Homonuclear Bonds Research Articles

Cleavage of Homonuclear Chalcogen‐Chalcogen Bonds in a Hybrid Platform in Response to X‐Ray Radiation Potentiates Tumor Radiochemotherapy

AbstractChalcogens are used as sensitive redox‐responsive reagents in tumor therapy. However, chalcogen bonds triggered by external ionizing radiation, rather than by internal environmental stimuli, enable site‐directed and real‐time drug degradation in target lesions. This approach helps to bypass chemoresistance and global systemic toxicity, presenting a significant advancement over traditional chemoradiotherapy. In this study, we fabricated a hybrid monodisperse organosilica nanoprodrug based on homonuclear single bonds (disulfide bonds (S−S, approximately 240 kJ/mol), diselenium bonds (Se−Se, approximately 172 kJ/mol), and tellurium bonds (Te−Te, 126 kJ/mol)), including ditelluride‐bond‐bridged MONs (DTeMSNs), diselenide‐bond‐bridged MONs (DSeMSNs) and disulfide‐bond‐bridged MONs (DSMSNs). The results demonstrated that differences in electronegativities and atomic radii influenced their oxidation sensitivities and reactivities. Tellurium, with the lowest electronegativity, showed the highest sensitivity, followed by selenium and sulfur. DTeMSNs exhibited highly responsive cleavage upon exposure to X‐rays, resulting in oxidation to TeO32−. Furthermore, chalcogen‐hybridized organosilica was loaded with manganese ions (Mn2+) to enhance the release of Mn2+ during radiotherapy, thereby activating the the stimulator of interferon genes (STING) pathway and enhancing the tumor immune response to inhibit tumor growth. This investigation of hybrid organosilica deepens our understanding of chalcogens response characteristics to radiotherapy and enriches the design principles for nanomedicine based on prodrugs.

Cleavage of Homonuclear Chalcogen-Chalcogen Bonds in a Hybrid Platform in Response to X-ray Radiation Potentiates Tumor Radiochemotherapy.

Chalcogens are used as sensitive redox-responsive reagents in tumor therapy. However, chalcogen bonds triggered by external ionizing radiation, rather than by internal environmental stimuli, enable site-directed and real-time drug degradation in target lesions. This approach helps to bypass chemoresistance and global systemic toxicity, presenting a significant advancement over traditional chemoradiotherapy. In this study, we fabricated a hybrid monodisperse organosilica nanoprodrug based on homonuclear single bonds (disulfide bonds (S-S, approximately 240 kJ/mol), diselenium bonds (Se-Se, approximately 172 kJ/mol), and tellurium bonds (Te-Te, 126 kJ/mol)), including ditelluride-bond-bridged MONs (DTeMSNs), diselenide-bond-bridged MONs (DSeMSNs) and disulfide-bond-bridged MONs (DSMSNs). The results demonstrated that differences in electronegativities and atomic radii influenced their oxidation sensitivities and reactivities. Tellurium, with the lowest electronegativity, showed the highest sensitivity, followed by selenium and sulfur. DTeMSNs exhibited highly responsive cleavage upon exposure to X-rays, resulting in oxidation to TeO32-. Furthermore, chalcogen-hybridized organosilica was loaded with manganese ions (Mn2+) to enhance the release of Mn2+ during radiotherapy, thereby activating the the stimulator of interferon genes (STING) pathway and enhancing the tumor immune response to inhibit tumor growth. This investigation of hybrid organosilica deepens our understanding of chalcogens response characteristics to radiotherapy and enriches the design principles for nanomedicine based on prodrugs.

Metal-Free B-B Dehydrocoupling Reaction of a Simple Borane Adduct: Convenient Access to a Nucleophilic Diborane(4).

The selective formation of homonuclear bonds is of key importance in synthetic chemistry. Especially, dehydrocoupling reactions are attractive as ecologically and economically friendly alternatives to established reductive bond forming reactions, since they do not require the use of stoichiometric amounts of a reducing reagent and produce only valuable dihydrogen as by-product. Here, we report on a metal-free B-B dehydrocoupling reaction that starts directly from a simple, easily accessible BH3 adduct, providing convenient access to a new nucleophilic dihydridodiborane in excellent yield. The dihydridodiborane in turn activates dihydrogen, allowing to obtain quantitatively the dideuteridodiborane from the dihydridodiborane by D2 activation. On the basis of detailed quantum-chemical calculations, the mechanism of this unprecedented reaction is elucidated. Some key points that are essential for metal-free dehydrocoupling are disclosed, paving the way for their systematic evaluation and application.

Metallfreie B−B Dehydrokupplung eines Einfachen Boran‐Addukts: Bequemer Zugang zu einem Nukleophilen Diboran(4)

AbstractDie selektive Bildung homonuklearer Bindungen ist in der synthetischen Chemie von zentraler Bedeutung. Insbesondere Dehydrokupplungsreaktionen sind als ökologische und ökonomische Alternativen zu etablierten reduktiven Bindungsbildungsreaktionen attraktiv, da sie nicht den Einsatz stöchiometrischer Mengen eines Reduktionsmittels erfordern und nur wertvollen Wasserstoff als Nebenprodukt erzeugen. Hier berichten wir über eine metallfreie B−B‐Dehydrokupplungsreaktion, die direkt von einem einfachen, leicht zugänglichen BH3‐Addukt startet und einen bequemen Zugang zu einem neuen nukleophilen Dihydridodiboran in exzellenter Ausbeute bietet. Das Dihydridodiboran wiederum aktiviert Wasserstoff, sodass durch D2‐Aktivierung quantitativ das Dideuteridodiboran aus dem Dihydridodiboran erhalten werden kann. Auf Grundlage detaillierter quantenchemischer Berechnungen wird der Mechanismus dieser beispiellosen Reaktion aufgeklärt. Einige für die metallfreie Dehydrokupplung essenzielle Kernpunkte werden aufgezeigt und dadurch der Weg für einen systematischen Zugang und Anwendung bereitet.

Optical spectroscopy study of damage in ion-irradiated 3C-SiC epilayers on a silicon substrate

Epitaxial cubic (100) 3C-SiC films on a (100) silicon wafer were irradiated at room temperature with 2.3-MeV Si+ or 3.0-MeV Kr+ ions up to a fluence of 1 × 1016 cm−2. The evolutions of the epilayer and the substrate were followed as a function of ion fluence by using micro-Raman spectroscopy, optical absorption, and diffuse reflectance spectroscopy in the UV-visible and near infrared range. Raman spectra evidence the amorphization of SiC films at an estimated dose of about 0.1 displacement per atom (dpa) for both ion irradiations. The narrow peaks of the Raman-allowed TO and LO modes of SiC and Si are recorded in the virgin sample, together with few peaks assigned to zone-edge modes of SiC arising from the intrinsic disorder in the strained films. Those crystal phonon peaks broaden or disappear with increasing fluence. The spectra finally exhibit broad extra peaks assigned to the formation of Si–Si and C–C wrong homonuclear bonds in the local order of the amorphous phase. The optical transmission and diffuse reflectance spectra feature interference fringe patterns in the SiC film that are smoothened out with irradiation due to the matching of refractive indices of the amorphous SiC film and Si substrate. The evolution of the refractive index of SiC and optical gap of Si are deduced from those spectra. The respective roles of ballistic effects and electronic excitations in the radiation damage of both SiC and Si are discussed for those two ions with about the same electronic stopping power and about one order-of-magnitude difference in nuclear stopping power. The damage is dominated by the nuclear collision processes and rather well correlated with the estimated irradiation dose in dpa. Optical spectra show that electronic excitations induce damage recovery of the amorphized substrate below the SiC/Si interface. Raman spectra and optical absorption/reflection spectra yield complementary pictures of the radiation damage.

The curious case of the crystalline tri-thorium cluster: cyclic delocalization without aromatic stabilization?

Actinides have been known to form extremely weak homonuclear bonds with their d-type orbitals, and one should therefore expect the superposition of cyclic resonance forms containing such bonds to bring rather marginal aromatic stabilization to the system, if any. It is for this very reason that the discovery of the cyclically delocalized Th3 σ-bonding in the crystalline cluster isolated by Liddle and co-workers has sparked such vigorous discussion on the actual role of molecular aromaticity on the periphery of the periodic table. It has recently been argued that the tri-thorium ring at the heart of the cluster features considerable aromatic stabilization energy comparable to the heterocyclic π-aromatic rings such as thiophene. However, previous investigations involved highly ionized model clusters like Th3Cl64+ or Th310+ in which aromatic stabilization associated with the cyclic delocalization of charge is dramatically exaggerated. In this work we investigate the model tri-thorium clusters at different geometries and ionization states to show that cyclic delocalization of electrons in the isolated crystalline cluster may be associated with rather marginal σ-aromatic stabilization energy thus strongly suggesting its non-aromatic character.

Helium and strontium co-implantation into SiC at room temperature and isochronal annealing: Structural evolution of SiC and migration behaviour of strontium

Understanding the structural evolution of SiC implanted with fission product surrogates in the presence of helium (He) is of importance for its application in both fission and fusion rectors. In this study, polycrystalline SiC wafers were sequentially co-implanted with 360 keV Sr and 21.5 keV He ions to a fluence of 2 × 1016 cm-2 and 1 × 1017 cm-2 at room temperature, respectively. The samples were then isochronally annealed in temperatures ranging from 1000 to 1300 °C for 5 h. Transmission electron microscopy (TEM) showed the formation of He-nanobubbles corresponding to helium's projected range. While scanning electron microscopy (SEM) revealed the formation of cornflower-like structures on the surface of the Sr + He–SiC samples after annealing. These were confirmed to be holes by atomic force microscopy (AFM), as a result of exfoliation and pressurized out-diffusion of helium gas from the samples. The Sr + He–SiC samples were completely amorphized characterized by the formation of the homonuclear bonds in Raman spectroscopy. The recovery process after annealing in the Sr + He–SiC samples resulted in the formation of graphite due to antisite defects driven by the growth of holes above the threshold chemical disorder. Time-of-flight heavy ions elastic recoil detection analysis (Tof-ERDA) showed that almost all helium out-diffused after annealing at 1000 °C and Sr atoms trapped in He cavities.

First-principles investigation of a-line Shockley partial dislocations in wurtzite GaN: core reconstruction and electronic structure

Αtomistic simulations based on density functional theory were performed to investigate Shockley partial dislocations, parallel to the <11–20> direction, in wurtzite gallium nitride. The cores of both 30° and 90° Shockley partial dislocations, their possible reconstructions and their electronic structures were analyzed and compared systematically. Shockley partial dislocations were found to have a variety of core structures while exhibiting different bonding states like: deformed Ga–N bonds, Ga–Ga and N–N homo-nuclear bonds, as well as Ga– and N–dangling bonds. We have demonstrated that the core reconstruction of these dislocations is not always energetically favourable in wurtzite GaN, as it is the case in elemental semiconductors. It results from interplay between the energy gain from eliminating dangling and forming homo-nuclear bonds and the energy excess from the induced strain of hetero-nuclear Ga–N bonds within the core. Depending on their core configurations, Shockley partial dislocations are expected to have different electronic behaviours in wurtzite GaN, as they can act either as sources of parasitic luminescence or non-radiative recombination centres, or give pathways to leakage currents. We have demonstrated that deep states associated with dislocations are not due to the dangling bonds, as generally believed, but rather to Ga–Ga bonds in their cores, and that N–N bonds do not lead to any gap states. These results are of high relevance as guidelines in defect engineering strategies aiming at producing dislocations with a particular electronic behaviour in wurtzite GaN.

Preparation, microstructure and ion-irradiation damage behavior of Al4SiC4-added SiC ceramics

Single-phase Al4SiC4 powder with a low neutron absorption cross section was synthesized and mixed with SiC powder to fabricate highly densified SiC ceramics by hot pressing. The densification of SiC ceramics was greatly improved by the decomposition of Al4SiC4 and the formation of aluminosilicate liquid phase during the sintering process. The resulting SiC ceramics were composed of fine equiaxed grains with an average grain size of 2.0 μm and exhibited excellent mechanical properties in terms of a high flexure strength of 593 ± 55 MPa and a fracture toughness of 6.9 ± 0.2 MPa m1/2. Furthermore, the ion-irradiation damage in SiC ceramics was investigated by irradiating with 1.2 MeV Si5+ ions at 650 °C using a fluence of 1.1 × 1016 ions/cm2, which corresponds to 6.3 displacements per atom (dpa). The evolution of the microstructure was investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The breaking of Si–C bonds and the segregation of C elements on the irradiated surface was revealed by XPS, whereas the formation of Si–Si and C–C homonuclear bonds within the Si–C network of SiC grains was detected by Raman spectroscopy.

On the nature of the chemical bond in valence bond theory.

This Perspective outlines a panoramic description of the nature of the chemical bond according to valence bond theory. It describes single bonds and demonstrates the existence of a "forgotten family" of charge-shift bonds (CSBs) in which the entire/most of the bond energy arises from the resonance between the covalent and ionic structures of the bond. Many of the CSBs are homonuclear bonds. Hypervalent molecules (e.g., XeF2) are CSBs. This Perspective proceeds to describe multiple bonded molecules with an emphasis on C2 and 3O2. C2 has four electron pairs in its valence shell and, hence, 14 covalent structures and 1750 ionic structures. This Perspective outlines an effective methodology of peeling the electronic structure to the minimal and important number of structures: a dominant structure that displays a quadruple bond and two minor structures with π + σ bonds, which stabilize the quadruple bond by resonance. 3O2 is chosen because it is a diradical, which is persistent and life-sustaining. It is shown that the persistence of this diradical is due to the charge-shift bonding of the π-3-electron bonds. This section ends with a discussion of the roles of π vs σ in the geometric preferences of benzene, acetylene, ethene, and their Si-based analogs. Subsequently, this Perspective discusses bonding in clusters of univalent metal atoms, which possess only parallel spins (n+1Mn), and are nevertheless bonded due to the resonance interactions that stabilize the repulsive elementary structure (all spins are up). The bond energy reaches ∼40 kcal/mol for a pair of atoms (in n+1Cun; n ∼ 10-12). The final subsection discusses singlet excited states in ethene, ozone, and SO2. It demonstrates the capability of the breathing-orbital VB method to yield an accurate description of a variety of excited states using merely 10 or few VB structures. Furthermore, the method underscores covalent structures that play a key role in the correct description and bonding of these excited states.

Irradiation-induced chemical disordering in ceramics: The case of SiC

Chemical disordering is one feature of damage that occurs in irradiation environments and is common in ceramic compounds. This paper reviews irradiation-induced chemical disordering in SiC-based materials at elevated temperatures. The results were obtained using advanced analytical tools to characterize atomistic defects and to evaluate the chemistry of defect clusters and homonuclear bond structures. This paper demonstrates that chemical disordering is crucial to understanding microstructural stability and establishing a mechanistic modeling of microstructural evolution of irradiated SiC.

A material point method based investigation on crack classification and transformation induced by grit geometry during scratching silicon carbide

The microcracks and residual stress are commonly regarded as non-negligible obstacles while realizing non-damage surfaces in the grinding of brittle materials. In this paper, with the combination of stress field analysis, material point method simulation and high-speed scratch experiment, a variety of cracks in the classical indentation-induced crack system were decoupled to determine the inducing conditions and coupling modes of different cracks. The tests on pressureless sintered silicon carbide (S–SiC) exhibited that the grit geometry was crucial in inducing different cracks. The Hertzian and lateral crack systems were mainly determined by the rake angle of scratching grit and can transform from one to another near the critical value of −60°. Further, a two-point stress field model was proposed to prove that large contact width is the key factor to induce radial cracks. By contrast, large contact depth is related to the appearance of bottom debris. Furthermore, Raman spectroscopy was used to investigate the velocity effect of grit on cumulative damage. Besides the stacking disorder and residual stress, even chemical disorders such as the fracture of the Si–C bond and the formation of the homonuclear bond were found to be variable at the bottom of the scratch under different contact speeds and depths. Based on the above findings, a multi-layer superposition model was proposed to re-couple the effect of different cracks and predict the workpiece's damage/force state scratched by spherical grits. It proved that blunt grit could also realize low damage machining under specific conditions.

Ammonia Mono Hydrate IV: An Attempted Structure Solution

The mixed homonuclear and heteronuclear hydrogen bonds in ammonia hydrates have been of interest for several decades. In this manuscript, a neutron powder diffraction study is presented to investigate the structure of ammonia monohydrate IV at 170 K at an elevated pressure of 3–5 GPa. The most plausible structure that accounts for all features in the experimental pattern was found in the P21/c space group and has the lattice parameters a=5.487(3) Å, b=19.068(4) Å, c=5.989(3) Å, and β=99.537(16) deg. While the data quality limits discussion to a proton-ordered structure, the structure presented here sheds light on an important part of the ammonia–water phase diagram.

Lauric acid adsorption on specific boron nitride fullerenes and the chemical influence of homonuclear bonds: a theoretical approach

The efficiency of boron nitride fullerenes as lauric acid transporters, seen from a density functional theory point of view.

Electron diffraction radial distribution function analysis of amorphous boron carbide synthesized by ion beam irradiation and chemical vapor deposition

Amorphous boron carbide (a-BxC) networks consist of light elements, and their low atomic scattering factors makes structural analysis by x-ray diffraction difficult. Electron diffraction has an advantage of detecting the light elements, because of the strong interaction between the matter and electrons. We prepared a-BxC by ion beam technologies and plasma-enhanced chemical vapor deposition, and characterized their structures via atomic pair-distribution functions derived from electron diffraction intensity profiles. It was found that a pentagonal pyramid is the most favorable cluster in a-B4C generated by ion irradiation, while CC homonuclear bonds were formed in the deposited a-BxC thin film. X-ray photoemission spectroscopy revealed that the a-BxC thin film possesses more carbon than B4C, which is responsible for the formation of the homonuclear bonds.

Effect of homonuclear boron bonds in the adsorption of DNA nucleobases on boron nitride nanosheets

Quantum-mechanics calculations were carried out within the density functional theory (DFT) scheme, to analyze the sorbate-sorbent interaction between DNA nucleobases (purines: guanine and adenine; pyrimidines: cytosine and thymine) and hexagonal boron nitride nanosheets (pristine and non-stoichiometric) in two phases: i) gas and ii) aqueous. The resulting molecular simulations for the pristine nanosheet indicate that the four molecular sorbates prefer to be oriented perpendicular and/or parallel respect to sorbent. This geometrical effect generates adsorption energies associated to non-covalent interactions (physisorption), being the preferential adsorption sites those of N atoms of the nanosheet. According to the calculated quantum descriptors, they exhibit low chemical reactivity and work function, as well as high polarity and semiconductor-like behavior. However, the homonuclear boron bonds in the nanosheet (negatively charged) induce a strong interaction, almost three times larger than pristine nanosheet in gas phase; except for cytosine, due to this is weakly adsorbed in both phases. Moreover, the chemical reactivity and work function are reduced, whereas its conductivity (energy LHgap) and polarity were increased, since the preferential interaction site is corresponding to a B atom. Since the magnetic behavior (1.0 bohr magneton) of BN nanosheet/rB is not altered, the nanosheets with homonuclear boron bonds might be used as potential drug delivery vehicles and sensors.

Intense ionizing irradiation-induced atomic movement toward recrystallization in 4H-SiC

An ultrafast thermal spike within a time interval of a few pico-seconds generated by intense ionizing energy deposited using 100 MeV Ag ions is utilized to study the atomistic details of damage recovery in 4H-SiC. Sequential single ion irradiations were performed using 300 keV Ar and 100 MeV Ag in ⟨0001⟩ 4H-SiC to invoke swift heavy ion (SHI) beam induced epitaxial recrystallization in samples with different degrees of pre-damaged conditions. SHI irradiation was carried out at both room temperature and a low temperature of ∼80K. Low-temperature irradiation was carried out to arrest thermal diffusion of defects and to isolate ionization-induced defect migration in 4H-SiC. Insights into the thermal spike generated by ionizing events in crystalline and amorphous regions at both the temperatures predict a SiC response to SHI. The results emphasize the role of different degrees of pre-damage induced physico-chemical conditions and irradiation temperatures against SHI-induced recrystallization as evaluated by Rutherford backscattering/channeling, Raman spectroscopy, and hard x-ray photoelectron spectroscopy. Understanding the dependence of ion-beam damage accumulation and their recovery on the inelastic to elastic energy loss ratio is important for the performance prediction of SiC intended for extreme environments such as space, defense, and nuclear radiation. We report substantial damage recovery even at a near liquid nitrogen temperature of ∼80K. The recovery gets impeded mainly by the formation of complex defects having homonuclear bonds. The results are explained in the framework of the inelastic thermal spike model, and the role of phonon in the damage recovery process is emphasized.

Chemical disorder of a-SiC layer induced in 6H-SiC by Cs and I ions co-implantation: Raman spectroscopy analysis

Single crystals of 6H-SiC wafers were sequentially co-implanted with 360 keV Cs and I ions to a fluence of 1 × 1016 cm−2 at room temperature. The Monte Carlo simulation code, Stopping and Range of Ions in Matter (SRIM) was used to simulate the implanted ions in SiC. The SRIM simulation showed that in the co-implanted 6H-SiC, the initial implantation of Cs ions at a dose of 36 displacements per atom (dpa) highly amorphized the SiC structure producing a shallow amorphous SiC (a-SiC) layer of about 200 nm, and the subsequent implantation of I ions at a dose of 35 dpa caused more amorphization, especially to partially distorted/disordered parts of the a-SiC layer. This was confirmed by Raman spectroscopy and imaging analysis which also revealed that ion implantation induced the appearance of several new SiSi and CC homonuclear and SiC heteronuclear bonds in the a-SiC network. The homonuclear bonds play a vital role in amorphization which proceeds through the accumulation of Frenkel pairs and antisite defects. The line-shapes of the Raman signals recorded for a-SiC layer resembles that of amorphous SiC, in the spectral region characteristic of homonuclear bonds in particular.

B12N12 nanocages with homonuclear bonds as a promising material in the removal/degradation of the insecticide imidacloprid

Abstract The isomers of B12N12 nanocage with eighteen (Hn18) and sixteen (Hn16) homonuclear bonds have been proposed to capture/degrade the insecticide imidacloprid (Im). Calculations based on Density Functional Theory (DFT) were performed to analyze the interactions and electronic properties of the B12N12-Im, Hn18-Im, and Hn16-Im systems at HSEh1PBE/6-311G(d,p) level of theory. The large values of adsorption energy for the whole systems indicate that the interaction is mediated by chemisorption; therefore, these could be efficient to capture this insecticide. The latter effect leads to high geometrical modification for the Hn18-Im and Hn16-Im models due to one oxygen migrates from the nitroimine group of the imidacloprid, suggesting a new way to degrade the imidacloprid through the formation of radical species. On the other hand, the quantum parameters such as the electron affinity, ǀHOMO-LUMOǀ gap energy, and work function their values decrease in both gas and water phases, compared to systems unsaturated. Interestingly, these nanocages have great adsorption capacity even with five imidacloprid molecules attached to pristine cases, the interaction energy remains in the interval of chemisorption. Moreover, their high polarity indicates excellent solubility, property very convenient for the removal of this insecticide.

Chemistry under high pressure

Thanks to the development of experimental high-pressure techniques and methods for crystal-structure prediction based on quantum mechanics, in the past decade, numerous new compounds, mostly binary, with atypical compositions have been predicted, and some have been synthesized. Differing from conventional solid-state materials, many of these new compounds are comprised of various homonuclear chemical species, such as dimers, trimers, pentagonal and heptagonal rings, polymeric chains, atomic layers and 3D networks. Strikingly, it has been shown that pressure can alter the chemistry of an element by activating its (semi)core electrons, unoccupied orbitals and even the non-atom-centred quantum orbitals located on the interstitial sites, leading to many new surprising phenomena. This Review provides a summary of atypical compounds that result from the effects of high pressure on either the chemical bonds or the local orbitals. We describe various unusual chemical species and motifs, show how the chemical properties of the elements are altered under pressure and illustrate how compound formation is favoured even in situations in which chemical bonds are not formed. An extraordinary new picture of chemistry emerges as we piece together these unexpected high-pressure phenomena. In marked contrast to the previously held beliefs regarding the behaviour of solids under pressure, we are learning that the quantum-mechanical features of electrons, such as those that lead to the formation of directional bonds, inhomogeneous distributions of electrons and atoms, as well as variations in symmetry, might be magnified under pressure. We discuss the influence of these phenomena on future studies that will probe chemistry at higher pressures and explore more complex chemical compositions than those that have been studied to date. High pressure leads to striking new chemistry. Many new compounds with atypical compositions and a plethora of novel chemical species can be stabilized by the formation of homonuclear bonds and the activation of core electrons, non-valence and non-atomic orbitals.