Weak Covalent Bonds Research Articles

Theoretical study on the electronic structure, mechanical property, and thermal expansion of yttrium oxysulfide

The electronic structure, mechanical property and thermal expansion of yttrium oxysulfide are calculated from first-principles using the theory of density functional. The calculated cohesive energy indicates its thermodynamic stable nature. From bond structure, the calculated bandgap is obtained as 2.7 eV; and strong covalent bonds exist between Y and O atoms intra 2D [ Y – O ] layer in material, while relatively weak covalent bonds also exist inter 2D [ Y – O ] layer and S atoms. From simulation, it is found that the bulk modulus is about 119.4 GPa for the elastic constants, and the bulk modulus shows weak anisotropy because the surface contours of them are close to a spherical shape. The calculated B/G clearly implies its ductile nature, and the Y 2 O 2 S phase can also be compressed easily. The temperature dependence of thermal expansions is mainly caused by the restoration of thermal energy due to lattice excitations at low temperature. When the temperature is very high, the thermal expansion coefficient increases linearly with temperature increasing. Meanwhile, the heat capacities are also calculated and discussed by thermal expansion and elasticity.

Donor-acceptor coordination of anions by chalcogen atoms of 1,2,5-chalcogenadiazoles

Synthetic, structural, and thermodynamic aspects of the recently discovered new reaction, donor-acceptor coordination of anions (A–) by chalcogen atoms (E) of 1,2,5-chalcogenadiazoles, are considered. According to the quantum chemical calculations, the charge transfer from A– to the heterocycle via the mechanism of negative hyperconjugation (i.e., from the MO of the lone pair of A– to the virtual σ* orbital of the E—N bond of chalcogenadiazole) depends on the nature of E and A–, being 0.42—0.52 and 0.30—0.44 e in terms of the Mulliken and NBO methods, respectively. According to the X-ray diffraction data, the E—A– coordinate bond is always longer than the sum of the covalent radii but shorter than the sum of the van der Waals radii of the atoms forming the bond. The E—A– bond energy varies in a wide range, from ~25 kcal mol–1 comparable to the energy of weak covalent bonds (e.g., internal N—N bond in organic azides) to ~86 kcal mol–1 comparable to the C—C bond energy in organic compounds. The quantum chemical estimations of the thermodynamics of the donor-acceptor coordination of the anions by the chalcogen atoms of 1,2,5-chalcogenadiazoles indicate that for E = Te and Se this reaction may be of general character also covering E = S in some cases.

High-pressure structure prediction of Hf-C system and first-principle simulation of their electronic properties

Hafnium carbides (Hf-C system), known as ultra-high temperature ceramics, have attracted growing attention because of their unique features. In this paper, we carry out researches on the stable crystal structures in the Hf-C system at high pressures, using a variable-composition ab initio evolutionary algorithm implemented in the USPEX code. In addition to the ambient-pressure structures HfC (Fm3m), there are two new compounds Hf3C2 and Hf6C5 and two high-pressure structures of HfC. When pressures are lower than 100 GPa, no new structures are found other than those at ambient pressure, and Hf3C2 and Hf6C5 become metastable at 20 GPa and 100 GPa, respectively. At 200 GPa, a new compound Hf2C is found, and the stable structure HfC has changed from Fm3m to C2/m. At 300 GPa, another new compound HfC2 is found. At 400 GPa, the stable structure of HfC has changed again to the space group Pnma. And at 500 GPa, the stable structures are Hf2C, HfC2 and HfC (Pnma), no new structures are found except those at 400 GPa. The composition-pressure phase diagram that shows the pressure range of stable structures in Hf-C system is simulated by calculation of their enthalpies. When the pressures are lower than 15.5 GPa and 37.7 GPa, Hf3C2 and Hf6C5 are stable, respectively, and their space groups are both of C2/m. And Hf2C and HfC2, with space group I4/m and Immm, respectively become stable structures when the pressure is higher than 102.5 GPa and 215.5 GPa, respectively. The phase-transition route of HfC is Fm3mC2/mPnma, and the two phase-transition pressures are 185.5 GPa and 322 GPa, respectively, which are different from the conclusion of Zhao. Then we will show and discuss the newly predicted high-pressure structures and their crystallographic data, such as volume, lattice constants and atom positions. The crystal structures of HfC are described in the literature. The structure of Hf2C contains 12 atoms in the conventional cell, and carbon atoms lie at the center of decahedron consisting of 8 hafnium atoms. In the structure of HfC2, carbon atoms form the quasi-graphite sheets and hafnium atoms lie betweent the two sheets. The dynamical and mechanical stabilities of the high-pressure structures have been verified by calculations of their phonon dispersion curves and elastic constants. And the bulk modulus and shear modulus of HfC2 are larger than those of the other three high-pressure structures. Finally we will study their electronic properties, band structures, density of states (DOS), electron localization functions (ELFs), and the Bader charge analyses of these structures are simulated based on the first-principle. The band structure and density of states show that these four high-pressure structures have weak metallic and strong Hf-C covalent bond. The Bader charge analysis further proves the strong Hf-C covalent bond and weak ionic bond. And ELF shows the existence of CC covalent bond. In summary, the HfC bond shows strong covalence, weak metallicity and ionicity, and the CC bond is covalent.

Mechanochemical Reactions Reporting and Repairing Bond Scission in Polymers.

The past 10 years have seen a resurgence of interest in the field of polymer mechanochemistry. Whilst the destructive effects of mechanical force on polymer chains have been known for decades, it was only recently that researchers tapped into these forces to realize more useful chemical transformations. The current review discusses the strategic incorporation of weak covalent bonds in polymers to create materials with stress-sensing and damage-repairing properties. Firstly, the development of mechanochromism and mechanoluminescence as stress reporters is considered. The second half focuses on the net formation of covalent bonds as a response to mechanical force, via mechanocatalysis and mechanically unmasked chemical reactivity, and concludes with perspectives for the field.

Chalcogen bonding between tetravalent SF4 and amines.

The N···S chalcogen bond between SF4 and a series of alkyl and arylamines is examined via ab initio calculations. This bond is a strong one, with a binding energy that varies from a minimum of 7 kcal/mol for NH3 to 14 kcal/mol for trimethylamine. Its strength derives in large measure from charge transfer from the N lone pair into the σ*(SF) antibonding orbitals involving the two equatorial F atoms, one of which is disposed directly opposite the N atom. Decomposition of the total interaction energy reveals that the induction energy constitutes more than half of the total attraction. The positive region of the molecular electrostatic potential of SF4 that lies directly opposite the equatorial F atoms is attracted to the N lone pair, but the magnitude of this negative region on each amine is a poor predictor of the binding energy. The shortness and strength of the N···S bond in the dimethylamine···SF4 complex suggest it may better be described as a weak covalent bond.

Detection of resonant impurities in graphene by quantum capacitance measurement

We investigated Ag-adatom-induced resonant impurities of graphene by quantum capacitance measurement. Different from charged impurities and other conventional resonant impurities, Ag atoms form very weak covalent bonds with graphene. The Ag-adatom-induced resonant peak as measured by quantum capacitance grows more intense at cryogenic temperatures, at higher impurity concentrations, and in stronger magnetic fields, in accordance with our theoretical calculations. The appearance of resonant states and the split of the zeroth Landau level for Ag-adsorbed graphene are manifestations of the formation of a flat impurity band near the Dirac point.

Comparability of a Three-Dimensional Structure in Biopharmaceuticals Using Spectroscopic Methods

Protein structure depends on weak interactions and covalent bonds, like disulfide bridges, established according to the environmental conditions. Here, we present the validation of two spectroscopic methodologies for the measurement of free and unoxidized thiols, as an attribute of structural integrity, using 5,5′-dithionitrobenzoic acid (DTNB) and DyLight Maleimide (DLM) as derivatizing agents. These methods were used to compare Rituximab and Etanercept products from different manufacturers. Physicochemical comparability was demonstrated for Rituximab products as DTNB showed no statistical differences under native, denaturing, and denaturing-reducing conditions, with Student's t-test P values of 0.6233, 0.4022, and 0.1475, respectively. While for Etanercept products no statistical differences were observed under native (P = 0.0758) and denaturing conditions (P = 0.2450), denaturing-reducing conditions revealed cysteine contents of 98% and 101%, towards the theoretical value of 58, for the evaluated products from different Etanercept manufacturers. DLM supported equality between Rituximab products under native (P = 0.7499) and denaturing conditions (P = 0.8027), but showed statistical differences among Etanercept products under native conditions (P < 0.001). DLM suggested that Infinitam has fewer exposed thiols than Enbrel, although DTNB method, circular dichroism (CD), fluorescence (TCSPC), and activity (TNFα neutralization) showed no differences. Overall, this data revealed the capabilities and drawbacks of each thiol quantification technique and their correlation with protein structure.

Theoretical Mechanism Studies on the Enantioselectivity of aza-MBH-type Reaction of Nitroalkene to N-tosylimine Catalyzed by Thiourea-tertiary Amine

The enantioselective aza-Morita Baylis Hillman reaction of nitroalkene and N-tosylimine catalyzed by thiourea-tertiary amine has been investigated using density functional theory. Enantioselectivity is dominated by the cooperative effect of non-covalent and weak covalent interactions imposed by different units of catalyst. As Lewis base, the tertiary amine unit activates nitroalkene via weak covalent bond. The weak covalent interaction orients the reaction in a major path with smaller variations of this bond. The aromatic ring unit activates N-tosylimine via <TEX>${\pi}-{\pi}$</TEX> stacking. The non-covalent interaction selects the major path with smaller changes of the efficient packing areas. Thiourea unit donates more compact H-bonded network for species of the major path. The calculated ee value in xylene solution phase (97.6%) is much higher than that in N,N-Dimethylformamide (27.2%). Our conclusion is also supported by NBO analysis.

Interfacial bond dependence of damping properties of carbon nanotubes enhanced polymers

The effect of interfacial bond on damping properties of carbon nanotubes (CNTs) enhanced polymers is investigated. To acquire different interfacial bonds (including van der Waals, weak, and strong covalent bond) of CNTs enhanced polymers, different functionalizations of CNTs have been designed through acid treatment and silane termination. CNTs with silanization show superior dispersion state in the matrix regardless of CNTs content. When the CNTs content is lower than 0.2 wt%, a strong interfacial bond of CNTs/epoxy composites has a deleterious effect on the damping properties. When the CNTs content is greater than 0.2 wt%, the strong covalent bond becomes the dominant factor to enhance damping properties and slow down the decreasing rate of the damping properties. POLYM. COMPOS., 35:548–556, 2014. © 2013 Society of Plastics Engineers

Effect of crystal structure of manganese dioxide on response for electrolyte of a hydrogen sensor operative at room temperature

Sensoring properties of a hydrogen sensor that used electrolytes made of different crystal type manganese dioxides were compared. An electrolyte made of a manganese dioxide, which has a high purity of ramsdellite-type crystal structure, provided the best characteristics for the hydrogen sensor. To explain the sensor property, network model of oxygen-pairs to store protons with a weak covalent bond and to conduct protons along the network in the ideal crystal structure of ramsdellite manganese dioxide was proposed. The inter-atomic distance of those oxygen-pairs in the high purity of ramsdellite manganese dioxide was estimated between 2.57 and 2.60Å using inelastic neutron scattering measurements. The property of the hydrogen sensor supported the unique proton conduction based on the network model.

Density functional calculations on 13-atom Pd12M (M = Sc—Ni) bimetallic clusters

The geometric structures, electronic and magnetic properties of the 3d transition metal doped clusters Pd12M (M = Sc—Ni) are studied using the semi-core pseudopots density functional theory. The groundstate geometric structure of the Pd12M cluster is probably of pseudoicosahedron. The Ih-Pd12M cluster has the most thermodynamic stability in five different symmetric isomers. The energy gap shows that Pd12M cluster is partly metallic. Both the absolutely predominant metal bond and very weak covalent bond might exist in the Pd12M cluster. The magnetic moment of Pd12M varies from 0 to 5 μB, implying that it has a potential application in new nanomaterials with tunable magnetic properties.

Reversible multi polyelectrolyte layers on gold nanoparticles

Gold nanoparticles surface can be easily modified by different molecules such as polyelectrolytes. In a typical multilayer system made of polyethyleneimine and poly(styrene sulfonate)sodium alternated layers around gold nanoparticles, we have evaluated the interactions between the different layers and the relative strength of interfacial properties. By means of UV–Visible and FTIR spectroscopies, we have shown that due to its amine functionalities, the bonding of polyethyleneimine to gold particles is stronger than the one implied with the sulfonate anion in the PSS inducing a clean removal of this latter after the last polyethyleneimine deposition. Considering that polyethyleneimine is cytotoxic and that only weak covalent bonds are concerned in polyelectrolyte multilayer, this last point is of main importance since external degradation thus exposing polyethyleneimine sub-layer of multilayer films to in vivo tissue cells can occur by many ways.

First principles investigation of interaction between impurity atom (Si, Ge, Sn) and carbon atom in diamond-like carbon system

The interaction between impurity atom (Si, Ge, and Sn) and carbon atom in diamond-like carbon (DLC) system was investigated by the first principles simulation method based on the density functional theory. The tetrahedral configuration was selected as the calculation model for simplicity. When the bond angle varied in a range of 90°–130° from the equivalent state of 109.471°, the distortion energy and the electronic structures including charge density of the highest occupied molecular orbital (HOMO) and partial density of state (PDOS) in the different systems were calculated. The results showed that the addition of Si, Ge and Sn atom into amorphous carbon matrix significantly decreased the distortion energy of the system as the bond angles deviated from the equilibrium one. Further studies of the HOMO and PDOS indicated that the weak covalent bond between Si(Ge, Sn) and C atoms was formed with the decreased strength and directionality, which were influenced by the electronegative difference. These results implied that the electron transfer behavior at the junction of carbon nano-devices could be tailored by the impurity element, and the compressive stress in DLC films could be reduced by the incorporation of Si, Ge and Sn because of the formation of weaker covalent bonds.

High-pressure behavior of dense hydrogen up to 3.5 TPa from density functional theory calculations

Structural behavior and equation of state (EOS) of atomic and molecular crystal phases of dense hydrogen at pressures up to 3.5 TPa are systematically investigated with density functional theory. The results indicate that the Vinet EOS model that fitted to low-pressure experimental data overestimates the compressibility of dense hydrogen drastically when beyond 500 GPa. Metastable multi-atomic molecular phases with weak covalent bonds are observed. When compressed beyond about 2.8 TPa, these exotic low-coordinated phases become competitive with the ground state and other high-symmetry atomic phases. Using nudged elastic band method, the transition path and the associated energy barrier between these high-pressure phases are evaluated. In particular for the case of dissociation of diatomic molecular phase into the atomic metallic Cs-IV phase, the existent barrier might raise the transition pressure about 200 GPa at low temperatures. Plenty of flat and broad basins on the energy surface of dense hydrogen have been discovered, which should take a major responsibility for the highly anharmonic zero point vibrations of the lattice, as well as the quantum structure fluctuations in some extreme cases. At zero pressure, our analysis demonstrates that all of these atomic phases of dense hydrogen known so far are unquenchable.

Graphene on Ir(111): Physisorption with Chemical Modulation

The nonlocal van der Waals density functional approach is applied to calculate the binding of graphene to Ir(111). The precise agreement of the calculated mean height h = 3.41 Å of the C atoms with their mean height h = (3.38±0.04) Å as measured by the x-ray standing wave technique provides a benchmark for the applicability of the nonlocal functional. We find bonding of graphene to Ir(111) to be due to the van der Waals interaction with an antibonding average contribution from chemical interaction. Despite its globally repulsive character, in certain areas of the large graphene moiré unit cell charge accumulation between Ir substrate and graphene C atoms is observed, signaling a weak covalent bond formation.

The mechanics of charge-shift bonds: A perspective from the electronic stress tensor

The mechanism of charge-shift bonding is discussed from the perspective of the electronic stress tensor. In charge-shift bonds, the tensile mode of the stress tensor (attraction of electrons to the nuclei) is stronger, relative to the compressive modes (attraction of electrons toward the bond axis) than in conventional covalent bonds. Similar bonding indicators based on the eigenvalues of the tensor of second derivatives of the electron density and bond metallicity measures also correlate with charge-shift binding. These indicators seem preferable to the electron-density Laplacian and the local energy density because they can distinguish between weak covalent bonds and charge-shift bonds.

Experimental and therotical study on the mechanical properties of AlMgB&amp;lt;sub&amp;gt;14&amp;lt;/sub&amp;gt;

Al-Mg-B films have been the subject of numerous investigations because their attractive properties, including high hardness, high melting point, low density, chemical inertness and thermal stability at high temperature. These materials may be useful in the fields of mechanical, aerospace and military. In this paper, using multi-target magnetron sputtering, we prepared Al-Mg-B amorphous film with atomic ratio approach to Al:Mg:B =1:1:14 on the silicon (100) substrate at room temperature by tuning the sputtering conditions. For the Al-Mg-B films along the AlMg isocontent line, nanoindentation test indicated that the hardness of films increase from 13 GPa to 32 GPa with the contents of B increasing, and the value would reached to 25-32 GPa for AlMgB14. Meanwhile, using first-principles calculations, we estimated the Vickers hardness of AlMgB14 crystal is 27.6 GPa, which agreed to the experimental value. Based on the electron density of states and Mulliken population analysis, the origination of hardness of AlMgB14 crystal other B12-based ternary compounds were discussed. The crystal hardness is primarily determined by the B12 icosahedral skeleton, whereas the contributions of metal atoms manifest as the electron transfer from metal to B atoms, showing weak covalent bond of Al-B and the ionic characteristics of Mg-B bonds, which is also demonstrated by our experimental XPS core level spectra. Moreover, we also investigated the thermodynamic property of AlMgB14 crystal by Quasi-harmonic Debye model, and found that the bulk modulus decrease with the temperature rising. Such tendency was opposite to the experiment results of Al-Mg-B films have higher hardness under high substrate temperature, which implying the role of temperature is to strengthen the adhesion between film and substrate rather than improving the true hardness of the materials.

A DFT investigation of the effects of doped Pb atoms on Pd n clusters (13 ⩽ n ⩽ 116)

Basic properties of cuboctahedral Pd–Pb clusters ranging from 13 to 116 atoms were studied by computational analysis, and the effects of doped Pb atoms on Pd clusters were investigated. The average bond length of Pd–Pb clusters was found to be ∼0.1 Å longer than pure ones, but doped Pb atoms did not constantly expand the Pd–Pd bond lengths in Pd–Pb clusters owing to size effect. In addition, doped Pb atom changed the thickness of surface phase and surface morphology of Pd cluster. The chemical activity of Pd clusters could be enhanced by the doped Pd atoms, although an enough size was required. Lead atoms essentially changed the surface charge population of Pd clusters. The Pd atoms in Pd–Pb clusters were seen to become more electrophilic than in pure Pd clusters, and the probability of being attacked by nucleophilic reagent could be comparable with that of being attacked by electrophilic reagent. Total electron density and deformation electron density revealed that the nature of interaction between metal atoms was metal bond. Weak covalent bond might exist when clusters were in small size.

Formation of Organoxenon Dications in the Reactions of Xenon with Dications Derived from Toluene

The bimolecular reactivity of xenon with C(7)H(n)(2+) dications (n=6-8), generated by double ionization of toluene using both electrons and synchrotron radiation, is studied by means of a triple-quadrupole mass spectrometer. Under these experimental conditions, the formation of the organoxenon dications C(7)H(6)Xe(2+) and C(7)H(7)Xe(2+) is observed to occur by termolecular collisional stabilization. Detailed experimental and theoretical studies show that the formation of C(7)H(6)Xe(2+)+H(2) from doubly ionized toluene (C(7)H(8)(2+)) and xenon occurs as a slightly endothermic, direct substitution of dihydrogen by the rare gas with an expansion to a seven-membered ring structure as the crucial step. For the most stable isomer of C(7)H(6)Xe(2+), an adduct between the cycloheptatrienyldiene dication and xenon, the computed binding energy of 1.36 eV reaches the strength of (weak) covalent bonds. Accordingly, electrophiles derived from carbenes might be particularly promising candidates in the search for new rare-gas compounds.

Adsorption of nitrogen oxides on graphene and graphene oxides: Insights from density functional calculations

The interactions of nitrogen oxides NO(x) (x = 1,2,3) and N(2)O(4) with graphene and graphene oxides (GOs) were studied by the density functional theory. Optimized geometries, binding energies, and electronic structures of the gas molecule-adsorbed graphene and GO were determined on the basis of first-principles calculations. The adsorption of nitrogen oxides on GO is generally stronger than that on graphene due to the presence of the active defect sites, such as the hydroxyl and carbonyl functional groups and the carbon atom near these groups. These active defect sites increase the binding energies and enhance charge transfers from nitrogen oxides to GO, eventually leading to the chemisorption of gas molecules and the doping character transition from acceptor to donor for NO(2) and NO. The interaction of nitrogen oxides with GO with various functional groups can result in the formation of hydrogen bonds OH⋅⋅⋅O (N) between -OH and nitrogen oxides and new weak covalent bonds C⋅⋅⋅N and C⋅⋅⋅O, as well as the H abstraction to form nitrous acid- and nitric acidlike moieties. The spin-polarized density of states reveals a strong hybridization of frontier orbitals of NO(2) and NO(3) with the electronic states around the Fermi level of GO, and gives rise to the strong acceptor doping by these molecules and remarkable charge transfers from molecules to GO, compared to NO and N(2)O(4) adsorptions on GO. The calculated results show good agreement with experimental observations.