Covalent Bond Energy Research Articles

Oxindolyl-Based Radicals with Tunable Mechanochromic and Thermochromic Behavior.

Organic radicals based dynamic covalent chemistry is promising in preparing stimuli-responsive chromic materials, due to their simplicity of dissociation/association, accompanied with distinct color changes during the process. However, suitable organic radicals for dynamic covalent chemistry have not been widely explored yet. Herein, a series of oxindolyl-based mono-radicals (OxRs) with different substituents were successfully synthesized and studied systematically as potential building blocks for stimuli-responsive chromic materials. These OxRs would dimerize spontaneously to form their corresponding dimers. The structures of dimers were unambiguously confirmed through low-temperature 1H NMR and single-crystal X-ray diffraction analyses. Dynamic interconversion between monomers and dimers was achieved by reversible cleavage and recovery of the σ-bond upon soft external stimuli (temperature, pressure, and solvent polarity), accompanied by significant color changes. It is interesting that the stability of the mono-radical could be tuned through changing different substituents, and consequently altering the bond dissociation energy of the dynamic covalent bond between monomers. These new OxRs characterized by appreciable properties are entitled to more opportunities in developing mechanochromic and thermochromic materials, where their responsiveness to stimuli can be readily controlled by the substituents adhered.

Oxindolyl‐based Radicals with Tunable Mechanochromic and Thermochromic Behavior

Organic radicals based dynamic covalent chemistry is promising in preparing stimuli‐responsive chromic materials, due to their simplicity of dissociation/association, accompanied with distinct color changes during the process. However, suitable organic radicals for dynamic covalent chemistry have not been widely explored yet. Herein, a series of oxindolyl‐based mono‐radicals (OxRs) with different substituents were successfully synthesized and studied systematically as potential building blocks for stimuli‐responsive chromic materials. These OxRs would dimerize spontaneously to form their corresponding dimers. The structures of dimers were unambiguously confirmed through low‐temperature 1H‐NMR and single‐crystal X‐ray diffraction analyses. Dynamic interconversion between monomers and dimers was achieved by reversible cleavage and recovery of the σ‐bond upon soft external stimuli (temperature, pressure, and solvent polarity), accompanied by significant color changes. It is interesting that the stability of the mono‐radical could be tuned through changing different substituents, and consequently altering the bond dissociation energy of the dynamic covalent bond between monomers. These new OxRs characterized by appreciable properties are entitled to more opportunities in developing mechanochromic and thermochromic materials, where their responsiveness to stimuli can be readily controlled by the substituents adhered.

Lignin-based vitrimer containing dynamic borate ester bonds with intrinsic photoconversion and excellent photothermal remoldability

The development of photoresponsive shape memory materials based on the photothermal conversion properties of lignin and the low activation energy of the dynamic covalent borate bond is of great importance. In this paper, a kind of lignin-based vitrimer polymer (LBP) containing dynamic boronic ester bonds was prepared by a “sulfhydryl-epoxy” click reaction and etherification reaction. The results show that the rigid segment EP-EL (lignin-based epoxy resin) and BDB (2,2′-(1,4-phenylene)-bis-[4-mercapto-1,3,2-dioxaneborane]) with benzene ring structure can impart tensile strength (20.8 MPa) to the LBP, while the flexible segment PEG imparts good elongation at break (15 %). The dynamic binding and dissociation exchange mechanism of the boronic ester bonds enables LBP to exhibit thermal remodelling properties (up to 36.2 %) and water-assisted self-healing properties at room temperature (up to 49.0 %). In addition, LBP exhibits excellent thermal and light-responsive shape memory properties due to its own photothermal conversion performance (photothermal conversion efficiency up to 18.2 %) and the dynamic boronic ester bond thermal activation bond exchange mechanism. The insulating properties of LBP make it suitable for use in high temperature protection circuit devices and light-responsive circuit devices. This study provides new insights into the design and application of Vitrimer and photoresponsive shape memory polymers, and also offers a new avenue for high-value utilization of lignin.

Self-indicating polymers: a pathway to intelligent materials.

Self-indicating polymers have emerged as a promising class of smart materials that possess the unique ability to undergo detectable variations in their physical or chemical properties in response to various stimuli. This article presents an overview of the most important mechanisms through which these materials exhibit self-indication, including aggregation, phase transition, covalent and non-covalent bond cleavage, isomerization, charge transfer, and energy transfer. Aggregation is a prevalent mechanism observed in self-indicating polymers, where changes in the degree of molecular organization result in variations in optical or electrical properties. Phase transition-induced self-indication relies on the transformation between different phases, such as liquid-to-solid or crystalline-to-amorphous transitions, leading to observable changes in color or conductivity. Covalent bond cleavage-based self-indicating polymers undergo controlled degradation or fragmentation upon exposure to specific triggers, resulting in noticeable variations in their structural or mechanical properties. Isomerization is another crucial mechanism exploited in self-indicating polymers, where the reversible transformation between the different isomeric forms induces detectable changes in fluorescence or absorption spectra. Charge transfer-based self-indicating polymers rely on the modulation of electron or hole transfer within the polymer backbone, manifesting as changes in electrical conductivity or redox properties. Energy transfer is an essential mechanism utilized by certain self-indicating polymers, where energy transfer between chromophores or fluorophores leads to variations in the emission characteristics. Furthermore, this review article highlights the diverse range of applications for self-indicating polymers. These materials find particular use in sensing and monitoring applications, where their responsive nature enables them to act as sensors for specific analytes, environmental parameters, or mechanical stress. Self-indicating polymers have also been used in the development of smart materials, including stimuli-responsive coatings, drug delivery systems, food sensors, wearable devices, and molecular switches. The unique combination of tunable properties and responsiveness makes self-indicating polymers highly promising for future advancements in the fields of biotechnology, materials science, and electronics.

C60 adsorption on defective Si (1 0 0) surface having one missed dimer from atomic simulations at electrical level

The adsorption geometry and electronic information of C60 onto reconstructed Si (100) surface having two types of missed dimers were investigated from self-consistent charge density functional tight-binding (SCC-DFTB) simulations. The C60 molecular orientation being relative to the defective Si surface, bonding form, and the number and distribution of dimers in the silicon surface, greatly affect the packing configurations in the C60/Si (100) systems. Charge-difference density and Mulliken populations analysis indicated that C60 was chemically bound to the silicon substrate by C-Si covalent bonds and the bond energy of short CSi covalent bonds is higher than others. The different adsorption configurations of C60/Si systems have different chemical potentials, implying their various applications in the redox reactivity.

Mechanism of CO2 Capture and Release on Redox-Active Organic Electrodes

The mechanism of faradaic electro-swing for CO2 capture/release on a redox-active organic electrode is studied from the point of view of realizing a reversible process for CO2 separation. First, the cyclic voltammograms (CVs) of two redox-active organic monomers, anthraquinone (AQ) and 2,1,3-benzothiadiazole (BTZ), were measured under a CO2 atmosphere. The waveforms of CVs of the two redox-active organic monomers are altered under a CO2 atmosphere relative to a N2 atmosphere. There is a change in the number of redox waves from two to one for AQ and a change from reversible to irreversible waves for BTZ. To further understand the mechanisms of CO2 capture/release on redox-active organic compounds, redox-active polymer electrodes coated with polyanthraquinone (PAQ) and polybenzothiadiazole (PBTZ) were investigated using a spectroscopic analysis known as in situ attenuated total reflectance–surface-enhanced infrared absorption spectroscopy (ATR–SEIRAS) as well as density functional theory (DFT) calculations. The CVs measured with two redox-active polymer electrodes have more positive shifts of reduction potentials for PAQ and PBTZ under a CO2 atmosphere than under an N2 atmosphere, as measured versus Ag/AgNO3: −1.8 and −1.4 V → −1.4 and −0.9 V for PAQ and −2.5 V → −2.1 and −1.9 V for PBTZ. In addition, the oxidation current on the PBTZ electrode disappears only under a CO2 atmosphere. DFT calculations indicate that the positive shifts of reduction potentials in the two electrodes under CO2 conditions are due to an exergonic adsorption reaction of CO2 onto the redox-active organic compounds. To clarify the reaction behavior between CO2 and the redox-active organic electrode, an ATR–SEIRAS spectroscopic analysis was performed. Infrared peaks are observed at 2200 and 2100 cm–1 for PAQ and PBTZ electrodes, respectively, under a CO2 atmosphere, which have been confirmed by measurements under a 13CO2 atmosphere to have adsorbed CO2. The wavenumbers corresponding to the CO2 molecules adsorbed on the two electrodes are different. These findings indicate that one electron reduction and CO2 adsorption for each redox-active organic compound are occurring simultaneously. The calculated adsorption energy of CO2 for two redox-active organic electrodes indicates that the adsorption energies of two CO2 molecules for AQ and BTZ are −7.3 and −36.3 kJ/mol. The larger adsorption energy in BTZ than that in AQ is clearly related to the disappearance of the oxidation current. However, both adsorption energies indicating adsorption of CO2 onto organic units are less than the covalent bond energies of common organic compounds, indicating that such weak adsorptions are suitable for the faradaic electro-swing of CO2 capture/release on the redox-active organic units.

The simplest Criegee intermediate CH2OO reaction with dimethylamine and trimethylamine: kinetics and atmospheric implications.

We have used the OH laser-induced fluorescence (LIF) method to measure the kinetics of the simplest Criegee intermediate (CH2OO) reacting with two abundant amines in the atmosphere: dimethylamine ((CH3)2NH) and trimethylamine ((CH3)3N). Our experiments were conducted under pseudo-first-order approximation conditions. The rate coefficients we report are (2.15 ± 0.28) × 10-11 cm3 molecule-1 s-1 for (CH3)2NH at 298 K and 10 Torr, and (1.56 ± 0.23) × 10-12 cm3 molecule-1 s-1 for (CH3)3N at 298 K and 25 Torr with Ar as the bath gas. Both reactions exhibit a negative temperature dependence. The activation energy and pre-exponential factors derived from the Arrhenius equation were (-2.03 ± 0.26) kcal mol-1 and (6.89 ± 0.90) × 10-13 cm3 molecule-1 s-1 for (CH3)2NH, and (-1.60 ± 0.24) kcal mol-1 and (1.06 ± 0.16) × 10-13 cm3 molecule-1 s-1 for (CH3)3N. We propose that the electronegativity of the atom in the co-reactant attached to the C atom of CH2OO, in addition to the dissociation energy of the fragile covalent bonds with H atoms (H-X bond), plays an important role in the 1,2-insertion reactions. Under certain circumstances, the title reactions can contribute to the sink of amines and Criegee intermediates and to the formation of secondary organic aerosol (SOA).

Scalability and Performance of a Program that Uses Domain Decomposition for Monte Carlo Simulation of Molecular Liquids

The main factors hindering the development of supercomputer programs for molecular simulation by the Monte Carlo method within the framework of classical physics are considered, and possible ways to eliminate the problems that arise in this case are discussed. Thus, the use of molecular models with moderate stiffness of covalent bonds between fragments makes it possible not only to increase the efficiency of scanning the configuration space of the model, but also to abandon the complex apparatus of kinematics with rigid links, which significantly limits the possibilities of domain decomposition. Based on the domain decomposition strategy and a simplified treatment of the deformation energy of covalent bonds and angles, an original parallel algorithm for calculating the properties of large all-atomic models of aqueous solutions of biopolymers by the Monte Carlo method was developed. To speed up computations within the framework of this approach, each domain is assigned its own group of processors/cores using local data replication and splitting the loop over the interacting partners. The article discusses the logical scheme of the computational algorithm and the main components of the software package (fortran77, MPI 1.2). Test calculations performed for water and n-hexane demonstrated the high performance and scalability of the program in which the proposed algorithm was implemented.

Advanced descriptors for long-range noncovalent interactions between SARS-CoV-2 spikes and polymer surfaces.

The recent pandemic triggered numerous societal efforts aimed to control and limit the spread of SARS-CoV-2. One of these aspects is related on how the virion interacts with inanimate surfaces, which might be the source of secondary infection. Although recent works address the adsorption of the spike protein on surfaces, there is no information concerning the long-range interactions between spike and surfaces, experimented by the virion when is dispersed in the droplet before its possible adsorption. Some descriptors, namely the interaction potentials per single protein and global potentials, were calculated in this work. These descriptors, evaluated for the closed and open states of the spike protein, are correlated to the long-range noncovalent interactions between the SARS-CoV-2 spikes and polymeric surfaces. They are associated with the surface's affinity towards SARS-CoV-2 dispersed in respiratory droplets or water solutions. Molecular-Dynamics simulations were performed to model the surface of three synthetic polymeric materials: Polypropylene (PP), Polyethylene Terephthalate (PET), and Polylactic Acid (PLA), used in Molecular Mechanics simulations to define the above potentials. The descriptors show a similar trend for the three surfaces, highlighting a greater affinity towards the spikes of PP and PLA over PET. For closed and open structures, the long-range interactions with the surfaces decreased in the following order PP ∼ PLA > PET and PLA > PP > PET, respectively. Thus, PLA and PP interact with the virion quite distant from these surfaces to a greater extent concerning the PET surface, however, the differences among the considered surfaces were small. The global potentials show that the long-range interactions are weak compared to classic binding energy of covalent or ionic bonds. The proposed descriptors are useful most of all for a comparative study aimed at quickly preliminary screening of polymeric surfaces. The obtained results should be validated by more accurate method which will be subject of a subsequent work.

Cation-Anion Interactions, Stability, and IR Spectra of Dicationic Amino Acid-Based Ionic Liquids Probed Using Density Functional Theory.

In this work, we have theoretically studied the dicationic ionic liquids (DILs) constructed from geminal methylimidazolium dication with varying amino acid anions and spacers using density functional theory. Amino acid-based DILs form via strong C-H···O hydrogen bonds. These hydrogen bonds have a significant role in stabilizing the DILs. The higher cation-anion interaction energy in the order of covalent bond energy and liquid density of DILs imply higher thermal stability than their mono analogues. The C-H stretching frequencies are above 3100cm-1 in all complexes and form a signature for DILs. Interestingly, aliphatic and aromatic amino acid anions show similar molecular properties. Overall, the DILs formed from amino acids exhibit high stability and large surface tension and are chemically non-toxic; hence, they can replace inorganic DILs.

An S10-Symmetric 5-Fold Interlocked [2]Catenane.

The reaction of sym-pentakis(4-aminothiophenyl)corannulene with 2-formyl-6-methylpyridine and CuI or 2-formyl-1,10-phenanthroline and MII (M = Co, Zn) yields an S10-symmetric 5-fold interlocked [2]catenane of two interpenetrating [CuI5L2]5+ cages or D5-symmetric [MII5L2]10+ cages, respectively. The new structures were characterized by X-ray crystallography, NMR spectroscopy, and mass spectrometry. Density functional theory computations point to dispersive energies on par with traditional covalent bond energies. Subcomponent exchange reactions favored formation of the [CoII5L2]10+ cage over the [CuI10L4]10+ catenane. The single cage and catenane each cocrystallized with a corannulene guest to form a bowl-in-bowl substructure.

On fractional charge in molecules and materials

Determining the charge on an atom in molecules and materials is a long-standing open issue. Quantitative analysis with clear physical picture of charge transfer, electrostatic and covalent contributions to bond energy is highly desirable to advance the understanding of both charge and ionicity. However, the consensus among computations from quantum mechanics to semiempirical methods and state-of-the-art experimental measurement on fractional charge remains elusive. In this work, we generalize Born–Haber thermochemical cycle to compute fractional charge on an atom by delicately balancing the interplay of all interacting components, including charge transfer, electrostatic–covalent interactions and bond energy in molecules. The fractional charge of an atom in solid materials can be also computed straightforwardly with this scheme.

Theoretical Calculation and Analysis of Coherent Interfacial Energy between B1-Type Carbides and FCC Iron Matrix

The valence electron structures (VES) of unit cells of the FCC iron matrix (γ) and B1-type carbides (ξ) in γ-Fe-M-C alloy systems (M=Nb, Ti, and V) were calculated using the empirical electron theory of solids and molecules (EET). Based on the results, the coherent interfacial energy between B1-type carbides and FCC iron matrix in γ-Fe-M-C alloy system was calculated and analyzed by combining EET and the discrete lattice plane-nearest neighbor broken bond (DLP-NNBB) method with covalent bond energy. The results showed that the solid solution strengthening effect was produced by the solute phase boundary segregation of the alloy elements at the γ/ξ interface, which enhanced the covalent bond network. The Nb alloy system had the largest segregation effect in γ-Fe-M-C alloy system. Moreover, the γ/ξ interfacial energy varies with different alloy elements over the range of 0.4-1.3 J/m2, and the γ-Fe-Nb-C alloy system has the largest coherent interfacial energy.The proposed theoretical method is consistent with the theoretical values calculated using other models, as well as empirical values. This indicates that the method proposed in this work is feasible for providing theoretical analysis and guidance.

Effect of Zr and Sc on microstructure and properties of 7136 aluminum alloy

We have linked experimental results to theoretical calculations and discussed the precipitation behavior of Zr and Sc in 7136 alloy and the influence of precipitation on the microstructure and mechanical properties of the alloy. The experimental results show that the added Sc atom can replace the Zr atom in the cubic Al3Zr phase with a lattice constant of a = 0.4417 nm, and form Al3(ScxZr1-x), which is also cubic and has a lattice constant of a = 0.4212 nm. A heterogeneous nucleation core promotes crystal-grain refinement, and the average size of the as-cast crystal grains is reduced from 82.4 μm to 51.7 μm. The valence electron structure and strongest bond energy of the Al3Zr phase with different structures was analyzed by the empirical electron theory. For Ll2-Al3Zr, the covalent electron number and bond energy of the strongest bonds are 0.3226 and 52.8 kJ/mol, respectively, and for DO23-Al3Zr, the numerical values are 0.4471 and 71.0 kJ/mol, respectively. Combined with the ab initio calculation, Ll2-Al3Zr has a good stability. Under suitable conditions, Ll2-Al3Zr will transform into the more stable DO23-Al3Zr. Quantitative analysis of the effect of strengthening mechanism such as fine grain strengthening and dispersion strengthening caused by the addition of Sc element on the yield strength of the alloy. The calculation results show that 0.2% elemental Sc addition can increase the alloy strength by 36.63 MPa, which is very similar to the measured result of 38.61 MPa.

Description of the Donor—Acceptor Bond in Terms of the Theory of Generalized Charges

Previously, a universal relation for the energy and length of homopolar covalent bond has been derived with the use of the theory of generalized charges, which represents an asymptotic approximation for interatomic forces of the quantum-statistical model of a multicomponent electron gas. The article discusses the new implications of the theory, obtained to describe the donor-acceptor bond. The bond energy includes the contribution of the donor atom characteristic of a homopolar bond and the Coulomb interaction of the atoms, with allowance for the probability of fixation of the bond electrons on the acceptor. Regularities for the donor-acceptor bond length and energy have been discovered. In particular, for the particular case realized in hydrides, expressions are obtained for the length and bond energy versus the atomic number of the acceptor.

Performance analysis of Fe-N compounds based on valence electron structure

In this paper, the valence electron structure and the bond energy of covalent bonds of α-Fe and ε-Fe2N, ε-Fe3N and γ′-Fe4N were calculated by means of the empirical electron theory (EET). It was put forward that the valence electron structure parameter related to hardness is the covalent electron density, the valence electron structure parameters related to plasticity are the lattice electron density and the symmetry factor of the atomic bonding strength, the valence electron structure parameters related to corrosion resistance are the atomic total bonding energy and lattice electron density, and the valence electron structure parameter related to stability is the average atomic total bonding energy. The hardening and corrosion resistance mechanism of ion-based nitriding layer was discussed and the influence of three Fe-N compounds on the hardness, brittleness, corrosion resistance and stability were compared and analyzed from the perspective of electron structure. The calculation results show that the three Fe-N compounds, which have larger covalent electron density and average atomic total bonding energy and smaller lattice electron density than α-Fe, can improve the hardness and corrosion resistance of the iron-based surface. Among the three compounds, ε-Fe2N has the greatest hardness, corrosion resistance and stability, and the least brittleness. Therefore, during the nitriding process, efforts shall be made to prompt the formation of ε-Fe2N as much as possible.

Calculations of stability of alloyed cementite from valance electron structure

Based on the empirical electronic theory of solids and molecules (EET), the actual model for unit cell of cementite (θ-Fe3C) was built and the valence electron structures (VES) of cementite with specified site and a number of Fe atoms substituted by alloying atoms of M ( M=Cr, V, W, Mo, Mn ) were computed by statistical method. By defining P as the stability factor, the stability of alloyed cementite with different numbers and sites of Fe atoms substituted by M was calculated. Calculation results show that the density of lattice electrons, the symmetry of distribution of covalent electron pairs and bond energy have huge influence on the stability of alloyed cementite. It is more stable as M substitutes for Fe2 than for Fe1. The alloyed cementite is the most stable when Cr, Mo, W and V substitute for 2 atoms of Fe2 at the sites of Nos. 2 and 3 (or No. 6 and No. 7). The stability of alloyed cementite decreases gradually as being substitutional doped by W, Cr, V, Mo and Mn.

Research on adiabatic shear failure character of pure copper and aluminum bronze based on empirical electron theory

The valence electron structure (VES) parameters affecting adiabatic shearing failure under high speed impact load using split Hopkinson pressure bar (SHPB) were studied by empirical electron theory (EET) of solids and molecules. There is a problem of multiple solutions about VES parameters in EET. The statistics values of VES parameters related to adiabatic shearing sensitivity were calculated to substitute for the most probable value among the multiple solutions according to the view that the microstate statistics values can reflect the macro physical quantity. The research shows that the adiabatic shearing sensitivity is growing with the rise of the statistics value of bond energy of the strongest covalent bond, and is decreasing with the rise of the statistics value of the lattice electron number. The statistics value of bond energy of the strongest covalent bond in aluminum bronze (QAl9-4) is larger than that in pure copper, and the statistics value of the lattice electron number in QAl9-4 is smaller than that in pure copper. Therefore, QAl9-4 is prone to adiabatic shearing failure, and the grains were only elongated due to the large deformation for pure copper without any adiabatic shear band (ASB). It is of great significance for the selection and design of material with different adiabatic shearing sensitivity to research the effect of alloy elements on adiabatic shearing sensitivity from the electronic structure perspective.

Atomistic prediction of plane stress behavior of glassy thermosets

When any covalent bond energy reached a threshold indicative of incipient bond scission, the simulation was stopped. This method was used to examine the responses of three highly-crosslinked epoxy systems. Systems were large enough to include 110 to 480 crosslink sites. Both elastic and yield properties show good agreement with the experiments of others. Continuum yield theories commonly applied to polymers are compared with the data. A Drucker-Prager pressure-dependent yield function applied best in the second and third quadrants of the σ1σ2 domain. In the first quadrant, however, data more closely match a normal-stress-yielding criterion. In biaxial tension and simple tension, plastic behavior and large growth in nano-porosity were observed. Ductility was lowest in simple tension and biaxial compression. In simple tension, the bonds at crosslink sites and in ether linkages were the most highly strained whereas carbon-carbon backbone bonds between phenyl groups were highly strained in other cases. When system energy at imminent bond rupture was divided on a per-atom basis, consistency with Peterlin’s theory for molecular rupture was found.

Interpretation of Methane and Hydrogen Evolution in Coal Pyrolysis from the Bond Cleavage Perspective

Pyrolysis of 34 coals of different rank with carbon contents of 73.7–91.9% was studied in a TG-MS system to analyze CH4 and H2 evolution. On the basis of the shape of evolution curves and dissociation energy of covalent bonds in coals, the CH4 formation is attributed to the cleavage of a Cal–Cal bond in a low temperature range and that of Cal–Car bond in a high temperature range, while the H2 formation was attributed to the cleavage of H–Cal bond in a low temperature range and that of Cal-Car bond in a high temperature range. The yields of CH4 and H2 corresponding to cleavage of each of these bonds are quantified by deconvolving each of the evolution curves into two subcurves. It is found that the cleavage of bonds containing only aliphatic carbon, such as Cal–Cal and H–Cal peaking at 549 and 540 °C, respectively, contributes to small fractions of CH4 and H2 generation. The cleavage of bonds containing an aromatic carbon, such as Cal–Car and H–Car peaking at temperatures of 618 and 774 °C, respectively, c...