogen bonding, as revealed by crystal analysis. Furthermore, the addition of acid and the change of solvent allowed turning on/off the intramolecular cyclization from the amide group to create fused ring scaffolds. Finally, the reversible covalent reactions with amine nucleophile reagents maintained the intermolecular reactivity successfully. The molecular and pathway diversity reported herein sets the scene for future design and applications. [Display omitted] [ABSTRACT FROM AUTHOR]

• 2-(2-pyridyl)-phenalene-1,3-dione (PPD) exists in diketo form in the solid state. • large Stokes shifts (> 160 nm in DMSO and ethanol) indicate on possible ESIPT process. • DFT calculations in ground state and excited singlet state suggest reverse stability order of the OH and NH tautomers. • photostability of PPD was compared with that of 2-acetyl-1,3-indandione (2AID). • PPD exhibits moderate photostability both in ethanol and in DMSO. Tautomerism of cyclic β-diketones has been of scientific interest for decades due to the practical applications it can render, from sensors to photostable materials. In this work the tautomerism of 2-(2-pyridyl)-phenalene-1,3-dione (PPD) was studied in solid state, in solution, and in gas phase through a combination of experimental and theoretical methods. Single-crystal X-ray diffraction analysis of PPD revealed its existence in diketo form in the solid state with protonated pyridyl nitrogen that is stabilized by the intramolecular hydrogen bond of N-H···O type. A very large Stokes shift was observed in the emission spectra in DMSO (ca. 177 nm) and in ethanol (ca. 160 nm), indicating on possible excited-state intramolecular proton transfer (ESIPT) process. DFT calculations in ground state (gas phase and water medium) and in the excited singlet state (B3LYP-6311++G(d,p)) agree very well with the experimental data and suggest reverse order of the possible OH and NH tautomers in the ground and the excited singlet state, namely the diketo NH tautomer is the most stable one in the ground state whereas in the excited singlet state the enol OH form is more stable. The photostability of PPD was estimated by irradiation experiments that were performed in comparison with 2-acetylindan-1,3-dione (2AID), which is a compound with proven high photostability in ethanol and fast ESIPT. The data showed that unlike 2AID, the 2-(2-pyridyl)-phenalene-1,3-dione exhibits lower photostability in ethanol. These results could suggest that either the ESIPT process is not the only transformation that takes place in the excited state of PPD, or it is not as fast as the one of 2AID. [Display omitted] [ABSTRACT FROM AUTHOR]

A series 4,6-diaryl-2-(1-hydroxy-4-nitronaphthalen-2-yl)pyridines were prepared by Kröhnke pyridine synthesis method. The tautomerism and fluorescent properties of the synthesized compounds are considered depending on the structure, the polarity of medium under neutral conditions and in the presence trifluoroacetic acid in DMSO. The resulting ESIPT fluorophores are characterized by dual emission and exist in two tautomeric forms under neutral conditions in DMSO. [Display omitted] • New ESIPT fluorophores of 2-(ortho -hydroxyaryl)pyridine series. • A simple synthetic procedure is described. • Double emission of resulting compounds. • Enol-imino keto-amine tautomerism in polar solvent. • Effect of 4-nitro-α-naphthol on the fluorescence of ESIPT fluorophores. [ABSTRACT FROM AUTHOR]

[Display omitted] • p -Hydroxyazobenzocrowns have been characterized with spectroscopic methods. • For the first time quantum yields values for p -hydroxybenzocrowns are given. • Fluorescence properties are dependent on macrocycle size. • Substituents in benzene rings affect the quantum yield values. The spectroscopic properties of a series of para -hydroxyazobenzocrowns, including three novel compounds, were investigated using UV–Vis absorption and emission spectroscopy. This study presents, for the first time, determined quantum yield (QY) values for macrocycles of this category, ranging between 0.122 and 0.195. The highest values were obtained for crowns bearing two phenyl substituents in benzene rings. The impact of aromatic ring substituents and macroring size on the spectral characterization (1H NMR and FTIR) of p -hydroxyazobenzocrowns was examined in consideration of the azophenol ⇄ quinone-hydrazone tautomeric equilibrium. Dipole moments of p -hydroxyazobenzocrowns in the ground and excited states have been determined. The alignment between experimental findings and theoretical studies was established. [ABSTRACT FROM AUTHOR]

• Ru-catalyzed high chemoselectivity hydroformylation. • New application of thiazolinyl-based phosphine in carbonylation. • Wide substrates scope including linear, internal and cyclic olefins in hydroformylation. A catalytic system comprised of Ru 3 (CO) 12 , thiazolinyl-based phosphine (L1), and HBF 4 enables the efficient hydroformylation, which the tautomer L1b is more favorable for the occurrence of the reaction as well as the additive HBF 4 is beneficial for the formation of ruthenium hydrogen active species. The formation and stabilization of the critical Ru-complex intermediates were verified by the DFT calculations. The hydroformylation proceeds with high chemical selectivity especially for the linear, internal and cyclic olefins (conversion up to 99 %, selectivity of aldehysdes up to 92 %). [Display omitted] [ABSTRACT FROM AUTHOR]

The azo-hydrazone tautomerism of four arylazoresorcinols, where aryl is phenyl, naphthyl, p -nitrophenyl and o -nitrophenyl group, was studied using the UV–vis spectroscopy of solutions and solid substances, the X-ray crystallography and DFT calculations. In solutions, all the studied compounds exist predominantly as the azo tautomers. In crystals, the tautomeric equilibrium shifts toward the keto hydrazone form under the effect of intermolecular O–H⋯O hydrogen bonds. This conclusion is supported by analysis of the spectroscopic and crystallographic literature data. • In solutions, all the studied azoresorcinols exist predominantly as azo tautomers. • In crystals, azoresorcinols can transform to keto hydrazone tautomers. • The keto hydrazone form is stabilized by intermolecular hydrogen bonds. [ABSTRACT FROM AUTHOR]

15N NMR data (1 J (15N, 1H) coupling constants and 15N chemical shifts) were used to characterize quantitatively azo-hydrazo tautomerism in some industrially produced azo dyes. Both 1 J (15N, 1H) coupling constants and 15N chemical shifts were obtained at the natural abundance level of 15N using inverse detection of resonances. All three possible types of azo dye existence were found: (i) dyes existing completely in azo forms, (ii) dyes existing completely in hydrazo forms or (iii) dyes forming azo-hydrazo tautomeric equilibria. [Display omitted] • 15N NMR spectra were measured at the natural abundance of 15N isotope with the aim to characterize azo-hydrazo tautomerism • This article focusses on characterization of azo-hydrazo tautomerism in some industrially produced azo dyes. • Dyes can exist completely in azo forms, or completely in hydrazo forms or forming azo-hydrazo tautomeric equilibria. • 15N NMR spectra are an ideal tool for such a differentiation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Present work explores contrasting photodynamic modulations of 1HAN upon interaction with HPβCD and HPγCD host molecules. • Both 1HAN-HPβCD and 1HAN-HPγCD systems display significant fluorescence enhancement, and the stochiometry obtained is 1:1. • Rotational relaxation times for the 1HAN-HP(β/γ)CD systems obtained were substantially longer compared to the free 1HAN. • The equilibrium between the K1* and K2* tautomers of 1HAN bound to HPβCD shifts towards the K1* form of 1HAN. • In contrast, the ES equilibrium between keto tautomers shifts towards the K2* form of 1HAN bound to HPγCD host. Confined environments, such as cyclodextrins, offer a convenient method to modulate the excited-state (ES) photodynamics of a probe molecule, and studies on such topics have attracted tremendous attention in numerous biological and chemical fields. Current study explores the photodynamic modulations of an ESIPT probe, 1′-hydroxy-2′-acetonaphthone (1HAN), by two homologous cyclodextrin derivatives having different cavity sizes, namely hydroxypropyl-β-cyclodextrin (HPβCD) and hydroxypropyl-γ-cyclodextrin (HPγCD), using multispectroscopic techniques. 1HAN shows substantial fluorescence enhancement after forming 1:1 inclusion complexes with both of these host molecules. Time-resolved (TR) anisotropy studies indicate substantially longer rotational relaxation times for the 1HAN-HP(β/γ)CD systems compared to the free 1HAN, further supporting the formation of inclusion complexes in the current systems. Intriguingly, TR fluorescence studies reveal contrasting modulation of the ES equilibrium between keto (K1*) and twisted keto (K2*) tautomers of 1HAN by HPβCD and HPγCD hosts, respectively. In the presence of the HPβCD host, the ES equilibrium between keto tautomers of 1HAN is shifted more towards the K1* form. On the contrary, the ES equilibrium shifts more towards the K2* form of the dye in the presence of the HPγCD host. This contrasting modulation in the ES equilibrium between K1* and K2* tautomers of 1HAN indicates that the dimension of the interior cavity of the host and hydrophobic interactions play a decisive role in regulating the photodynamics of the excited 1HAN dye. [ABSTRACT FROM AUTHOR]

Edaravone is a heterocyclic pyrazolone compound. It has pronounced effect against free radicals, however renal and hepatic disorders have been reported. Isoxazolones are considered bioisosteric analogues of pyrazolones and may have comparable properties. Thus, we investigated the structural and electronic influences for edaravone, isoxazolone, and their tautomers on antioxidant process. Structure and tautomerism study among edaravone, isoxazolone and their heterocycles derivatives were related to antioxidant mechanisms by using the hybrid DFT method B3LYP with the basis sets 6-31++G(2d,2p). The C H tautomer was the most stable and energetically favored among them. Intramolecular N H N hydrogen bonds and polar medium were responsible for the low energy differences among all possible tautomers. N H tautomers in both systems proved to be better antioxidant by SET (single electron transfer), while O H tautomers were better antioxidant on HAT (homolytic hydrogen atom transfer) mechanism. Theoretical calculation showed that edaravone is more potent than phenylisoxazolone, however, both has similar antioxidant scavenging on experimental DPPH. The carbonyliminic system played a very important role in the antioxidant activity for both studied classes. [ABSTRACT FROM AUTHOR]

Imidazopyridines can exist in several tautomeric forms such as benzimidazole or purine condensed systems. Regioselectivities were determined for N -alkylations of 2-(3,4-dimethoxyphenyl)- imidazopyridines and their 4 and 5-oxides (2 – 4 , 13 , 14) with n -butyl and 4-fluorobenzyl bromides under basic conditions (K 2 CO 3 in DMF). It was observed that N -4 (5 – 8) and N -5 (15 – 17) regioisomers were mainly formed. Compounds 7 (N 4) and 7a (N 1) were separated from the mixtures of regioisomers in a 50 : 1 ratio. Their structural assignments were made with the use of two-dimensional 1H-1H NOE (nuclear overhauser effect spectroscopy [NOESY]) enhancements between the N -CH 2 and protons on the C-4, 5, 6, and 7 positions of the pyridine moiety. To verify the NOESY data, synthesis of compounds 7a and 7b was achieved by the selective method. Complementary structural information was provided by 2D-HMBC spectra of the compounds. Image 1 • N -benzylation of imidazo[4,5- b ]pyridine 4-oxide under basic condition in DMF afforded the mix of N -4 and N- 1,3 regiosomers. • N -benzylation of imidazo[4,5- c ]pyridine 5-oxide under basic condition in DMF afforded only N- 5 regiosomers. • NOESY experiment is the best available method for the structural elucidation of these regioisomers. • HMBC experiment could be effective for assignment of the regisomers as alternative way. [ABSTRACT FROM AUTHOR]

A series of N -substituted-4-thiocarbamoyl-5-pyrazolone derivatives (HL1-HL4) is presented as chelating agents for complexation with Fe(III), Ni(II) and Cu(II) metal ions. The synthesized pyrazolone ligands and their newly metal complexes are characterized by different spectral and analytical methods such as UV–Vis, IR, 1H NMR, 13C NMR, ESR, MS, magnetic measurement, and TGA. The spectral data reveal that ligands coordinated to metal ions in a bidentate pattern via O & N atoms of the OH group at C(5) and thiocarbamoyl (–CSNHR) at C(4) of the pyrazolone ring. Also, the analytical data suggest the stoichiometries 2:3 (M:L) for both Cu(II) & Ni(II) complexes and 1:3 for Fe(III) complexes. Besides, the normal magnetic moments values for Fe(III) complexes confirm high spin octahedral structure while the diamagnetic nature of all Ni(II) complexes is consistent with square planar geometry. However, the subnormal magnetic values for Cu(II) complexes suggest the proposal of their binuclear structures. The ESR spectra of the Cu(II) complexes support the distorted square planar geometry with a considerably strong intradimeric spin-exchange interaction. Moreover, the anticancer, antibacterial and antifungal activities are screened. Among the synthesized compounds, HL4 ligand exhibits a significant broad spectrum of action against Gram-positive (S. aureus), Gram-negative bacteria (P. vulgaris), and antifungal potency against A. fumigatus & C. albicans in comparison with gentamicin and ketoconazole drug. Such potency of HL4 could be related to the insertion of the p -chloro in the phenyl group attached to the pharmacophoric thiocarbamoyl group at C(4). Furthermore, IC 50 values of two Cu(II) complexes derived from HL2 and HL3 display nearly twofold or threefold more cytotoxicity impact against three cell lines (MCF-7, HCT116 and HepG-2) compared with cis -platin as positive control. [ABSTRACT FROM AUTHOR]

In this work, we provide a realistic description of the tautomers of [2,2′-bipyridyl]-3,3′-diol (BPOH2) and their relative populations in water solution, eventually validating the computational predictions through the comparison of the simulated absorption spectrum with the one experimentally measured at ambient conditions. To this end, we present an original computational protocol, which exploits accurate and specific quantum mechanically derived force-fields in a MD/QM sequential procedure, allowing for a first principle characterization of all BPOH2 tautomers, including their thermalized conformational dynamics and relative Boltzmann weights. To our knowledge, this represents an unprecedented result demonstrating that the lower energy bands, appearing in water, are due to the absorption of the diketo tautomer of BPOH2, which is stabilized by a strong and specific hydrogen bond network. Considering its general applicability, the proposed computational procedure paves the way to future application on more complex systems of biological or technological relevance, as for instance protic solvents micro-confined in large organic aggregates. • Water stabilizes the diketo tautomer of [2,2'bipyridyl]-3,3'-diol. • Only an explicit solvation model delivers realistic populations for the tautomers. • The simulated absorption spectrum is in very good agreement with experiments. [ABSTRACT FROM AUTHOR]

[Display omitted] • The article contains a literature survey of metal Schiff base complexes formed primarily by the ligation behavior of Schiff bases having tridentate ONO pyridyl hydrazone donor sites with different metals. • Various documented perspectives on the synthesis and structural elucidation of these complexes have been discussed. • The structural aspects documented in the literature have been analyzed and illustrated for a better understanding. • The applications of these Schiff base metal complexes have been included for deciphering the significance of these complexes. Schiff bases are an extensive group of materials which have a carbon–nitrogen double bond (–C = N–). The present review gives a perspective on this class of compounds by highlighting the significance of various types of tridentate ONO pyridyl Schiff base ligands and their corresponding metal complexes. The review focuses on the reactions of salicylaldehyde and its derivatives with different acylhydrazides, such as picolinohydrazide, nicotinohydrazide and isonicotinohydrazide, in which the single crystal structures of the resulting products have been reported in the last two decades (2002–2022). This class of Schiff bases show amide-iminol tautomerism, in which the carbon–nitrogen double bond is a component of the iminol tautomer. The molecular structure, spectral analysis and applications of ONO pyridyl Schiff base complexes are covered in this article. [ABSTRACT FROM AUTHOR]

Eight structurally diverse rearranged sesquiterpenoids, including seven undescribed sesquiterpenoids (1a/1b and 3 – 8) were obtained from the aerial parts of Daphne penicillata. 1a/1b , 3 , 5 and 6 possess rare rearranged guaiane skeletons and 4 represents the first example of rearranged carotene sesquiterpenoids. Their structures and absolute configurations were determined by extensive spectroscopic analyses, NMR and ECD calculations. Interestingly, 1a and 1b were a pair of magical interconverting epimers that may interconvert by retro-aldol condensation. The mechanism of interconversion has been demonstrated indirectly by 9-OH derivatization of 1a/1b and a hypothetical biogenetic pathway was proposed. All compounds were evaluated for anti-inflammatory and cytotoxic activities. Among them, 1a/1b and 2 exhibited potential inhibitory activities on the production of NO against LPS-induced BV2 microglial cells. Eight structurally diverse rearranged sesquiterpenoids, including a pair of magical interconverting epimers (Daphneterpenoids A, 1a/1b) that may interconvert by retro-aldol condensation, were isolated from the aerial parts of Daphne penicillata. [Display omitted] • Eight rare rearranged sesquiterpenes were isolated from Daphne penicillata. • Their structures were determined by extensive spectroscopic, NMR and ECD analysis. • Daphneterpenoids A (1a/1 b) were a pair of magical interconverting epimers. • A hypothetical biogenetic pathway of 1–6 was proposed. • All compounds were evaluated for anti-inflammatory and cytotoxic activities. [ABSTRACT FROM AUTHOR]

We explore the excited-state intramolecular proton transfer (ESIPT) phenomenon in hydroxy-substituted aza-[5]-cycloparaphenylene (HA-[5]CPP). Computed excitation energies show well-separated S 1 and S 2 states, with the latter possessing a relatively higher oscillator strength. Upon photoexcitation to S 2 , the molecule undergoes an intramolecular proton transfer over a barrierless pathway to relax at the proton-transferred tautomeric form, demonstrating anti-Kasha photochemistry. Further, well-separated S 1 and S 2 state minima of the tautomers allow a direct fluorescence emission from S 2 , thus making it a potential anti-Kasha emitter. [Display omitted] • Barrierless ESIPT. • Anti-Kasha photochemistry. • Potential anti-Kasha emitter. [ABSTRACT FROM AUTHOR]

• Synthesis of a new derivative of mono-cyclopentyl-substituted bismuthiol. • X-ray crystallography demonstrated the existence of C=S and C–N–H for the new product. • Investigate the differences in thermal stability and phase transitions of three bismuthiol derivatives. • Potential of mono-cyclohexyl-substituted bismuthiol as an anticancer and antitumor compound. • A more potent agent than cefazolin against negative and positive bacteria. A new derivative of 2-cyclohexyl-substituted bismuthiol was synthesized via a sequential one-pot procedure in ethanol under reflux conditions. After many trials, the long colorless needle-like crystals could be isolated, suitable for X-ray crystallography. The chemical structure of the new bismuthiol derivative was characterized by spectroscopic techniques. The characteristic vibrations for differentiation of thiol-thione tautomers were defined regarding the comparative FTIR study of bismuthiol and two of its derivatives viz mono- and bis-cyclohexyl-substituted bismuthiol. The presence of a hyper-conjugative interaction between the S–H and π bond of C=N was indicated. In addition, the existence of C=S and C–N–H for the new product was demonstrated through single crystal XRD analysis, which supports thiol or thione tautomer determination using IR spectra. The TGA/DTA and DSC profiles of bismuthiol and mono- and bis-cyclohexyl-substituted bismuthiol were studied to determine the influence of hydrogen bonding, packing, and symmetric and unsymmetric structure on the transition phases and thermal stability. Finally, the biological activity of bismuthiol and bis- and mono-cyclohexyl-substituted bismuthiols were screened in silico by SwissADME tool and in vit ro against the gram-negative (Escherichia coli ATCC 25,922) and gram-positive (Staphylococcus aureus ATCC 25,923) bacterial species. [Display omitted] [ABSTRACT FROM AUTHOR]

Hydrogen bonds between solute and protic solvent molecules by explicitly adding hydrogen bonding solvent molecules significantly affect the energies of hydrogen-bonded clusters. [Display omitted] • Solvation study of the photophysics of 2-(2′-Hydroxyphenyl)oxazolo[4,5- b ]pyridine (HPOP) in the ground and first excited singlet states taken into account the polarity and hydrogen bonding capability of the solvent. • HPOP exists in cis -closed enol, cis -open enol and cis -keto and their corresponding trans forms. • cis -Closed enol and cis -keto form are the most stable forms in the ground state and the excited state, respectively in vacuum, acetonitrile and methanol. • Polarity of the solvent and explicit solute–solvent hydrogen bond play important role in the relative stability of the different conformer and tautomers both in the ground and excited states. • Intramolecular proton transfer occurs between cis -closed enol and cis -keto forms in a single OH stretching vibration. The photophysics of 2-(2′-hydroxyphenyl)oxazolo[4,5- b ]pyridine (HPOP) was theoretically examined by employing the integral equation formalism variant of polarizable continuum model (PCM) at the level of density functional theory with M06 functional. Polar protic methanol and water and aprotic acetonitrile and tetrahydrofuran, which have varied dielectric constants, were considered to study the role of polarity on the photophysics of HPOP and was compared with the results obtained in vacuum. Six different structural forms of HPOP were observed: cis and trans -closed enol, cis and trans -open enol, and cis and trans -keto. cis -closed enol is the most stable and trans -keto the least in the ground state. In the S 1 state, cis -keto is the most stable and cis -open enol the most unstable. The relative stabilities of the different structures increase with the polarity of the solvent. The calculations were extended to vacuum and methanol environments by adding two methanol molecules, and similarly done with two water molecules in vacuum and water environments which are explicitly hydrogen bonded to HPOP at different hydrogen bonding centres of HPOP to study the role of solute–solvent hydrogen bond on its photophysics and the changes in results obtained under the PCM formalism. The hydrogen bond between the solute and solvent molecules further increases the relative stabilities of the different hydrogen bonded clusters. The calculations show that the intramolecular proton transfer (IPT) between cis -closed enol and cis -keto is a barrierless process with the transition state displaying a submerged barrier both in S 0 and S 1 states where the IPT occurs within a single OH stretching vibration. The IPT trajectory is shorter in the S 1 state. [ABSTRACT FROM AUTHOR]

Ultrafast photophysical processes of normal and proton-transferred tautomers associated with the excited-state intramolecular proton transfer (ESIPT) in 5-sulfanylchromen-4-one (5-SC) are explored theoretically. The normal tautomer undergoes the proton transfer predominantly via S 1 , populating the proton-transferred tautomer in its lowest singlet excited state, i.e., S 1 ′. Subsequently, the S 1 ′ -T 2 ′ intersystem crossing pathway efficiently induces triplet formation in the latter tautomer. Our findings show that the energetics and dynamics associated with the triplet formation are similar to that of respective tautomers of positional isomer, i.e., of 3-sulfanylchromen-4-one (3-SC). However, the proton transfer might be more efficient in 5-SC than in 3-SC as this process takes place via a barrierless pathway in 5-SC. [Display omitted] • Six-membered ESIPT cyclic transition state. • Barrierless proton transfer via S 1. • Rapid triplet generation in proton-transferred tautomer. • Potential photosensitization via ESIPT. [ABSTRACT FROM AUTHOR]

• New isomeric 4-(methoxyphenyl)amino-1,3-thiazol-2(5 H)-one derivatives were synthesized. • The phenomena of proton tautomerism and stereoisomerism were investigated. • Compounds were characterized by X-ray analysis, FT-IR, and NMR spectroscopy. • A theoretical analysis of the intermolecular interactions was performed. In this article, we have reported the most recent results of our systematic studies of phenyl-ring substituent effect on proton tautomerism and stereoisomerism in 4-phenylamino-1,3-thiazol-2(5 H)-one derivatives. The work concerns the synthesis, analysis of 1H and 13C NMR and FT-IR spectra, and X-ray crystal studies of three isomeric compounds, i.e. 5-dimethylaminomethylidene-4-(o -, m -, p -methoxyphenyl)amino-1,3-thiazol-2(5 H)-ones. All three isomers were found to solely exist in the amino tautomeric form, both in the DMSO solution and the solid phase. In the molecules of all the title compounds, the o -, m -, p -methoxyphenylamine residue has synperiplanar disposition with respect to the thiazolone system, while the 5-dimethylaminomethylidene residue adopts the Z -configuration. DFT calculations correctly predict that the synperiplanar arrangement is favored in all investigated materials [ABSTRACT FROM AUTHOR]

Six half-sandwich Ru(II) complexes (Ru1-Ru6), integrated with 5-phenyl-2-(pyridin-2-yl)-2,4-dihydro-3H-pyrazol-3-one (PDPO1-PDPO6) ligands, were synthesized and spectroscopically characterized. The structure of Ru3 that crystallized as a monoclinic crystal with P21/c space group was further confirmed by X-ray single crystal diffraction. Prototropic tautomerism within the complexes transformed OH-form ligands to NH-form, forming a hydrogen bond (Cl 1 ---H-N 3). The complexes and ligands' cytotoxicity was assessed against several cancerous (HepG2, A549, MCF-7) and normal Vero cell lines. Relative to the ligands and Cisplatin, complexes (Ru2 , Ru3 , Ru5 , Ru6) exhibited potent cytotoxicity against cancer cells, with IC 50 values from 2.05 to 15.69 μM/L, excluding Ru1 and Ru4. Specifically, Ru2 , Ru3 , and Ru5 demonstrated superior anti-HepG2 properties. Compared to Cisplatin, Ru2 and Ru5 were less toxic to Vero cells, highlighting their enhanced selectivity in toxicity. Structure-activity relationship (SAR) studies indicated that t-butyl substitution (in Ru2) or -Cl (in Ru5) on the benzene ring significantly improved the selective toxicity. These complexes manifested substantial lipophilicity, cellular uptake, and were quickly hydrolyzed to Ru-H 2 O species. Roughly positive correlations were observed between hydrolysis rate, lipophilicity, cellular uptake, and anticancer activities. Ru2 , investigated specifically, induced apoptosis in HepG2 cells at concentrations of 10 and 20 μM/L through ROS-mediated mitochondrial dysfunction and G0/G1phase arrest, associated with altered P21, cyclin D, and CDK4 expression levels. In half-sandwich Ru(II) complexes, OH-form of PDPO ligands converted into NH-form ones. Complexes displayed good lipophilicity and cellular uptake, and stronger anti-cancer actives, and caused apoptosis by inducing mitochondria dysfunction and G0/G1phase arrest. [Display omitted] • Complexes exhibited good lipophilicity and cellular uptake, and strong cytotoxicity. • Complexes Ru2 and Ru5 displayed better selectivity in toxicity than Cisplatin. • Apoptosis was based on ROS-mediated mitochondria dysfunction and G0/G1 arrest. • In complexes, the ligands transformed from OH-form to NH-form. • Ru3 's structure was further confirmed by X-ray single crystal diffraction. [ABSTRACT FROM AUTHOR]

• A two-step strategy for FDCA production without HMF isolation using dioxane as bridging solvent. • A biphasic-solvent system built by TmaCl with a 90.6 % HMF yield for fructose dehydration. • TmaCl promoting effect for the increasing fructofuranoses proportion in fructose tautomers. • An alkali-free method for HMF oxidation with higher initial HMF concentration and a 93 % FDCA yield. 2,5-Furandicarboxylic acid (FDCA) is a crucial bio-platform molecule for renewable polymers and fine chemicals. Here, a two-step method including fructose dehydration and 5-hydroxymethylfurfural (HMF) oxidation has been reported as an effective strategy to produce FDCA with high yield from fructose directly. In this method, for fructose dehydration over CH 3 SO 3 H, tetramethylammonium chloride (TmaCl) was first used as phase modifier and co-solvent in a H 2 O-dioxane biphasic-solvent system, affording HMF yield up to 90.6 % under mild condition (383 K) with relatively high fructose concentration (e.g. 5–10 wt %). Detailed 13C NMR studies verified the interaction between TmaCl and fructose, which significantly promoted the proportion of fructofuranoses and helped to improve the fructose dehydration activity and HMF yield. After dehydration step, more than 97.8 % of CH 3 SO 3 H were recovered in reaction phase for catalyst reuse, while the dioxane solution of extracted HMF was separated via simple phase separation. Followed by water commixing, this HMF dioxane solution (with the HMF concentration of 6.3 wt %) was directly oxidized over Ru/C (393 K, 1 MPa O 2) to produce FDCA without HMF isolation and alkali addition, affording FDCA yield near 90 %. This study integrates the fructose dehydration and HMF oxidation rationally via dioxane as the bridging solvent to achieve high FDCA yield without complex HMF purification steps, and thus proposes an applicable model for high-efficient production of FDCA in practice. This study integrates the fructose dehydration and HMF oxidation rationally via dioxane as the bridging solvent to achieve high overall FDCA yield over 80 % without complex HMF purification steps. [Display omitted] [ABSTRACT FROM AUTHOR]

• Three tautomers of 5-methylbenzotriazole (5MeBTA) have similar stability. • Vibrational spectra show the presence of 1 H - and 3 H -5MeBTA tautomers in the solid state. • Formation of N H···N hydrogen bonds between the 1 H - and 3 H -5MeBTA tautomers is confirmed. • 5MeBTA is a strong agent against A. niger, C. albicans and L. monocytogenes. Three tautomeric forms (1 H -, 2 H -, 3 H -) of 5-methylbenzotriazole (5MeBTA) and the tetramer, which includes the 1 H -5MeBTA and 3 H -5MeBTA tautomers connected by N3− H ⋅⋅⋅N3 and N1− H ⋅⋅⋅N2 intermolecular hydrogen bonds, have been studied by density functional theory (DFT) using B3LYP, B3LYP-D3 and ωB97XD methods combined with the 6–31++ G (d,p) basis set. Calculations for the tetramer have revealed that intermolecular N3⋅⋅⋅N3 distances range from 2.881 to 2.851 Å, while the N1⋅⋅⋅N2 distances vary between 2.965 and 2.915 Å. These distances are very similar to those reported for the crystal of 5MeBTA (R. K. Belter, F. R. Fronczek, CCDC 1971683, 2019). The theoretical Raman and infrared spectra of the tetramer are in a very good agreement with the FT-Raman and FT-IR spectra of 5MeBTA in the solid state, measured in this work. A detailed interpretation of the experimental vibrational spectra has been made on the basis of the calculated potential energy distribution (PED). Microbiological studies in vitro were conducted for 5MeBTA and benzotriazole (BTA) against six bacterial strains (E. coli, K. pneumoniae, P. aeruginosa, B. subtilis, L. monocytogenes and S. aureus) and two fungal strains (A. niger and C. albicans). The results have shown that the substitution by the methyl group at the C-5 atom of BTA leads to an increase in activity against tested strains, in comparison to BTA. For 5MeBTA, the inhibition zones for bacteria and fungi range from 12 mm to 20 mm, and from 13 mm to 24 mm, respectively. The title compound has revealed particularly strong activity against A. niger, C. albicans and L. monocytogenes (with inhibition zones ≥ 20 mm). [Display omitted] [ABSTRACT FROM AUTHOR]

• Vibrational spectroscopy shows oxocyclohexan[ b ]indoles are ketoindole tautomers. • Computational data confirms oxocyclohexan[ b ]indoles are most stable as ketoindoles. • Vibrational spectroscopy differentiates isomeric oxocyclohexan[ b ]indoles. • Mass spectrometry gives complementary information on oxocyclohexan[ b ]indoles. • Fragmentation of ionised oxocyclohexan[ b ]indoles involves stepwise mechanisms. The vibrational and mass spectra of a representative set of nine oxocyclohexan[ b ]indoles (tricyclic indoles with a 6,5,6 ring pattern) with a carbonyl group in each of the four positions of the third non-aromatic ring and up to two methyl substituent(s) attached to the carbon atoms in the other positions of this ring are reported and discussed. Particular attention is focused on the use of infrared spectroscopy to determine whether these compounds may best be described as ketoindoles or hydroxyindolenines. Most of these spectra indicate that the tricyclic heterocycles exist preferentially at ketoindoles, but the solid state infrared spectra of oxocyclohexan[ b ]indoles with the oxygen function in the 4-position are highly unusual, giving the initial impression that these compounds are hydroxyindolenines. Computational modelling supports the interpretation that the 4-ketoindole tautomer has a lower energy than either of its most stable isomeric indolenines conformers. Mass spectrometry is shown to provide valuable confirmatory evidence to differentiate the isomeric ionised oxocyclohexan[ b ]indoles, which dissociate via distinctive and informative fragmentation routes, in which the stability of intermediate distonic ions appears to be profoundly influenced by participation of the lone pair of electrons on the nitrogen atom. Many of these fragmentations are better described by stepwise mechanisms, rather than as concerted cycloreversions. [Display omitted] [ABSTRACT FROM AUTHOR]