Weak Covalent Bonds Research Articles

A Strong Supramolecular Mechanophore with Controlled Mechanical Strength.

Supramolecular mechanophores typically exhibit much lower mechanical strengths than covalent counterparts, with strengths usually around 100 pN, which is significantly lower than the nN-scale strength of covalent bonds. Inspired by the slow dissociation kinetics of the cucurbit[7]uril (CB[7])-hexanoate-isoquinoline (HIQ) complex, we discovered that charge-dipole repulsion can be utilized to create strong supramolecular mechanophores. When activated at its -COO- state, the CB[7]-HIQ complex exhibits a high mechanical strength of ~700 pN, comparable to weak covalent bonds such as Au-S bonds or thiol-maleimide adducts. The strength of the CB[7]-HIQ complex can also be tuned with pH in a gradual manner, with a minimum value of ~150 pN at its -COOH state, similar to an ordinary supramolecular conjugate. This research may pave the way for the development of supramolecular architectures that combine the advantages of covalent and supramolecular systems.

Deformability, inherent mechanical properties and chemical bonding of Al11Nd3 in Al-Nd target material

Microstructure uniformity of the Al-Nd target materials with Al11Nd3 significantly affects the performance of the fabricated Al-Nd film, which is widely used as wiring material in large-size thin-film transistor liquid crystal display (TFT-LCD) panels. Understanding the inherent mechanical properties and chemical bonds of Al11Nd3 is crucial for homogenizing the Al-Nd target. Here, by a combined experimental and ab-initio theoretical study, the microstructure and deformability of the Al-3wt%Nd alloy and the inherent mechanical properties and chemical bonds of Al11Nd3 were investigated comprehensively. The Al-3wt%Nd alloy is composed of the pre-eutectic α-Al matrix and the eutectic α-Al and a high stable α-Al11Nd3 phase. During the plastic deformation, the eutectic microstructure transforms from a cellular to a lamellar shape, while the morphology and dimension of α-Al11Nd3 are not changed significantly. By examining ideal tensile strength, elastic moduli, hardness and brittleness-ductility, the hardness-brittleness of α-Al11Nd3 is quantitatively evaluated, accounting for its difficulties of plastic deformation and fragmentation. Combining band structure, population analysis, topological analysis and crystal orbital Hamilton population, we found that α-Al11Nd3 possesses two types of chemical bonds: the Nd-Al and Al-Al bonds. The former is a typical ionic bond with electron transfer from Nd to Al, while the latter, dominated by both 3s-3p and 3p-3p interactions, is a weak covalent bond. The mixed chemical bond is responsible for the high hardness-brittleness of α-Al11Nd3. This work is expected to lay a foundation for Al-Nd alloy and catalyze the fabrication of high-quality Al-Nd target materials.

A theoretical study on the electronic and optical properties of M-TiO2/ZnO (M=Li, Na, K, Fe) toward application in photocatalysis

This study employs the tight-binding density functional method GFN1-xTB to investigate the structural and electronic properties of TiO2-ZnO and TiO2-ZnO composite materials modified by Li, Na, K, and Fe metals. Computational analyses reveal the formation of weak covalent bonds between the TiO2 and ZnO components within the composite system. Doping TiO2-ZnO with metals induces significant alterations in the electronic structure, particularly in terms of ionization energy and global electrophilic index. The UV-VIS spectra are calculated using real-time time-dependent density functional theory with xTB Hamiltonians. The obtained results demonstrate minimal impact of metal presence on the absorption spectrum of TiO2-ZnO, but significant influence on the recombination potential of photogenerated charge carriers. It is suggested that Fe/TiO2-ZnO and Li/TiO2-ZnO will demonstrate higher photocatalytic activity than the pristine TiO2-ZnO material.

Exploring the molecular characteristics of organic matter in low-rank coals using GC×GC/TOF-MS plus data mining

In-depth understanding the form of organic matter in low-rank coals contributes to the clean and efficient utilization of coals. Two low-rank coals were successively dissolved with cyclohexane, acetone and methanol at 300 °C, respectively. Six thermal dissolution (TD) extracts were comparatively analyzed using comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry (GC×GC/TOF-MS). Cyclohexane tends to extract low-polar compounds inherent in coal, such as aliphatic hydrocarbons, arenes and phenols. Acetone can extract monocyclic aromatic hydrocarbons containing alkyl side chains. Meanwhile, acetone can form N-H⋯O hydrogen bonds with nitrogen-containing organic compounds (NCOCs), facilitating the extraction of a greater quantity of NCOCs. Methanol breaks the ArCH2OArR structural unit within coal molecules and forms the dominant alkyl-substituted phenol. It was revealed that sequentially TD at 300 °C using different solvents only breaks intermolecular forces and weak covalent bonds. Additionally, a series of isomers of tricyclic aromatic hydrocarbons and alkylphenols were authenticated by GC×GC/TOF-MS. Principal component analysis and hierarchical clustering analysis were applied to mine effective molecular information from the extensive MS data. The statistical results indicate that solvent had a significant impact on the structure and composition of the TD extracts, more so than the variations in coal species. Thus, the conjunction of GC×GC/TOF-MS and chemometrics provided methodological guidance for an enhanced comprehension of the molecular structure of coals.

Accurate classification of the molecular characteristics of soluble portions from various lignites: Joint analysis of thermal dissolution experiments and data mining methods

Gas chromatography-mass spectrometry (GC-MS) was used to analyze the soluble portions (SPs) of eight lignites which were obtained by extracting using cyclohexane, methanol and ethanol, respectively, as the solvent. For thermal dissolution at 300 °C using cyclohexane, the intermolecular interactions between mobile phase and macromolecules in coal were weakened. Consequently, non- and low-polar compounds such as alkanes and aromatics were well dissolved. Methanol and ethanol were more likely to closely interact with oxygen-containing functional groups and promoted the breakage of hydrogen bonds and weak covalent bonds. Hierarchical cluster analysis classified all the 24 SPs, and the SPs from cyclohexane and ethanol solvents exhibited more similarity. Partial least squares discriminant analysis was established to reproduce the classification results of SPs entirely and accurately. The SPs were further classified by combining a small number of geochemical characteristic compounds rather than massive GC-MS data.

A study on the structures and primary reaction products of kerogens in Longkou oil shale with different densities through supercritical ethanolysis

The difference in the density of oil shale significantly affects the structural characteristics of the kerogen, which brings the change in the pyrolysis behavior, thus leading to different retorting oil yields. The primary reaction (the cleavage of weak covalent bonds into free radicals) in the pyrolysis of oil shale greatly affects the retorting oil yield and quality. In this work, the structural characteristics and primary reaction products of the kerogens in Longkou oil shale (LKOS) with three different densities (<1.3, 1.4 − 1.5, 1.8 − 1.9 g/cm3) were investigated by supercritical ethanolysis, 13C nuclear magnetic resonance (13C NMR) and Fourier transform infrared spectroscopy (FTIR). The weak covalent bonds in the kerogens were broken by ethanolysis at 350 °C to obtain small molecular substances (SMSs) with the carbon conversions of 28 %, 47 % and 54 %, respectively. The SMSs can be divided into aliphatic acid esters, aromatics, alkanes and N-containing organic compounds (NCCs). Among the SMSs, the relative contents of the aliphatic acid esters, whose aliphatic chain lengths distribute in C3-C23, C1-C24 and C1-C28, are the highest. The aromatics are mainly monocyclic, and the NCCs mainly exist as amines. The possible pathways for these ethanolysis products were proposed. The 13C NMR and FTIR analyses of the kerogens and their ethanolysis residues show that the aliphaticity (fal) and average methylene chain length (Cn) increase, while the aromaticity (fa) and average aromatic cluster size (Xb) decrease with the increasing oil shale density. After ethanolysis, the value of fa gradually increases, while that of fal and Cn decrease. Furthermore, the contents of C=O and O-C=O decrease, while that of O-C-O and C-OH increase. The results suggest that the SMSs are connected to the kerogen matrix by weak covalent bonds. The cleavage of C=O, O-C=O and C-O bonds dominates the early stage of the oil shale pyrolysis. During the pyrolysis of oil shale, these weak covalent bonds are first broken and the above SMSs are the most primary reaction products and then undergo the second reactions to yield oil and gas.

A GSH-resistant FK228 analogue containing a stable disulfide bond

FK228 is a potent natural pan HDAC inhibitor approved by the FDA for the treatment of cutaneous T-cell lymphoma as well as peripheral T-cell lymphoma. It is generally believed that the mechanism of FK228 acting on HDACs is by reducing its disulfide bond after entering the cell, and the dithiol group may chelate with Zn2+ and form a weak reversible covalent bond with cysteine in the catalytic pocket of HDACs, therefore inhibiting the activity of HDACs. However, due to the weak stability of the disulfide bond in FK228, it has been difficult to obtain direct evidence for the above conjecture. Thus, improving the stability of the FK228 disulfide bond will help to explore the exact mechanism of FK228. In this study, based on the stability and target-induced covalent properties of the Cysteine-Penicillamine (Cys-Pen) disulfide bond reported previously, the Pen was introduced into the modification of FK228. Specifically, the d-Cys in FK228 was replaced by d-Pen, the total synthetic pathway was optimized, and the novel synthetic FK228 analogue (FK-P) stability was verified. FK-P can also be used as a new drug molecule in the future to participate in the research of related biological mechanisms or the treatment of diseases.

Investigation of Interface Interactions Between Monolayer MoS2 and Metals: Implications on Strain and Surface Roughness.

Achieving a low contact resistance has been an important issue in the design of two-dimensional (2D) semiconductor-metal interfaces. The metal contact resistance is dominated by interfacial interactions. Here, we systematically investigate 2D semiconductor-metal interfaces formed by transferring monolayer MoS2 onto prefabricated metal surfaces, such as Au and Pd, using X-ray photoelectron spectroscopy (XPS), atomic force microscopy, and Raman spectroscopy. In contrast to the MoS2/HOPG interface, the interfaces of MoS2/Au and MoS2/Pd feature the formation of weak covalent bonds. The XPS spectra reveal distinct peak positions for S-Au and S-Pd, indicating a higher doping concentration at the S-Au interface. This difference is a key factor in understanding the electronic interactions at the metal-MoS2 interfaces. Additionally, we observe that the metal surface roughness is a critical determinant of the adhesion behavior of transferred monolayer MoS2, resulting in different strains and doping concentrations. The strain on transferred MoS2 increases with an increase in substrate roughness. However, the strain is released when the roughness of metal surface surpasses a certain threshold. The dependence of the contact material and the influence of the substrate roughness on the contact interface provide critical information for improving 2D semiconductor-metal contacts and device performance.

Activation and Stabilization of Mn‐Based Positive Electrode Materials by Doping Nonmetallic Elements

AbstractDisordered rock‐salt (DRS) type active materials are highly significant because of their large reversible capacities, which are due to their unique Li+ diffusion pathway and the redox capabilities of cationic transition metals (TMs) and anionic O ions. Loosely crystalline DRS materials have weak covalent bonds between TMs and O, increasing the O redox contribution and thereby enhancing their capacities. In this study, Mn‐based positive electrode materials with DRS structures are activated and stabilized by mechanochemical doping of nonmetallic elements P and B into interstitial sites. Synthesized Li0.90Mn0.84P0.04O2 (LMPO5) exhibits an initial discharge capacity of 346 mAh g−1 (1050 Wh kg−1) during charging/discharging. Li0.91Mn0.83B0.10O2 (LMBO5) has a moderately expanded lattice size, which facilitates high‐capacity retention during cycling (≈284 mAh g−1 at the 30th cycle). The structural properties of the synthesized active materials are extensively characterized. By introducing nonmetallic elements into the interstitial sites of Mn‐based materials, inexpensive, high‐capacity, and long‐cycling/calendar‐life Co/Ni‐free monometallic positive electrode materials may be further developed.

Ultrasound-responsive copolymeric micelles prepared by RAFT polymerization using a difunctional spiropyran-based RAFT agent: Investigation of mechanochromism kinetics, responsivities, and enhanced spiropyran mechanoactivation within micelles

An in-depth investigation of the ultrasonic response of mechanophores containing weak covalent bonds within nanosized aggregates is vital for ultrasound imaging, drug delivery, and sensors. In this work, well-defined amphiphilic block SP-[poly(methyl methacrylate-b-N-isopropylacrylamide)]2SP-[p(MMA-b-NIPAM)]2 (P1) and amphiphilic random, SP-[poly(methyl methacrylate-r-N-isopropylacrylamide)]2SP-[p(MMA-r-NIPAM)]2 (P2) copolymers with SP mechanophore embedded at midpoint polymer chain are prepared by RAFT (reversible addition-fragmentation chain transfer) polymerization using a pre-synthesized novel functional bi (trithiocarbonate) and spiropyran (SP)-based RAFT reagent. Both P1 and P2 copolymers self-assemble in spherical micelles in AcCN/water mixed solvents, and several assemblies with different water contents are fabricated. The structural differentiation between block P1 and random P2 dominates the inverse influence of water content on the aggregate sizes of micelles. Under ultrasound irradiation, the SP mechanophore is activated within both micelles, and the amount of activated SP mechanophore increases progressively with prolonged sonication time. For P1, the mechanochemical reaction (mechanochromism) kinetics in an aggregated state and a single chain state are different, while that of P2 micelles and liner are similar, inferred to be associated with the structure difference between both micelles. The mechanoactivation of SP mechanophore within micelles in AcCN/water mixed solvents is higher than in dissolved state due to the micellization of P1 and P2, entangling the hydrophobic PMMA segments and partially swelling in the micellar core. Moreover, an increase in the medium's dielectric constant around the core's SP is also observed, promoting the SP mechanoactivation. This approach may provide a new strategy for preparing novel mechanophore-centered amphiphilic copolymers with colorful structures and high hydrophilicity. These can self-aggregate into nanoparticles in aqueous or mixed solutions, exhibiting effective effects of ultrasonic responsiveness. We expect this work to push forward the in-depth studies on the ultrasonic responsive mechanism of assembly and help open new research avenues into the mechanochemical reactions within nanosized aggregates.

Effect of solvent quality on covalent bonds cleavage behaviors of lignite during pyrolysis/liquefaction processes

Effect of solvent quality on coal conversion and product distribution during lignite pyrolysis/liquefaction were investigated at the temperatures ranging from 180 to 460 °C, and the covalent bond cleavage behaviors in different solvents were comparatively elucidated. Results showed that at the temperatures below 280 °C, weak covalent bonds, such as methoxy and carboxyl groups, are cleaved to generate a small amount of gaseous products (mainly CO2) and liquefied oil. At the temperatures of 330–380 °C, ether bridges that connecting the aromatic clusters are cleaved and contributed significantly to the production of preasphaltene (PA). Meanwhile, oil yield and coal conversion increased to 27.50% and 64.58% in tetralin. At the temperatures of 430–460 °C, the cleavage of CH2 bridges, alkyl substituent groups and aromatic heterocyclic structures proceeded markedly, and the scission of CH2 bridges and some naphthenic CH2 bonds significantly contributed to oil production. Consequently, oil yield and coal conversion greatly increased to 43.90% and 84.13% in tetralin. Hydrogen donor solvent is conducive to the cleavage of strong covalent bonds, such as Car–O–Car and Car-Cal bonds, to realize the efficient liquefaction of coal.

Effects of deep eutectic solvents pretreatment on coal physicochemical characteristics

ABSTRACT Coal is a major resource in China both for chemicals production and power generation, but its impact on the environment demands attention. Solvent pretreatment is an effective method to realize cleaner utilization of coal. Shendong sub-bituminous coal was treated with three different deep eutectic solvents (DESs) – choline chloride (ChCl)-urea, ChCl-glycerol, and ChCl- ethylene glycol – at temperature as low as 120°C. Proximate/ultimate analyses, scanning electron microscopy, thermogravimetric analysis, and Fourier transform infrared spectroscopy were used to examine the physicochemical changes that occurred in coal after DESs treatment. Results showed that DES treatment led to coal fragmentation and decreased average particle size. Compared with raw coal (33.1%), DES-treated coal showed greater mass loss (35.2 − 37.7%), showing that the amount of low molecular mass species increased after treatment. This was confirmed by the observation that DES-treated coal had more volatile components and less minerals and functional groups that contained oxygen (especially C−O). The content of elemental sulfur also decreased from 0.26% to 0.10 − 0.15% after DES treatment, along with a reduction in aliphatic chain length and aromatic substitution degree, which were from 10.73 to 9.33−10.08 and 16.70% to 10.73−15.36%, respectively. The macromolecular structure of the coal was destroyed as a result of DES’s interactions with the weak covalent bonds and hydrogen bonds in the coal. These results suggest that DES has the potential to be used as a pretreatment solvent for coal conversion.

1‐Butyl‐3‐methylimidazolium chloride involved in sequential extraction of lignite to separate group components and its effect on functional groups of residues

AbstractBACKGROUNDIn view of the low economic benefit for direct utilization of lignite, the ionic liquid 1‐butyl‐3‐methylimidazolium chloride ([Bmim]Cl) has been used to extract Inner Mongolia lignite (IML) in order to efficiently separate lignite, develop the technology for clean and value‐added utilization of lignite, and alleviate the enormous pressure of ecological environment pollution. Subsequently, sequential extraction of IML by [Bmim]Cl/N‐methyl‐2‐pyrrolidone (NMP), acetonitrile and ether was employed to separate group components in lignite.RESULTSFunctional group results show that hydrogen bonds of IML including OHether, COOH dimers and SHN, and weak covalent bonds such as CalO, CalN, CalS and SS, can be cleaved by [Bmim]Cl. Extraction experiments using mixed solvents of [Bmim]Cl and NMP or water indicate that the synergistic effect of NMP and [Bmim]Cl plays a vital role in increasing lignite extraction yield, but for water an opposite effect is observed. Importantly, the group components extracted by [Bmim]Cl/NMP and consecutively by acetonitrile and ether show a clear difference in composition according to gas chromatography–mass spectrometry. Specifically, polycyclic aromatic hydrocarbons and long‐chain alkanes are present in the acetonitrile phase, with relative content of 8% and 88%, respectively, while most of esters, aliphatic acids and phenols are dissolved in ether with proportions of 35.24%, 35.53% and 16.57%, respectively.CONCLUSIONThis work demonstrates that sequential extraction of lignite by [Bmim]Cl/NMP, acetonitrile and ether may provide new insights for the separation of group components and clean utilization of lignite in industry. © 2023 Society of Chemical Industry (SCI).

Coal/NH3 interactions during co-pyrolysis and their effects on the char reactivity for NO-reduction: A ReaxFF MD study

In this work, the interactions between NH3 and coal during co-pyrolysis and their influences on char structure and char reactivity for NO reduction are investigated using reactive molecular dynamic (ReaxFF MD) simulations, revealing that coal promotes the thermal decomposition of NH3 because it provides H radicals to attack the N-H bonds in NH3. The primary pyrolysis reactions (thermal breaking of weak covalent bonds) of coal are inhibited by NH3·NH3 also influences the secondary reactions in coal pyrolysis through two aspects: (1) promotes tar cracking to generate gas, and (2) inhibits the polymerization of tar to form char. Compared with coal pyrolysis, the H/C ratio of char and number of O and N atoms in char decrease more slowly during coal/NH3 co-pyrolysis. However, NH3 has little influence on the release of H2S. The effect of NH3 on the physicochemical properties of the char is then analyzed. The oxygen atoms are present mainly as hydroxyl, ether, and carbonyl forms, and the nitrogen atoms are present mainly as cyanide and amino forms in both coal pyrolysis char and coal/NH3 co-pyrolysis char. The coal/NH3 co-pyrolysis char has lower helium density, lower graphitization degree, and larger pores compared with coal pyrolysis char. Finally, the reactivity of coal pyrolysis char and coal/NH3 co-pyrolysis char for NO reduction are examined. The coal/NH3 co-pyrolysis char is more conducive to NO chemisorption and N2 generation. Using a first-order kinetics model, the activation energy of NO reduction reactions on coal pyrolysis char and coal/NH3 co-pyrolysis char are determined as 180 and 123 kJ/mol, respectively.

Designing ternary superconducting hydrides with A15-type structure at moderate pressures

Ever since hydrogen-based superconductors have been actively considered as candidates for room-temperature superconductors, binary hydrides have been almost completely explored theoretically. Now researchers are turning their interest to ternary hydrides. However, the enormous compositional and structural possibilities in the ternary hydride space limit the search efficiency. Here, using an advanced crystal structure search method combined with first-principles calculations, a series of ternary hydrides AXH6 (MgPH6, MgAsH6, CaSbH6, CaSnH6, NaSbH6 and ScSiH6) with the A15-type structure are predicted to be stable at around one million atmospheres. Among them, CaSnH6 presents the lowest thermodynamically stable pressure of 110 GPa. CaSbH6, CaSnH6 and ScSiH6 may be dynamically stable down to 41 (70), 71 (90) and 86 (70) GPa, respectively, without (with) anharmonic effects and temperature effects. Hydrogen atoms in these A15-type structures are found to form weak covalent bonds to X atoms. More interestingly, we find that ternary hydrides with the AXH6 stoichiometry may assume the A15-type structure under pressure when the sum of the chemical valences of the two non-hydrogen atoms is six or seven. Based on electron-phonon coupling (EPC) calculations, all these ternary hydrides are estimated to be superconductors. CaSbH6, CaSnH6 and ScSiH6 have superconducting transition temperatures (Tcs) of 65, 53 and 116 K at 45, 75 and 90 GPa, respectively, while MgPH6 has the highest Tc of 148 K at 135 GPa. Our study will provide guidance for designing high Tc ternary hydrides near ambient pressures.

Green two-step oxidation for efficient utilization of Huolinhe lignite under the vision of carbon neutrality

To realize carbon neutrality in energy transition process, a two-step oxidation of Huolinhe lignite (HL) with less CO2 emissions is proposed, including pretreatment with H2O2 (PT) or pretreatment with Fenton’s reagent (PTF) and further oxidation in aqueous sodium hypochlorite (ASHC). The results indicate that the insoluble macromolecular moieties in HL are mainly oxidized into alkanedioic acids (ADAs) and non-substituted benzenecarboxylic acids (NSBCAs) and the yield of soluble portion is elevated by PTF from 60.56 % to 74.99 %. The increase in the ADA carbon number indicates that PT and PTF could enhance the oxidation ability of ASHC to break bridged linkages with longer chains. As for NSBCAs, the carboxylic group number (CGN) ranges from 2 to 5 in E1 and 2 to 6 in E2 and E3, respectively. The absence of mellitic acid in E1 states the limited oxidation capacity of ASHC to degrade highly condensed aromatic rings, which could be improved by PT and PTF. Moreover, the highest yield of NSBCAs is achieved at CGN = 2 in E2 and CGN = 4 in E3, implying that PTF is more effective to destruct highly condensed aromatic species in HL. According to the characterizations of coal samples, PT could increase the volatile matter and oxygen contents of HL and create new micropores to corrode HL viaOH attack, which would be enhanced by the introduction of Fe2+ during PTF to break weak covalent bonds, resulting in the preliminary degradation of macromolecular networks in HL. Then, cleavage of strong bonds undergoes easily via the attack of O2− generated from ASHC, leading to the deep decomposition of macromolecular moieties in HL.

Pyrolysis behaviors of model compounds with representative oxygen-containing functional groups in carbonaceous feedstock over calcium

Compounds with different functional groups and bridged bonds have been extensively used as model compounds of carbonaceous feedstock to investigate the mechanism of catalytic pyrolysis. Aiming to understand the structure evolution mechanism of different oxygen-containing functional groups and weak-covalent bonds during catalytic pyrolysis of carbonaceous feedstocks over calcium, polyacrylic acid, polyvinyl alcohol, polyethylene terephthalate, and polymethyl methacrylate were used as model compounds. The structural characteristics, reactivity, and pyrolysis products of model compounds with and without calcium addition were investigated by using X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, thermal gravimetric analysis, and in-situ pyrolysis vacuum ultraviolet photoionization time-of-flight mass spectrometry (in-situ Py-VUV-PIMS). The results show that calcium can form calcium complexes with the carboxyl or alcoholic hydroxyl structures in the long aliphatic carbon chain, but it has no obvious effect on the structure and reactivity of the ester groups in different compounds. The pyrolysis products analysis showed that the chemical structure and decomposition pathway of the model compounds containing carboxyl or alcoholic hydroxyl groups changed significantly after calcium addition, and a large number of carbonyl compounds appeared in the pyrolysis products. This result provides a theoretical basis for understanding the mechanism of calcium-catalyzed carbonaceous feedstock pyrolysis.

UV-irradiation reduces the mechanical performance of wood-adhesive interphase

ABSTRACT UV irradiation leads to various usage issues of engineered wood products (EWPs), including discoloration and declined mechanical performance. The radical generation and physico-chemical changes of the wood-phenol formaldehyde (PF) adhesive interphase during alternating UV irradiation were investigated in this paper, aiming to understand the oxidation and degradation of EWPs in daily applications. Electron spin resonance spectroscopy demonstrated that UV irradiation activates the phenol ring in lignin and PF adhesive, generating the reactive oxygen species (ROSs) composed of phenoxyl radicals, singlet oxygen, superoxide anion, and hydroxyl radicals. Experiments also showed that ROSs cleavage the weak covalent bonds in the cell wall and PF adhesive, triggering photon-oxidation and degradation. The cavities in the UV irradiated cell wall were observed after UV irradiation by scanning electron microscopy. Nanoindentation analysis also demonstrated the reduced mechanical performance of cell walls in the wood-adhesive interphase and bulk wood, leading to the significant reduction in the shearing and compression strength of wood-PF adhesive bonds. In contrast, the chemical structure and mechanical performance of PF adhesive remained after UV irradiation, suggesting better stability to UV exposure. It has been demonstrated that the degradation of wood has priority during the alternating UV irradiation.

The nature of supermolecular bonds: Investigating hydrocarbon linked beryllium solvated electron precursors.

Beryllium ammonia complexes Be(NH3)4 are known to bear two diffuse electrons in the periphery of a Be(NH3)4 2+ skeleton. The replacement of one ammonia with a methyl group forms CH3Be(NH3)3 with one peripheral electron, which is shown to maintain the hydrogenic-type shell model observed for Li(NH3)4. Two CH3Be(NH3)3 monomers are together linked by aliphatic chains to form strongly bound beryllium ammonia complexes, (NH3)3Be(CH2)nBe(NH3)3, n = 1-6, with one electron around each beryllium ammonia center. In the case of a linear carbon chain, this system can be seen as the analog of two hydrogen atoms approaching each other at specific distances (determined by n). We show that the two electrons occupy diffuse s-type orbitals and can couple exactly as in H2 in either a triplet or singlet state. For long hydrocarbon chains, the singlet is an open-shell singlet nearly degenerate with the triplet spin state, which transforms to a closed-shell singlet for n = 1 imitating the σ-covalent bond of H2. The biradical character of the system is analyzed, and the singlet-triplet splitting is estimated as a function of n based on multi-reference calculations. Finally, we consider the case of bent hydrocarbon chains, which allows the closer proximity of the two diffuse electrons for larger chains and the formation of a direct covalent bond between the two diffuse electrons, which happens for two Li(NH3)4 complexes converting the open-shell to closed-shell singlets. The energy cost for bending the hydrocarbon chain is nearly compensated by the formation of the weak covalent bond rendering bent and linear structures nearly isoenergetic.

A method to evaluate hydrogen donation ability of hydrogen-storage solvent in hydrogenation process by radical-precursor compounds

Hydrogen storage using organic solvent has attracted worldwide attention. Such solvents, generally named hydrogen storage solvent (HS), are expected to release H2 but can also be used directly in hydrogenation reactions. Hydrogen donation capacity (QHS-H) and initial hydrogen donation temperature (Ti) of HSs are important parameters in hydrogenation reaction. This work develops a simple method to determine these two parameters using radical-precursors (RP) containing weak covalent bonds. The test is carried out in an ambient-pressure glass microreactor system. The idea is estimating QHS-H by the amount of hydrogen obtained by RPs (QRP+H) and determining Ti by the first-order kinetics of RP hydrogenation reaction. Reliability of the method is verified by evaluating QHS-H of the structure–known HSs, 9,10-dihydrophenanthrene (DHP), 9,10-dihydroanthracene (DHA) and tetralin (THN) with benzyl phenyl ether (BPE) and bibenzyl (BBZ) as the RPs. Results show that QRP+H is close to QHS-H, and the p-value of F-test is higher than 0.05 at a confidence level of 95%. The Ti can be determined by the first-order kinetics of BPE hydrogenation.