Intramolecular Hydrogen Bond Research Articles

Synthesis and structure of trans-bis-(4-amino-3-nitro-benzoato-κO)bis-(4-amino-3-nitro-benzoic acid-κO)di-aqua-manganese(II) dihydrate.

The manganese title complex, [Mn(C7H5N2O4)2(C7H6N2O4)2(H2O)2]·2H2O, is one of the first 4-amino 3-nitro-benzoic acid (4 A3NBA) monoligand metal complexes to be synthesized. It crystallizes in the centrosymmetric monoclinic space group P21/n with the complex mol-ecules located on inversion centers. Four 4 A3NBA ligand mol-ecules are monodentately coordinated by the Mn2+ ion through the carb-oxy-lic oxygen atoms while the other two positions of the inner coordination sphere are occupied by water mol-ecules, giving rise to a distorted octa-hedron, and two water mol-ecules are in the outer coordination sphere. There are two intra-molecular hydrogen bonds in the complex mol-ecule. The first is of the common N-H⋯O=N type, while the second is a rarely occurring very strong hydrogen bond in which a common proton is shared by two uncoordinated oxygen atoms of neighboring carboxyl-ate groups. In the crystal, an intricate system of inter-molecular hydrogen bonds links the complex mol-ecules into a three-dimensional-network.

Characterization of Various Noncovalent Polyphenol-Starch Complexes and Their Prebiotic Activities during In Vitro Digestion and Fermentation.

This study explores the structural characterization of six noncovalent polyphenol-starch complexes and their prebiotic activities during in vitro digestion and fermentation. Ferulic acid, caffeic acid, gallic acid, isoquercetin, astragalin, and hyperin were complexed with sweet potato starch (SPS). The polyphenols exhibited high binding capacity (>70%) with SPS. A partial release of flavonoids from the complexes was observed via in vitro digestion, while the phenolic acids remained tightly bound. Molecular dynamics (MD) simulation revealed that polyphenols altered the spatial configuration of polysaccharides and intramolecular hydrogen bonds formed. Additionally, polyphenol-SPS complexes exerted inhibitory effects on starch digestion compared to gelatinized SPS, owing to the increase in resistant starch fraction. It revealed that the different complexes stimulated the growth of Lactobacillus rhamnosus and Bifidobacterium bifidum, while inhibiting the growth of Escherichia coli. Moreover, in vitro fermentation experiments revealed that complexes were utilized by the gut microbiota, resulting in the production of short-chain fatty acids and a decrease in pH. In addition, the polyphenol-SPS complexes altered the composition of gut microbiota by promoting the growth of beneficial bacteria and decreasing pathogenic bacteria. Polyphenol-SPS complexes exhibit great potential for use as a prebiotic and exert dual beneficial effects on gut microbiota.

Crystal structure, tautomerism and photostability of 2-(2-pyridyl)-phenalene-1,3-dione

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.

Effects of Chalcogen Atoms on Excited-State Double-Proton Transfer Behavior for 3,6-bis(4,5-Dihydroxyoxazo-2-yl)benzene-1,2-diol Derivatives: A Computational Investigation.

The impact of the chalcogen atomic electronegativity (O, S, and Se atoms) of new organic molecules on excited-state dynamical reactions is self-evident. Inspired by this kind of distinguished photochemical characteristic, in this work, we performed a computational investigation of chalcogen-substituted 3,6-bis(4,5-dihydroxyoxazo-2-yl)benzene-1,2-diol (BDYBD) derivatives (i.e., BDYBD-O, BDYBD-S, and BDYBD-Se). In this paper, we pay close attention to characteristic BDYBD derivatives that contain intramolecular double hydrogen bonds (O1-H2···N3 and O4-H5···N6). The main goal of this study was to explore how changes in atomic electronegativity affect the way hydrogen bonds interact and how excited molecules affect transfer protons. We go into further detail in the main text of the paper. By fixing our attention to geometrical variations and infrared (IR) vibrational spectra between the S0 and S1 states, exploring hydrogen bonding behaviors using the core-valence bifurcation (CVB) index, and simulating hydrogen bonding energy (EHB) via the atom in molecule (AIM) method, we clarified the photo-induced strengthened dual hydrogen bonding interactions that facilitate the excited-state dual-proton transfer (ESDPT) behavior of BDYBD derivatives. The reorganization of charge stemming from photoexcitation further verifies the tendencies of ESDPT reactions. We relied on constructing potential energy surfaces (PESs) by adopting a restrictive optimization approach, and herein, we finally clarify the gradual ESDPT mechanism of BDYBD derivatives. Particularly, we confirm that the variation in chalcogen atomic electronegativity has a regulatory effect on the ESDPT behavior of BDYBD derivatives; that is, the lower the atomic electronegativity, the more favorable it is for the gradual ESDPT reaction.

Sono-assembly of ellagic acid into nanostructures significantly enhances aqueous solubility and bioavailability

IntroductionEllagic acid (EA), commonly found in foods, offers significant health benefits in combating chronic diseases. However, its therapeutic potential is hindered by its extremely poor solubility and bioavailability. MethodIn this study, EA nanoparticles (EA NPs) were produced using a sono-assembly method, without additional agents. ResultsEA NPs exhibited stick-like nanoparticle structures with an average size of 147.3 ± 0.73 nm. EA NPs likely adopt a tunnel-type solvate structure, with 4 water participating in disruption of intramolecular hydrogen bonds in EA and establishment of intermolecular hydrogen bonds between EAs. Importantly, EA NPs exhibited remarkable enhancements in water solubility, with 120.7-fold increase in water, and 97.8-fold increase in pH 6.8 buffer. Moreover, ex vivo intestinal permeability studies demonstrated significant improvements (P < 0.5). These findings were further supported by in vivo pharmacokinetic studies, where EA NPs significantly enhanced the relative bioavailability of EA by 4.69 times.

How Permanent Are the Permanent Macrodipoles of Anthranilamide Bioinspired Molecular Electrets?

Dipoles are ubiquitous, and their impacts on materials and interfaces affect many aspects of daily life. Despite their importance, dipoles remain underutilized, often because of insufficient knowledge about the structures producing them. As electrostatic analogues of magnets, electrets possess ordered electric dipoles. Here, we characterize the structural dynamics of bioinspired electret oligomers based on anthranilamide motifs. We report dynamics simulations, employing a force field that allows dynamic polarization, in a variety of solvents. The results show a linear increase in macrodipoles with oligomer length that strongly depends on solvent polarity and hydrogen-bonding (HB) propensity, as well as on the anthranilamide side chains. An increase in solvent polarity increases the dipole moments of the electret structures while decreasing the dipole effects on the moieties outside the solvation cavities. The former is due to enhancement of the Onsager reaction field and the latter to screening of the dipole-generated fields. Solvent dynamics hugely contributes to the fluctuations and magnitude of the electret dipoles. HB with the solvent weakens electret macrodipoles without breaking the intramolecular HB that maintains their extended conformation. This study provides design principles for developing a new class of organic materials with controllable electronic properties. An animated version of the TOC graphic showing a sequence of the MD trajectories of short and long molecular electrets in three solvents with different polarities is available in the HTML version of this paper.

Creating Efficient Red Thermally Activated Delayed Fluorescence Materials with Cyano-Substituted 11,12-Diphenyldipyrido[3,2-a:2',3'-c]phenazine Acceptors.

Red luminescent materials are essential components for full color display and white lightening based on organic light-emitting diode (OLED) technology, but the extension of emission color towards red or deep red region generally leads to decreased photoluminescence and electroluminescence efficiencies. Herein, we wish to report two new luminescent molecules (2CNDPBPPr-TPA and 4CNDPBPPr-TPA) consisting of cyano-substituted 11,12-diphenyldipyrido[3,2-a:2',3'-c]phenazine acceptors and triphenylamine donors. As the increase of cyano substituents, the emission wavelength is greatly red-shifted and the reverse intersystem crossing process is promoted, resulting in strong red delayed fluorescence. Meanwhile, due to the formation of intramolecular hydrogen bonds, the molecular structures become rigidified and planarized, which brings about large horizontal dipole ratios. As a result, 2CNDPBPPr-TPA and 4CNDPBPPr-TPA can perform as emitters efficiently in OLEDs, furnishing excellent external quantum efficiencies of 28.8 % at 616 nm and 20.2 % at 648 nm, which are significantly improved in comparison with that of the control molecule without cyano substituents. The findings in this work demonstrate that the introduction of cyano substituents to the acceptors of delayed fluorescence molecules could be a facile and effective approach to explore high-efficiency red or deep red delayed fluorescence materials.

Intermolecular hydrogen bond interactions in water clusters of zwitterionic glycine: DFT, MESP, AIM, RDG, and molecular dynamics analysis

The putative global minima geometries of zwitterionic glycine-water complexes [zGly.Wn (1 = 1–10)] are controlled by the cyclic network of water molecules bonded to the zwitterionic glycine. Their stability is governed by the number of water-water interactions along with the existence of the intramolecular glycine-water bonds. The computed intramolecular distances suggest that water molecules tend to coordinate within themselves. The computed association energy attains saturation at 6, implying that up to 6 water molecules are coordinated to the zwitterionic glycine. Further addition of water molecules happens to be an intramolecular addition to the existing water. The computed incremental association energy decreases with the increase in the number of water molecules, except for the six and nine water molecules. This implies that compared to the water clusters, the zwitterionic clusters are less stable. The higher stability of n = 6 and 9 can be marked to the resemblance of the cyclic network of water molecules to the bare global minima water clusters. Benchmarking calculations suggest that M06-2X-D3 functional provides binding energy close to the PW6B97D3(BJ) method. The PGM clusters' thermodynamic parameters show that the reaction's enthalpy remains constant and is negative, indicating that the hydration process is exothermic and independent of the number of water molecules. The electron density difference (EDD) diagram shows the build-up of charge is more pronounced in the intramolecular hydrogen bonds (HBs) between water molecules than between zwitterionic glycine and water. QTAIM analysis shows that the water-water HBs are comparatively weaker than zwitterionic glycine-water interactions.

Bio‐Disintegrable Elastic Polymers for Stretchable Piezoresistive Strain Sensors

AbstractThe fast‐growing usage of electronics is creating large amounts of e‐waste (electronic waste), most of which are directly sent into landfills or incinerated as recycling is either impractical or too costly. Therefore, it is believed that degradable, environmentally friendly materials are the solution to this pressing issue. Herein, disintegrable, durable, and highly stretchable strain sensors are developed based on elastic thin films made of sodium carboxymethyl cellulose, glycerol, and polyvinyl alcohol. The polymer thin films show a failure strain up to ≈ 330% and low hysteresis (5.74% at the second cycle) when subjected to 50% cyclic strain, due to the formation of inter or intramolecular hydrogen bonds. Carbon nanofibers with Au thin film are deposited on the elastic thin film, resulting in highly stretchable piezoresistive strain sensors with a maximum gauge factor of 1.7. More interestingly, the as‐developed sensors can be completely broken down in hot water (at ≈ 95 °C) within ≈ 25 min, indicating their remarkable disintegrability. This unique characteristic is expected to contribute to environmental sustainability. The applications of the sensor for joint bending recognition as well as physiological sign measurement have been demonstrated.

Investigating the role of thymol as a promising inhibitor of pyruvate dehydrogenase kinase 3 for targeted cancer therapy

Protein kinases have emerged as major contributors to various diseases. They are currently exploited as a potential target in drug discovery because they play crucial roles in cell signaling, growth, and regulation. Their dysregulation is associated with inflammatory disorders, cancer, and neurodegenerative diseases. Pyruvate dehydrogenase kinase 3 (PDK3) has become an attractive drug target in cancer therapeutics. In the present study, we investigated the effective role of thymol in PDK3 inhibition due to the high affinity predicted through molecular docking studies. Hence, to better understand this inhibition mechanism, we carried out a 100 ns molecular dynamics (MD) simulation to analyse the dynamics and stability of the PDK3-thymol complex. The PDK3-thymol complex was stable and energetically favourable, with many intramolecular hydrogen bond interactions in the PDK3-thymol complex. Enzyme inhibition assay showed significant inhibition of PDK3 by thymol, revealing potential inhibitory action of thymol towards PDK3 (IC50 = 2.66 μM). In summary, we established thymol as one of the potential inhibitors of PDK3, proposing promising therapeutic implications for severe diseases associated with PDK3 dysregulation. This study further advances our understanding of thymol's therapeutic capabilities and potential role in cancer treatment.

Unlocking the potential of solvent polarity in directing ESIPT pathways of HHMB with dual hydrogen bond acceptors: a DFT/TD-DFT study

The excited-state intramolecular proton transfer (ESIPT) process involving dual hydrogen bond acceptors (HBAs) is of great interest in research and has significant potential for applications due to its flexible adjustability. In this work, the solvent polarity-dependent changes in structures, photophysical properties and ESIPT processes of 2-(2-hydroxy-3-hydroxyiminomethyl-5-methylphenyl)-benzimidazole (HHMB) with two asymmetric HBAs are investigated. Firstly, the orientation of intramolecular hydrogen bond (IHB) under different solvent polarities are explored. The computational results indicate that in non-polar solvents, the central hydroxyl group predominantly forms an IHB with the imine of the oxime group, resulting in large Stokes and near-infrared emission peaks. Conversely, in polar solvents, the central hydroxyl group links the imidazole group to form a stronger IHB with improved fluorescence efficiency. Additionally, the quantum chemical calculations are used to simulate the absorption–emission spectra and plot the frontier molecular orbitals, as well as the potential energy curves about ESIPT in different solvents. In conclusion, we elucidate the rationale behind the preferential formation of the biased IHBs and examine its impact on the ESIPT process by manipulating the solvent polarity. Theoretical studies on the dual HBAs molecule, exhibiting specific selectivity, pave the way for the development of higher-performance fluorescence probes and luminescent materials.

Green Starch-Based Hydrogels with Excellent Injectability, Self-Healing, Adhesion, Photothermal Effect, and Antibacterial Activity for Promoting Wound Healing.

Hydrogel materials have proven valuable in wound healing, but improving the safety of these hydrogels is still challenging. Therefore, designing multifunctional natural polymeric-based hydrogels with excellent mechanical properties to replace toxic or potentially risky, refractory chemical polymer-based hydrogels such as polyacrylamide and polyethylene glycol is of particular significance. Here, a green starch-based hydrogel (Starch@Ca/CGC hydrogel) with injectability, self-healing, and instant adhesion was constructed by coordination interaction, electrostatic interaction, and intramolecular and intermolecular hydrogen bonds. Therein, natural bioactive small molecules gallic acid (GA) and carvacrol (CA) were coordinated with metal ions by the ultrasonic-triggered self-assembly and ionic cross-linking codriven strategy to prepare Cu-gallic acid-carvacrol nanospheres (CGC NPs), which conferred the hydrogel with near-infrared light (NIR)-controlled CA release and photothermal synergistic sterilization properties, as well as antioxidant and anti-infection capabilities. More importantly, the multifunctional hydrogel platforms could completely cover an irregular wound shape to prevent secondary injury and significantly accelerate wound healing under NIR with more skin appendages like hair follicles and blood vessels appearing. Therefore, it is expected that this starch-based hydrogel could serve as a competitive multifunctional dressing in the biomedical field, including bacteria-derived wound infection and other tissue repair.

Insights into the role of RAHB and IHB interactions for the structure- Activity relationship of caged xanthone Morellic Acid: Spectroscopic and DFT exploration, molecular docking, and molecular dynamics simulation studies as an Antituberculosis agent

The current study elaborates on experimental and simulated methods using density functional theory (DFT) to analyze the structure, electronic characteristics, and vibrational features of morellic acid, an isolate from Garcinia wightii. The in-depth examination of the stabilized structure indicates that the compound achieves equilibrium through conjugative, hyperconjugative, resonance-assisted hydrogen bonding (RAHB), and intramolecular hydrogen bonding (IHB) interactions, resulting in a well-supported conformation. The vibrational spectra were recorded, and the wavenumbers were comprehensively assigned using Potential Energy Distribution with a scaled quantum mechanical force field. Natural bond orbital (NBO) studies were utilized to calculate donor and acceptor bond interaction energy and electron densities. The TD-DFT computations were performed in solvent (chloroform) and gaseous phase, which agreed with the experimental values. Molecular electrostatic potential (MEP), reduced density gradient (RDG), electron localized function (ELF), and localized orbital locator (LOL) of the molecule were also analyzed. The molecular docking studies revealed that the compound holds great promise as a drug candidate for tuberculosis treatment, with the most thermodynamically stable binding score of -11.542 kcal/mol with 4BFS (protein of Mycobacterium tuberculosis PanK). The molecular dynamics (MD) simulation studies examined the stability and hydrogen bonding system of the ligand-protein complex formed between the molecule and the essential target protein for 100 ns. The findings revealed that the complex remained stable with minimal fluctuations throughout the simulation, suggesting that the molecule may prevent tuberculosis and be used as a treatment candidate.

Implicit and Explicit Solvent Effects on the Global Reactivity and the Density Topological Parameters of the Preferred Conformers of Caespitate

In the search to cover the urgent need to combat infectious diseases, natural products have gained attention in recent years. The caespitate molecule, isolated from the plant Helichrysum caespititium of the Asteraceae family, is used in traditional African medicine. Caespitate is an acylphloroglucinol with biological activity. Acylphloroglucinols have attracted attention for treating tuberculosis due to their structural characteristics, highlighting the stabilizing effect of their intramolecular hydrogen bonds (IHBs). In this work, a conformational search for the caespitate was performed using the MM method. Posteriorly, DFT calculations with the APFD functional were used for full optimization and vibrational frequencies, obtaining stable structures. A population analysis was performed to predict the distribution of the most probable conformers. The calculations were performed in the gas phase and solution using the implicit SMD model for water, chloroform, acetonitrile, and DMSO solvents. Additionally, the multiscale ONIOM QM1/QM2 model was used to simulate the explicit solvent. The implicit and explicit solvent effects were evaluated on the global reactivity indexes using the conceptual-DFT approach. In addition, the QTAIM approach was applied to analyze the properties of the IHBs of the most energetically and populated conformers. The obtained results indicated that the most stable and populated conformer is in the gas phase, and chloroform has an extended conformation. However, water, acetonitrile, and DMSO have a hairpin shape. The optimized structures are well preserved in explicit solvent and the interaction energies for the IHBs were lower in explicit than implicit solvents due to non-covalent interactions formed between the solvent molecules. Finally, both methodologies, with implicit and explicit solvents, were validated with 1H and 13C NMR experimental data. In both cases, the results agreed with the experimental data reported in the CDCl3 solvent.

Water detection in organic solvents using a copolymer membrane immobilised with a fluorescent intramolecular charge transfer-type dye: effects of intramolecular hydrogen bonds.

Numerous fluorescent dye-based optical sensors have been developed to detect water in organic solvents. However, only a few such sensors can detect water in polar solvents such as methanol or dimethyl sulfoxide, and their detection range is generally narrow. Therefore, in this study, a copolymer membrane incorporated with a pyridinium betaine dye (denoted PB1), which exhibited intramolecular charge transfer (ICT) characteristics, was developed to realise simple water detection in organic solvents. The pyridinium betaine structure, comprising intramolecular hydrogen bonds between the oxygen in the maleimide moiety and the hydrogen in the pyridinium, was vital for achieving efficient fluorescence emission. The membrane was prepared by copolymerising PB1 with the N,N-dimethyl acrylamide/acrylamide monomer on a glass plate, and the fluorescence in water-mixed organic solvents was investigated (λabs = 490 nm, λfl = 630 nm). The fluorescence intensity of the dye-immobilised membrane decreased with increasing water content of the organic solvents. The detection ranges in tetrahydrofuran, ethanol, methanol, and dimethyl sulfoxide were approximately <40, <40, <40, and <60 vol% water, respectively. In contrast, membranes based on a quaternary pyridinium dye (without intramolecular hydrogen bonds) did not detect water in methanol and dimethyl sulfoxide, although it was more sensitive than PB1 in the narrow region of low water concentration in THF. Theoretical calculations corroborated the importance of the pyridinium betaine structure in detecting water in organic solvents, with the increase in polarity and the formation of intermolecular hydrogen bonds between PB1 and water found to induce molecular rotation and fluorescence quenching.

The role of an intramolecular hydrogen bond in the redox properties of carboxylic acid naphthoquinones.

A bioinspired naphthoquinone model of the quinones in photosynthetic reaction centers but bearing an intramolecular hydrogen-bonded carboxylic acid has been synthesized and characterized electrochemically, spectroscopically, and computationally to provide mechanistic insight into the role of proton-coupled electron transfer (PCET) of quinone reduction in photosynthesis. The reduction potential of this construct is 370 mV more positive than the unsubstituted naphthoquinone. In addition to the reversible cyclic voltammetry, infrared spectroelectrochemistry confirms that the naphthoquinone/naphthoquinone radical anion couple is fully reversible. Calculated redox potentials agree with the experimental trends arising from the intramolecular hydrogen bond. Molecular electrostatic potentials illustrate the reversible proton transfer driving forces, and analysis of the computed vibrational spectra supports the possibility of a combination of electron transfer and PCET processes. The significance of PCET, reversibility, and redox potential management relevant to the design of artificial photosynthetic assemblies involving PCET processes is discussed.

Aromatic foldamer-derived transmembrane transporters.

This review is the first to focus on transmembrane transporters derived from aromatic foldamers, with most studies reported over the past decade. These foldamers have made significant strides in mimicking the essential functions of natural ion channel proteins. With their aromatic backbones rigidified by intramolecular hydrogen bonds or differential repulsive forces, this innovative family of molecules stands out for its structural diversity and functional adaptability. They achieve efficient and selective ion and molecule transport across lipid bilayers via carefully designed helical structures and tunable large cavities. Recent developments in this field highlight the transformative potential of foldamers in therapeutic applications and biomaterial engineering. Key advances include innovative molecular engineering strategies that enable highly selective ion transport by fine-tuning structural and functional attributes. Specific modifications to macrocyclic or helical foldamer structures have allowed precise control over ion selectivity and transport efficiency, with notable selectivity for K+, Li+, H+ and water molecules. Although challenges remain, future directions may focus on more innovative molecular designs, optimizing synthetic methods, improving membrane transport properties, integrating responsive designs that adapt to environmental stimuli, and fostering interdisciplinary collaborations. By emphasizing the pivotal role of aromatic foldamers in modern chemistry, this review aims to inspire further development, offering new molecular toolboxes and strategies to address technological and biological challenges in chemistry, biology, medicine, and materials science.

Structural subtleties and catalytic activity of sodium aminophenolate complexes in polylactide degradation: towards sustainable waste management solutions.

This study explores the intricate coordination chemistry of sodium aminophenolate species and their significant role in the depolymerization of polylactide (PLA), offering novel insights into catalytic degradation processes. By examining sodium coordination entities, including dimers and larger aggregates such as tetramers, we reveal how structural modifications, particularly the manipulation of steric hindrances, influence the formation and stability of these complexes. The dimers, characterized by a unique four-center core (Na-O-Na-O), serve as a foundational motif, which is further elaborated to obtain complexes with varied coordination environments through strategic ligand design. Our research delves into the lability of the amino arm in these complexes, a critical factor that facilitates the coordination of PLA to the sodium center, thereby initiating the depolymerization process. Moreover, DFT studies have been pivotal in identifying the most energetically favorable structures for catalysis, highlighting a distinct preference for an eight-membered ring motif stabilized by intramolecular hydrogen bonds. This motif not only enhances the catalyst's efficiency but also introduces a novel structural paradigm for sodium-based catalysis in PLA degradation. Experimental validation of the theoretical models was achieved through NMR spectroscopy, which confirmed the formation of the active catalyst forms and monitored the progress of PLA degradation. The study presents a comprehensive analysis of the influence of ligand structure on the catalytic activity, underscoring the importance of the eight-membered ring motif. Furthermore, we demonstrate how varying the steric bulk of substituents on the amino arm affects the catalyst's performance, with benzyl-substituted ligands exhibiting superior activity. Our findings offer a profound understanding of the structural factors governing the catalytic efficiency of sodium aminophenolate complexes in PLA degradation. This research not only advances the field of coordination chemistry but also presents a promising avenue for the development of efficient and environmentally friendly catalysts for polymer degradation.

A rotational spectroscopy study of microsolvation effects on intramolecular proton transfer in trifluoroacetylacetone-(H2O)1-3.

Trifluoroacetylacetone (TFAA) has two enol forms, which can switch to each other via proton transfer. While much attention has been paid to their conformational preferences, the influence of microsolvation on regulating the proton position remains unexplored. Herein, we report the rotational spectra of trifluoroacetylacetone-(water)n (n = 1-3) investigated by chirped pulse Fourier transform microwave spectroscopy in the 2-8 GHz frequency range. Two conformers were identified for both TFAA-H2O and TFAA-(H2O)2, while only one conformer was characterized for TFAA-(H2O)3. The results indicate that water binding on the CH3 side stabilizes the enolF form, whereas water binding on the CF3 side stabilizes the enolH form. The enolF form predominates over the enolH form in these hydrated complexes, which contrasts with the fact that only enolH exists in isolated TFAA. EnolH becomes preferred only when water inserts itself into the intramolecular hydrogen bond. Instanton theory calculations reveal that the proton transfer reaction is dominated by quantum tunneling at low temperatures, leading to the stable existence of only one enol form in each configuration of the hydrated clusters.

Intramolecular hydrogen-atom tunneling in matrix-isolated heterocyclic compounds: 2-thiouracil and its analogues.

Hydrogen-atom tunneling leading to spontaneous tautomeric conversion in monomeric heterocyclic molecules without intramolecular hydrogen bonds has been experimentally detected for the first time. For monomers of 2-thiouracil, 6-aza-2-thiouracil and 1-methyl-2-thiouracil isolated in low-temperature matrices, higher-energy thiol forms were generated upon UV (λ = 305 nm) excitation of the most stable thione tautomers. When the matrices were subsequently kept in the dark and at low temperature, hydrogen-atom tunneling occurred, leading to the thiol → thione conversion. During this process, the photoproduced thiol form spontaneously converted into the lowest energy thione tautomer.