Symmetric Bond Research Articles

Unimolecular net heterolysis of symmetric and homopolar σ-bonds

The unimolecular heterolysis of covalent σ-bonds is integral to many chemical transformations, including SN1-, E1- and 1,2-migration reactions. To a first approximation, the unequal redistribution of electron density during bond heterolysis is governed by the difference in polarity of the two departing bonding partners1–3. This means that if a σ-bond consists of two identical groups (that is, symmetric σ-bonds), its unimolecular fission from the S0, S1, or T1 states only occurs homolytically after thermal or photochemical activation1–7. To force symmetric σ-bonds into heterolytic manifolds, co-activation by bimolecular noncovalent interactions is necessary4. These tactics are only applicable to σ-bond constituents susceptible to such polarizing effects, and often suffer from inefficient chemoselectivity in polyfunctional molecules. Here we report the net heterolysis of symmetric and homopolar σ-bonds (that is, those with similar electronegativity and equal leaving group ability3) by means of stimulated doublet–doublet electron transfer (SDET). As exemplified by Se–Se and C–Se σ-bonds, symmetric and homopolar bonds initially undergo thermal homolysis, followed by photochemically SDET, eventually leading to net heterolysis. Two key factors make this process feasible and synthetically valuable: (1) photoexcitation probably occurs in only one of the incipient radical pair members, thus leading to coincidental symmetry breaking8 and consequently net heterolysis even of symmetric σ-bonds. (2) If non-identical radicals are formed, each radical may be excited at different wavelengths, thus rendering the net heterolysis highly chemospecific and orthogonal to conventional heterolyses. This feature is demonstrated in a series of atypical SN1 reactions, in which selenides show SDET-induced nucleofugalities3 rivalling those of more electronegative halides or diazoniums.

Chlorine bridge bond-enabled binuclear copper complex for electrocatalyzing lithium–sulfur reactions

Engineering atom-scale sites are crucial to the mitigation of polysulfide shuttle, promotion of sulfur redox, and regulation of lithium deposition in lithium–sulfur batteries. Herein, a homonuclear copper dual-atom catalyst with a proximal distance of 3.5 Å is developed for lithium–sulfur batteries, wherein two adjacent copper atoms are linked by a pair of symmetrical chlorine bridge bonds. Benefiting from the proximal copper atoms and their unique coordination, the copper dual-atom catalyst with the increased active interface concentration synchronously guide the evolutions of sulfur and lithium species. Such a delicate design breaks through the activity limitation of mononuclear metal center and represents a catalyst concept for lithium–sulfur battery realm. Therefore, a remarkable areal capacity of 7.8 mA h cm−2 is achieved under the scenario of sulfur content of 60 wt.%, mass loading of 7.7 mg cm−2 and electrolyte dosage of 4.8 μL mg−1.

Influence of Stone-Wales defects on the structural and electronic properties of single-walled silicon nanotubes by SCC-DFTB calculations

In this work, using the self-consistent charge density functional tight-binding (SCC-DFTB) approach, we explored the effects of Stone-Wales (SW) defects on the structure and electronic properties of perfect single-walled silicon nanotubes(SWSiNTs), including zigzag (n1,0) (8≤n1≤16), armchair (n2,n2) (5≤n2≤10), and chiral (2n3,n3) (3≤n3≤8). We found that the presence of SW defects leads to changes in bond length and bond angle, resulting in structural changes at the defect site. However, nanotubes still exhibit tubular and hexagonal structures. Also, the defect structure of zigzag and armchair SW-II defective tubes is symmetrical, which means that the bond length of the Si-Si bond is equivalent to that of its symmetric bond. Besides, after introducing the SW-II defect, zigzag and chiral tubes become more stable, but the stability of armchair tubes with small diameter is reduced. The introduction of SW-Ⅰ and SW-Ⅱ defects transforms armchair SiNTs from semiconductor into semi-metallic properties. Moreover, for a perfect structure, the charge moves from inside out along the radial direction of the tube and the amount of charge is distributed in an axisymmetric manner. After introducing SW defects, the range of charge transfer is expanded, SW-Ⅱ defective structures appear inward deviation, and the atoms near the defect change from regular arrangement to inward aggregation. This work may provide a new way to adjust the electronic properties of SWSiNTs and open up wider applications in nanoelectronic devices.

Alkynyl Boosted High‐Performance Lithium Storage and Mechanism in Covalent Phenanthroline Framework

AbstractThe poor conductivity of the pristine bulk covalent organic material is the main challenge for its application in energy storage. The mechanism of symmetric alkynyl bonds (C≡C) in covalent organic materials for lithium storage is still rarely reported. Herein, a nanosized (≈80 nm) alkynyl‐linked covalent phenanthroline framework (Alkynyl‐CPF) is synthesized for the first time to improve the intrinsic charge conductivity and the insolubility of the covalent organic material in lithium‐ion batteries. Because of the high degree of electron conjugation along alkynyl units and N atoms from phenanthroline groups, the Alkynyl‐CPF electrodes with the lowest HOMO–LUMO energy gap (ΔE=2.629 eV) show improved intrinsic conductivity by density functional theory (DFT) calculations. As a result, the pristine Alkynyl‐CPF electrode delivers superior cycling performance with a large reversible capacity and outstanding rate properties (1068.0 mAh g−1 after 300 cycles at 100 mA g−1 and 410.5 mAh g−1 after 700 cycles at 1000 mA g−1). Moreover, by Raman, FT‐IR, XPS, EIS, and theoretical simulations, the energy‐storage mechanism of C≡C units and phenanthroline groups in the Alkynyl‐CPF electrode has been investigated. This work provides new strategies and insights for the design and mechanism investigation of covalent organic materials in electrochemical energy storage.

Alkynyl Boosted High-Performance Lithium Storage and Mechanism in Covalent Phenanthroline Framework.

The poor conductivity of the pristine bulk covalent organic material is the main challenge for its application in energy storage. The mechanism of symmetric alkynyl bonds (C≡C) in covalent organic materials for lithium storage is still rarely reported. Herein, a nanosized (≈80 nm) alkynyl-linked covalent phenanthroline framework (Alkynyl-CPF) is synthesized for the first time to improve the intrinsic charge conductivity and the insolubility of the covalent organic material in lithium-ion batteries. Because of the high degree of electron conjugation along alkynyl units and N atoms from phenanthroline groups, the Alkynyl-CPF electrodes with the lowest HOMO-LUMO energy gap (ΔE=2.629 eV) show improved intrinsic conductivity by density functional theory (DFT) calculations. As a result, the pristine Alkynyl-CPF electrode delivers superior cycling performance with a large reversible capacity and outstanding rate properties (1068.0 mAh g-1 after 300 cycles at 100 mA g-1 and 410.5 mAh g-1 after 700 cycles at 1000 mA g-1 ). Moreover, by Raman, FT-IR, XPS, EIS, and theoretical simulations, the energy-storage mechanism of C≡C units and phenanthroline groups in the Alkynyl-CPF electrode has been investigated. This work provides new strategies and insights for the design and mechanism investigation of covalent organic materials in electrochemical energy storage.

Finite projected entangled pair states for the Hubbard model

We adapt and optimize the projected-pair-entangled-state (PEPS) algorithm on finite lattices (fPEPS) for two-dimensional Hubbard models and apply the algorithm to the Hubbard model with nearest-neighbor hopping on a square lattice. In particular, we formulate the PEPS algorithm using projected entangled pair operators, incorporate SU(2) symmetry in all tensor indices, and optimize the PEPS using both iterative-diagonalization-based local bond optimization and gradient-based optimization of the PEPS. We discuss the performance and convergence of the algorithm for the Hubbard model on lattice sizes of up to $8\ifmmode\times\else\texttimes\fi{}8$ for PEPS states with U(1) symmetric bond dimensions of up to $D=8$ and SU(2) symmetric bond dimensions of up to $D=6$. Finally, we comment on the relative and overall efficiency of schemes for optimizing fPEPS.

Effect of silica-alumina co-introduction on structure as well as physical and spectroscopic properties of Nd3+-doped potassium phosphate glass

The effect of silica-alumina co-introduction on physical and spectroscopic properties of Nd3+-doped potassium phosphate glass was investigated. The Raman and X-ray photoelectron spectroscopy (XPS), physical parameters as well as spectroscopic characteristics were determined and analyzed in detail. Q2 units gradually increase while Q1 units have the opposite trend with the increase of SiO2/Al2O3 ratios. Meanwhile the total number of [AlO6] octahedrons and [AlO4] tetrahedrons continue to decrease with [AlO6]/ [AlO6]+ [AlO4] ratio keeping rise. As a result, the increase of more symmetrical bridge oxygen bonds and more ionic Si-O bonds lead to more centrosymmetric and stronger ionicity of Nd3+ ion coordination environment. The physical and spectroscopic properties of glasses benefit from silica-alumina co-introduction. When SiO2/Al2O3 ratio was 1.8, the glass showed a high emission cross-section of 3.05 × 10−20 cm2, a high optical gain parameter of 10.8 × 10−24 cm2s, together with wider gain bandwidth parameter of 7.61 × 10−26 cm3.

Preparation and structure–property relationship of flexible aramid films with enhanced strength by introducing asymmetric and symmetric aromatic ether bond structures

The flexible molecular segments and aggregated structures of MAPB–PMIA and PAPB–PMIA endow polymers with soft properties with high extension and low modulus.

Structural, Spectroscopic, Cytotoxicity and Molecular Docking Studies of Charge Transfer Salt: 4-Aminiumantipyrine Salicylate

A pyrazolone based novel Schiff base, 4-aminiumantipyrine salicylate (4AAPSAL) was synthesized and single crystals had been grown by slow evaporation technique. It was characterized by using Single crystal XRD, FT-IR, FT-Raman and Thermogravimetric analyses. To obtain the ground state optimized geometry of the title molecule, Density Functional Theory (DFT) calculations were carried out by B3LYP method using 6-311++G(d,p) basis set. The title molecule was crystallized in the monoclinic system with the space group P21/c which was confirmed by single-crystal XRD. The protonation is evidenced by the elongated C–N bond distance in the N site of cation and COO- symmetric bond distances which confirms the deprotonation in the anion. Mulliken charges show that the H18 (0.556 e) atom have higher positive charge than other hydrogen, because this hydrogen atom is participating in the O-H…O intermolecular hydrogen bond in solid crystalline state. In addition, the thermal behavior and cytotoxicity test of the title compound were investigated. Cytotoxicity test results show that the 4AAPSAL is biologically active.

An integrated approach on extraction methodologies of nanosilica from cultivated agricultural wastes and microstructural characteristics – A review

In the present scenario, the significant environmental pollutants are from the cultivated agricultural waste by incineration process; in the process, more contaminants directly enter into the atmosphere. Therefore, the utilization of these wastes has been proved to be a better helpful product in geotechnical engineering applications. The primary product extracted from cultivated agricultural waste is nanosilica. It can be used as a supplement for stabilizing soft soils and improving the mechanical properties of problematic soils, polymer composites, pest control, biomedical applications, and increased concrete strength with durability properties. This paper deals with previous studies on the extraction of nanosilica from cultivated agricultural wastes with different methodologies such as precipitation, surface modification, calcination, and warm’s digestive process and characterized by microstructural analysis. Out of many available methodologies, precipitation has been proved to be an effective way to extract of nanosilica percentage, less consumption time, double reflexed, and economy. On the other hand, various studies on microstructural analysis proved that more silica, nanosized, amphorous form, and more symmetrical bonds between Si and O.

Mode-Specific Vibrational Analysis of Exciton Delocalization and Structural Dynamics in Conjugated Oligomers.

Exciton delocalization in organic semiconducting polymers, affected by structures at a molecular level, plays a crucial role in modulating relaxation pathways, such as charge generation and singlet fission, which can boost device efficiency. However, the structural diversity of polymers and broad signals from typical electronic spectroscopy have their limits when it comes to revealing the interplay between local structures and exciton delocalization. To tackle these problems, we apply femtosecond stimulated Raman spectroscopy in archetypical conjugated oligothiophenes with different chain lengths. We observed Raman frequency dispersions of symmetric bond stretching modes and mode-specific kinetics in the S1 Raman spectra, which underpins the subtle and complex interplay between exciton delocalization and bond length alternation along the conjugation coordinate. Our results provide a more general picture of exciton delocalization in the context of molecular structures for conjugated materials.

Mode‐Specific Vibrational Analysis of Exciton Delocalization and Structural Dynamics in Conjugated Oligomers

AbstractExciton delocalization in organic semiconducting polymers, affected by structures at a molecular level, plays a crucial role in modulating relaxation pathways, such as charge generation and singlet fission, which can boost device efficiency. However, the structural diversity of polymers and broad signals from typical electronic spectroscopy have their limits when it comes to revealing the interplay between local structures and exciton delocalization. To tackle these problems, we apply femtosecond stimulated Raman spectroscopy in archetypical conjugated oligothiophenes with different chain lengths. We observed Raman frequency dispersions of symmetric bond stretching modes and mode‐specific kinetics in the S1 Raman spectra, which underpins the subtle and complex interplay between exciton delocalization and bond length alternation along the conjugation coordinate. Our results provide a more general picture of exciton delocalization in the context of molecular structures for conjugated materials.

Superionic iron oxide–hydroxide in Earth’s deep mantle

Water ice becomes a superionic phase under the high pressure and temperature conditions of deep planetary interiors of ice planets such as Neptune and Uranus, which affects interior structures and generates magnetic fields. The solid Earth, however, contains only hydrous minerals with a negligible amount of ice. Here we combine high pressure and temperature electrical conductivity experiments, Raman spectroscopy and first-principles simulations to investigate the state of hydrogen in the pyrite-type FeO2Hx (x ≤ 1), which is a potential H-bearing phase near the core–mantle boundary. We find that when the pressure increases beyond 73 GPa at room temperature, symmetric hydroxyl bonds are softened and the H+ (or proton) becomes diffusive within the vicinity of its crystallographic site. Increasing temperature under pressure, the diffusivity of hydrogen is extended beyond the individual unit cell to cover the entire solid, and the electrical conductivity soars, indicating a transition to the superionic state, which is characterized by freely moving protons and a solid FeO2 lattice. The highly diffusive hydrogen provides fresh transport mechanisms for charge and mass, which dictate the geophysical behaviours of electrical conductivity and magnetism, as well as geochemical processes of redox, hydrogen circulation and hydrogen isotopic mixing in Earth’s deep mantle.

First-principles investigations of optoelectronic properties of ZnO$$\left( {11\overline{2}0} \right)$$ and ZnO(0001) monolayers

Devising the two-dimensional (2D) structures of low-cost and non-toxic semiconductors for nanoscale technological applications has attracted substantial interest since the past decade. In this work, we design two types of ZnO monolayers derived from polar 0001-plane and nonpolar $$11\overline{2}0$$ -plane of the wurtzite structure, and explore their physical properties using the first-principles approach. Both ZnO $$\left( {11\overline{2}0} \right)$$ and ZnO(0001) monolayers exhibited cohesive and formation energies comparable to that of the stable wurtzite-structured ZnO. However, both monolayers exhibited substantially different electronic structures of band gaps 1.56 eV for single-layered ZnO $$\left( {11\overline{2}0} \right)$$ and 0.71 eV for ZnO(0001) monolayer. The edges of the valence and conduction bands of ZnO $$\left( {11\overline{2}0} \right)$$ monolayer are formed by parabolic bands, whereas almost flat band gap edges have been seen for ZnO(0001) surface. As a result, charge carriers associated with ZnO $$\left( {11\overline{2}0} \right)$$ monolayer exhibited relatively lighter effective mass than ZnO(0001) monolayer. The ZnO(0001) monolayer exhibited symmetrical bond lengths and subsequently isotropic optical spectra, whereas asymmetrical bond lengths and anisotropic subsequent optical spectra have been recorded for ZnO $$\left( {11\overline{2}0} \right)$$ monolayer. The optical absorption recorded for the designed monolayers has been found higher than their bulk counterpart. The refraction spectra indicated these monolayers of transparent behavior over a significant range of the electromagnetic spectrum. These fascinating features of ZnO $$\left( {11\overline{2}0} \right)$$ and ZnO(0001) monolayers suggest them suitable for applications in electronic and optoelectronic devices.

Investigations of Surface Properties of SAW Fluxes Using CaO-SiO2-TiO2 & Al2O3-CaO-SiO2 Ternary Phase Systems

In the present paper, investigation on wettability and contact angle measurement of twenty-one SAW fluxes for CaO-SiO2-TiO2 & Al2O3-CaO-SiO2 welding flux system at a temperature of 1300 °C was studied. Wettability observed by measuring the contact angle properties of twenty-one SAW fluxes. The purity of flux constituents determined by the XRF technique. Si-O-Si and Al-O-Al symmetric bond behaviour and various crystalline phases observed using FTIR and XRD techniques. Lower contact angle gives higher wetting and spreading properties. The contact angle increased with the increase of TiO2/SiO2 ratio. For the composition range 1.5 to 2.0, there is a higher value of contact angle observed. A maximum increase of contact angle noticed for the composition range 1.5 to 2.0. There is a constant rise in the amount of contact angle for the composition that varies from 1.0 to 1.2.

2’-methylklavuzon causes lipid-lowering effects on A549 non-small cell lung cancer cells and significant changes on DNA structure evidenced by fourier transform infrared spectroscopy

Various chemical agents are used in the treatment of Non-Small Cell Lung Cancer (NSCLC). 2′-methylklavuzon was proposed as a potential chemotherapeutic agent in cancer treatment based on its topoisomerase inhibition activity. In this study the cellular effects of 2′-methylklavuzon was evaluated on A549 cancer cells using FTIR spectroscopy. 2′-methylklavuzon induced significant changes on both the whole cell lyophilizates and the lipid extracts of the A549 lung cancer cells. 2′-methylklavuzon caused significant structural changes in A549 cell DNA structure: T, A and G DNA breathing modes are lost after the drug application indicating the loss of topoisomerase activity. The level of transcription and RNA synthesis was enhanced. 2′-methylklavuzon induced single stranded DNA formation evidenced by the increase in the ratio of asymmetric/symmetric phosphate stretching modes. 2′-methylklavuzon induced band shifts only in the asymmetric mode of phosphate bonds not in the symmetrical phosphate bond stretching. 2′-methylklavuzon induced A form of DNA topography. In addition to the changes in the DNA structure and transcription 2′-methylklavuzon also caused lipid-lowering effect in A549 cancer cells. 2′-methylklavuzon suppressed lipid unsaturation, however, it induced formation of lipids with ring structures. 2′-methylklavuzon suppressed phosphate-containing lipids significantly and decreased carbonyl containing lipids and cholesterol slightly. 2′-methylklavuzon caused increases in the hydrocarbon chain length. Overall, 2′-methylklavuzon can be used as a lipid-lowering compound in the treatment of NSCLC and other cancer therapies.

The caesium phosphates Cs3(H1.5PO4)2(H2O)2, Cs3(H1.5PO4)2, Cs4P2O7(H2O)4, and CsPO3

The caesium phosphates Cs3(H1.5PO4)2(H2O)2 and Cs3(H1.5PO4)2 were obtained from aqueous solutions, and Cs4P2O7(H2O)4 and CsPO3 from solid state reactions, respectively. Cs3(H1.5PO4)2, Cs4P2O7(H2O)4, and CsPO3 were fully structurally characterized for the first time on basis of single-crystal X-ray diffraction data recorded at − 173 °C. Monoclinic Cs3(H1.5PO4)2 (Z = 2, C2/m) represents a new structure type and comprises hydrogen phosphate groups involved in the formation of a strong non-symmetrical hydrogen bond (accompanied by a disordered H atom over a twofold rotation axis) and a very strong symmetric hydrogen bond (with the H atom situated on an inversion centre) with symmetry-related neighbouring anions. Triclinic Cs4P2O7(H2O)4 (Z = 2, Pbar{1}) crystallizes also in a new structure type and is represented by a diphosphate group with a P–O–P bridging angle of 128.5°. Although H atoms of the water molecules were not modelled, O···O distances point to hydrogen bonds of medium strengths in the crystal structure. CsPO3 is monoclinic (Z = 4, P21/n) and belongs to the family of catena-polyphosphates (MPO3)n with a repetition period of 2. It is isotypic with the room-temperature modification of RbPO3. The crystal structure of Cs3(H1.5PO4)2(H2O)2 was re-evaluated on the basis of single-crystal X-ray diffraction data at − 173 °C, revealing that two adjacent hydrogen phosphate anions are connected by a very strong and non-symmetrical hydrogen bond, in contrast to the previously described symmetrical bonding situation derived from room temperature X-ray diffraction data. In the four title crystal structures, coordination numbers of the caesium cations range from 7 to 12.Graphic abstract

Experimental and Theoretical Investigation of Origin of Mechanical Properties and Fracture Mechanism of Pt3Zr High-Temperature Material

Pt3Zr is a promising high-temperature material. However, its mechanical properties and fracture mechanism are unknown. Pure Pt3Zr was prepared by the arc-melting method, and its mechanical properties and microstructure were measured by universal tester and scanning electron microscopy, respectively. In particular, its fracture mechanism was studied by first-principles and molecular dynamics (MD) methods. The measured fracture stress was 150.1 MPa. The experimental results revealed predominant cleavage fracture behavior. Although first-principles calculations showed that it is a ductile material due to the formation of symmetrical Pt-Zr bonds, the MD simulation results demonstrated that the fracture behavior of Pt3Zr can be attributed to the coalescence of numerous voids at high temperature, which is consistent with the experimental results.

Auxetic Properties of Chiral Hexagonal Cosserat Lattices Composed of Finite‐Sized Particles

This article is devoted to the study of auxetic properties of Cosserat hexagonal lattices composed of finite‐sized particles with complex connections. The description of complex connections is given; their mathematical model is elaborated and the properties are studied. The introduction of complex connections enables one varying their structure and component parameters. Due to that there arise possibilities for both simulation of nonchiral lattices with symmetrical bonds and with a chiral microstructure and construction of lattices with desired properties. The discrete and micropolar equations of the lattice are obtained. As a result, the macroparameters are expressed in terms of the lattice microparameters. The dependence of the Poisson's ratio on the lattice microparameters is obtained. It allows finding and analyzing parameters, for which the lattice possesses auxetic properties. The importance of rotational degrees of freedom of particles and chirality for the appearance of auxetic properties is shown. For verification, the results of the calculation of the Poisson's ratio obtained on the basis of theoretically obtained relations are compared with the results of numerical simulation of the stretching of the lattice.

Power spectrum and coherence of electroencephalograms of young people with the use of mnemotechnics

The increase of external information and the need for processing of large information arrays has been leading to the search for mechanisms of thinking in mental activity, as well as new methods of data protection and reproduction. Power spectrum and coherence of electroencephalograms (EEG) during memorizing of foreign words has been analyzed in many ways: in the usual one with the help of mnemotechnics. The research was conducted during the ovarian phase of the cycle of 11 female students aged 18–20. Registration of EEG activity was carried out according to the international system of H. Jasper,"10–20". The process of recollection of associative information led to the decrease in the power spectrum of the EEG in the alpha range in the frontal and temporal loci of the left hemisphere and the prefrontal sections of both hemispheres, in the beta range it was probable only in T6 zone of beta2 range, in the delta range it was in the prefrontal, posterior lower-frontal and posttemporal loci of both hemispheres, in the central, parietal and occipital regions of the left hemisphere, as well as in the anteriofrontal zone of the right hemisphere, in the theta-range – in the prefrontal and posterior lower-frontal regions of both hemispheres and in the posttemporal and occipital loci of the left hemisphere. Synchronization in the delta range is the reflection of the processes of figural information processing and manipulation. With the increase of attention concentration, the synchronization was observed in the theta range in the anteriofrontal and F4–P4 zones. Interhemispheric functional and symmetrical bonds in the alpha and beta ranges indicate the involvement of the corpus callosum in the process of memorizing foreign words, which facilitates their faster, more correct and easier reproduction, especially with the use of associative images. The decrease of power spectrum in the delta and theta ranges showed that less effort was required to reproduce associative information than to mention unsupported images of foreign words. The effective use of the association method is realized by reduction of the power spectrum of the waves in the alpha range in the frontal and temporal loci of the left hemisphere and the prefrontal sections of both hemispheres, which indicates the activation of mental activity in these zones in the processes of maintaining and reproducing associative information.