Single Covalent Bond Research Articles

Single Dynamic Covalent Bond Tailored Responsive Molecular Junctions.

Responsive molecular devices are one of the core units for molecular electronics, and dynamic covalent bonds (DCBs) provide the opportunity for the fabrication of responsive molecular devices. Herein we employ a single dynamic acyl hydrazone bond to fabricate tailored molecular devices using the scanning tunneling microscopy break-junction technique (STM-BJ) and the eutectic Ga-In technique (EGaIn). We found that the single-DCB-tailored molecular devices exhibited acid-base and/or photo-thermal response with three well-defined molecular conductance states. The reversible switching has the ON/OFF ratio of ≈10 between each state for single-molecule junctions and ≈3 for the SAMs-based molecular junctions. Combined with the density functional theory calculations, we revealed that the multiple conductance states of these molecular junctions originate from the dynamic acyl hydrazone bond exchange and C=N isomerization. Our work opens the avenue towards the design of tailored single-molecule electrical devices by implanting dynamic covalent bonds in molecular architectures.

Single Dynamic Covalent Bond Tailored Responsive Molecular Junctions

AbstractResponsive molecular devices are one of the core units for molecular electronics, and dynamic covalent bonds (DCBs) provide the opportunity for the fabrication of responsive molecular devices. Herein we employ a single dynamic acyl hydrazone bond to fabricate tailored molecular devices using the scanning tunneling microscopy break‐junction technique (STM‐BJ) and the eutectic Ga‐In technique (EGaIn). We found that the single‐DCB‐tailored molecular devices exhibited acid‐base and/or photo‐thermal response with three well‐defined molecular conductance states. The reversible switching has the ON/OFF ratio of ≈10 between each state for single‐molecule junctions and ≈3 for the SAMs‐based molecular junctions. Combined with the density functional theory calculations, we revealed that the multiple conductance states of these molecular junctions originate from the dynamic acyl hydrazone bond exchange and C=N isomerization. Our work opens the avenue towards the design of tailored single‐molecule electrical devices by implanting dynamic covalent bonds in molecular architectures.

Pyrolysis mechanism of phenylboronic acid modified phenolic resin

Phenylboronic acid (PBA) modified phenolic resin (PBPR) is one of the most important phenolic resins (PR) due to its good processability and high char yield. However, the pyrolysis mechanisms of PR and PBPR still remain poorly understood, which imposes limits on the synthesis, development and application of these materials. In this work, the pyrolysis process of PBPR has been studied through novolac resin (NR) and PBA crosslinked NR (PBNR) at high temperatures, and the reasons for the high char yield of PBPR were explored, as well. According to the results, the pyrolysis of NR begins with the cleavage reaction of the covalent single bonds in the molecular backbone. The release of volatile organic compounds (VOCs) such as monophenols and bisphenols formed by the cleavage of C—C single bonds is the major cause of the decreased char yield of NR at elevated temperatures. Meanwhile, increasing NR molecular weight and crosslinking NR could reduce the formation probability of VOCs, which is beneficial to obtaining the high char yield resin. In PBPR, the boronic ester linkages make the formation of VOCs more difficult and contribute to the high char yield of resin. The present work provides guiding significance for deeply understanding the pyrolysis mechanism of PR and PBPR to ensure their high thermal stability and char yield at high temperatures.

Stabilizing Black Phosphorus via Covalent Functionalization of Solvent Formamide

AbstractBlack phosphorus (BP) has been attracting increasing scientific and technological interest for its excellent materials properties, but suffers from the intrinsic easy oxidization due to the high reactivity of lone‐pair electrons in each phosphorus atom. Herein, solvent formamide (FM) is proposed for the covalent modification of BP by a one‐step ultrasonication without any additive. FM is demonstrated to functionalize BP by the formation of single covalent bond PN, thus occupying the active lone‐pair electrons of P atoms and leading to the significant enhancement of the antioxidative capacity of BP. Density functional theory calculations also rationalize the formation of stable PN bond and its contribution to the high solvent‐exfoliated efficiency during the exfoliation of BP. The plausible reaction mechanism of FM with BP and the final 5‐membered ring structure of products are also proposed to explain the formation process of the stable PN bonds.

Peroxides in metal complex catalysis

• Peroxides as versatile ligands in coordination chemistry. • Peroxides as valuable oxidants, initiators or substrates in metal complex catalysis. • Mechanisms of metal complex catalyzed peroxidative oxidation reactions. Peroxides form an important class of chemicals in which two oxygen atoms are linked together by a single covalent bond (R−O−O−R, R=H, SO 3 K, alkyl, etc.), with a wide range of applications in medicine, industry, environmental protection, etc. The current version of the Cambridge Structural Database reveals more than 1400 structurally characterized metal-peroxy/peroxo complexes with diverse coordination modes having specific biological, catalytical and other functional properties. Both inorganic and organic peroxides are versatile oxidants and extensively used reagents for metal complex catalyzed functionalization of organic compounds. The utilization of peroxides in metal complex catalysis typically requires prior activation of O−O or O−R bonds at a metal centre leading to radical species with a reactivity and lifetime that are dependent on the substituents R and reaction medium (pH, solvent, etc.). In fact, the cooperation of a metal centre (coordination) with a peroxide can lead to a multifunctional catalytic system having a high performance towards selective functionalization of organic substrates. This review is intended to focus on the auxiliary/crucial role of peroxides in the transformations of various classes of organic compounds such as alkanes, alkenes, alkynes, aromatics, heterocycles, aldehydes, ketones and alcohols, catalyzed by metal complexes.

Cutting-Edge Single-Molecule Technologies Unveil New Mechanics in Cellular Biochemistry.

Single-molecule technologies have expanded our ability to detect biological events individually, in contrast to ensemble biophysical technologies, where the result provides averaged information. Recent developments in atomic force microscopy have not only enabled us to distinguish the heterogeneous phenomena of individual molecules, but also allowed us to view up to the resolution of a single covalent bond. Similarly, optical tweezers, due to their versatility and precision, have emerged as a potent technique to dissect a diverse range of complex biological processes, from the nanomechanics of ClpXP protease-dependent degradation to force-dependent processivity of motor proteins. Despite the advantages of optical tweezers, the time scales used in this technology were inconsistent with physiological scenarios, which led to the development of magnetic tweezers, where proteins are covalently linked with the glass surface, which in turn increases the observation window of a single biomolecule from minutes to weeks. Unlike optical tweezers, magnetic tweezers use magnetic fields to impose torque, which makes them convenient for studying DNA topology and topoisomerase functioning. Using modified magnetic tweezers, researchers were able to discover the mechanical role of chaperones, which support their substrate proteinsby pulling them during translocation and assist their native folding as a mechanical foldase. In this article, we provide a focused review of many of these new roles of single-molecule technologies, ranging from single bond breaking to complex chaperone machinery, along with the potential to design mechanomedicine, which would be a breakthrough in pharmacological interventions against many diseases.

Be-Be Bond in Action: Lessons from the Beryllium-Ammonia Complexes [Be(NH3)0-4]20,2.

High-level electronic structure calculations are performed to elucidate the Be-Be chemical bond in the (NH3)nBe-Be(NH3)n species for n = 0-4. We show that the Be2 bond is explained as a resonance between two Lewis structures, where one beryllium atom donates an electron pair to the second one, and vice versa. The presence of ammonia ligands enhances the stability of this bond considerably. The ∼2.5 kcal/mol binding energy of Be2 becomes ∼30 kcal/mol for [Be(NH3)1-3]2 because of their more polarizable electron pairs. The larger Be(NH3)4 complex has been classified as a solvated electron precursor in the past and has an electron pair in the periphery of a Be(NH3)42+ core occupying a diffuse s-type orbital. The analogy of Be(NH3)4 to Be reflects into the electronic structure of their dimers. The two systems have identical bonding patterns and low-lying electronic states. The ground state binding energy of [Be(NH3)4]2 is 3 times larger than Be2, and its excitation energies are considerably lower by a factor of 3. We also studied the dimers of the cationic Be(NH3)n+ species, and we found that the Coulombic repulsion is counterbalanced by the formation of a single covalent bond in the cases of n = 1, 2 forming stable dicationic [(NH3)nBe-Be(NH3)n]2+ systems, unlike Be22+. We believe that our numerical results will allow the identification and characterization of these exotic species and their solid state (beryllium liquid metals) analogues in future experiments.

Quantum-well states for uniform Ag layers on the Ga-induced Si(111)–([formula omitted])R30° surface

The atomic and electronic structures are calculated for atomically uniform Ag layers on the Ga–induced Si(111)–(3×3)R30° surface by using the Density Functional Theory. It is found that when the amount of Ag atoms increases on the 1/3 monolayer Ga–induced Si(111)–(3×3)R30° surface, the equilibrium T4 adsorption site of Ga atom changes to the T1 site. We have determined a single covalent bond between Ga and Si atoms but there is some charge accumulation on the Ga–Ag layer, turning the surface to metallic nature. For 10 Ag monolayers, we have determined evolution of several quantum-well states within the energy range of 1 eV below the Fermi level. The energy separation between the quantum well states increases as their numbers develop below the Fermi level. The Ag film quantum-well states show in-plane parabolic dispersion, with splittings arising due to Umklapp features related to the Si(111)–(1×1) surface Brillouin zone. We have also identified Shockley type surface states, which show small Rashba-type spin-orbit splitting.

Fatigue-resistant adhesion I. Long-chain polymers as elastic dissipaters

Tough adhesion is often achieved by using inelastic dissipaters. However, inelastic dissipaters fail to enhance adhesion under cyclic loads. Here we achieve fatigue-resistant adhesion by using a particularly simple kind of elastic dissipater: long-chain polymers. Each polymer chain is elastic before rupture. When a single covalent bond of the chain breaks, the elastic energy stored in the entire chain dissipates, amplifying the adhesion energy by the number of links on the chain. So far as the adherends provide the stiffness of an adhered sample, the adhesive can be made of polymer chains of extremely long length. As a proof of concept, we use polyacrylamide hydrogels to adhere two pieces of polyester cloth through topological entanglement. We find that both the adhesion energy and fatigue threshold increase with the polymer chain length and can reach 1400 J/m2 and 300 J/m2, respectively. The measured fatigue threshold of adhesion is linearly proportional to the square root of the chain length, in agreement with the Lake–Thomas model. This fatigue-resistant design can be extended to a variety of adhesion topologies for different adherends and adhesives.

Dissipative Assembly of Macrocycles Comprising Multiple Transient Bonds.

Dissipative assembly has great potential for the creation of new adaptive chemical systems. However, while molecular assembly at equilibrium is routinely used to prepare complex architectures from polyfunctional monomers, species formed out of equilibrium have, to this point, been structurally very simple. In most examples the fuel simply effects the formation of a single short-lived covalent bond. Herein, we show that chemical fuels can assemble bifunctional components into macrocycles containing multiple transient bonds. Specifically, dicarboxylic acids give aqueous dianhydride macrocycles on treatment with a carbodiimide. The macrocycles are assembled efficiently as a consequence of both fuel-dependent and fuel-independent mechanisms; they undergo slower decomposition, building up as the fuel recycles the components, and are a favored product of the dynamic exchange of the anhydride bonds. These results create new possibilities for generating structurally sophisticated out-of-equilibrium species.

Profil Penguasaan Konsep Ikatan Kimia pada Siswa Kelas X SMA Negeri 4 Palangka Raya Tahun Ajaran 2017/2018

Konsep ikatan kimia merupakan salah satu materi pelajaran kimia SMA kelas X. Penelitian ini bertujuan untuk mendeskripsikan penguasaan konsep ikatan kimia pada siswa kelas X SMA Negeri 4 Palangka Raya Tahun Ajaran 2017/2018. Penelitian ini termasuk jenis penelitian kualitatif yang bersifat deskriptif. Subjek penelitian ini adalah 29 siswa kelas X SMA Negeri 4 Palangka Raya Tahun Ajaran 2017/2018 yang sudah menerima pembelajaran materi ikatan kimia. Data yang digunakan pada penelitian ini berupa dokumen RPP yang didapat dari guru yang bersangkutan. Data penguasaan konsep siswa diperoleh melalui tes uraian ikatan kimia menggunakan 7 butir soal esai yang mencakup 5 indikator. Data pendukung menggunakan hasil wawancara terhadap siswa. Penguasaan konsep siswa dalam menuliskan konfigurasi elektron dan menentukan elektron valensi unsur 94% dengan kategori sangat tinggi. Penguasaan konsep menggambarkan struktur Lewis 62% kategori sedang. Penguasaan konsep menggambarkan cara suatu unsur untuk mencapai kestabilan 74 % kategori tinggi. Penguasaan konsep mengelompokan unsur gas mulia dan bukan gas 22% kategori rendah. Penguasaan konsep menjelaskan aturan duplet dan aturan oktet 20% kategori rendah. Penguasaan konsep menjelaskan proses terbentuknya ikatan ion 49% berada pada kategori sedang. Penguasaan konsep menggambarkan struktur Lewis dan menentukan rumus kimia senyawa ikatan kovalen tunggal 51% kategori sedang. Penguasaan konsep menggambarkan struktur Lewis dan menentukan rumus kimia senyawa ikatan kovalen rangkap dua 65% berada pada kategori sedang. Penguasaan konsep menggambarkan struktur Lewis dan menentukan rumus kimia senyawa ikatan kovalen rangkap tiga 40% kategori rendah. Penguasaan konsep menjelaskan proses terbentuknya ikatan kovalen koordinasi 29% kategori rendah. Penguasaan konsep menjelaskan proses pembentukan ikatan logam dan menjelaskan sifat senyawa yang berikatan logam masing-masing 31% dan 45% kategori rendah.

Stabilization of Open-Shell Single Bonds within Endohedral Metallofullerene.

The open-shell single covalent bond composed of two electrons is unstable under normal conditions, because the closed-shell electronic configuration is generally beneficial to minimize the energy of the system. This classical rule always governs the chemical bonding of s- and p-block homonuclear diatomic molecules, such as the stable σ2 electron-pair bonds in hydrogen. In this work, surprisingly, we found that the diversified open-shell single bonds between two f-block atoms (e.g., thorium) can be stabilized within a tight "carbon-confined-space" using relativistic quantum chemical calculations. We first identified a stable dithorium endohedral metallofullerene (EMF), Th26+@Ih-C806-, with a Th-Th distance of 3.803 Å inside the Ih-C80 cage, which displays a unique spin-polarized σ1π1 2-fold single-electron Th3+-Th3+ bond that is collaboratively dominated by 5f6d7s7p orbitals. The Th3+-Th3+ bond can further evolve into a 5f6d dominated spin-polarized π2 configuration by compressing the Th-Th distance further down to 2.843 Å, within a smaller Ih-C60 cage. On the other hand, elongating the Th-Th distance to 4.063 Å by encapsulating Th2 into a long diametric D3h-C78 fullerene returns the Th3+-Th3+ bond to the normal closed-shell (6d7s7p)σ2 form. Hence, a new rule is unambiguously revealed through the carbon-confinement induced spin-polarization of a single bond. The key point of this rule is the size of the carbon cage, because the squeezed effect is conducive to the effective overlap of the Th(5f) orbitals, reducing and further reversing the original large singlet-triplet energy gap of the Th26+ unit. This discovery provides pioneering guidance for exploring new chemical bonds and thorium-based endofullerenes.

Filling the void: controlled donor-acceptor interaction facilitates the formation of an M-M single bond in the zero oxidation state of M (M = Zn, Cd, Hg).

The intriguing question of whether it is possible to form a genuine M0-M0 single bond for the M2 species (M = Zn, Cd, Hg) is addressed here. So far, all the bonds reported in the literature are exclusively MI-MI. Herein, we present viable M2(NHBMe)2 (M = Zn, Cd, Hg; NHBMe = (HCNMe)2B) complexes in which the controlled donor-acceptor interaction leads to an M0-M0 single bond. In these complexes, M2 in the 1∑g ground state with the (nσg+)2(nσu+)2 (n = 7, 10 and 14 for M = Zn, Cd and Hg, respectively) valence electron configuration forms donor-acceptor bonding with singlet 2NHBMe ligands where a combined effect of dominant (+,-) σ-backdonation from the antibonding (nσu+)2 orbital of M2 to the 2NHBMe ligands and a somewhat weaker (+,+) σ-donation from the 2NHBMe ligands to the bonding (n + 1)σg+ orbital leads to the unorthodox bonding situation of forming an M-M single bond in the zero oxidation state by eventually nullifying one effect by another. This is an unprecedented situation in the sense that the NHBMe ligand acts as a strong σ-acceptor and a weaker σ-donor. A comparison with the experimentally reported M2(PhDipp)2 complexes reveals the uniqueness of the NHBMe ligand in exhibiting such a bonding scenario. The M2(NHBMe)2 complex is thermochemically viable with respect to possible dissociation channels at room temperature, except for metal extrusion processes, M2(NHBMe)2 → M + M(NHBMe)2 and M2(NHBMe)2 → M2 + (NHBMe)2. Although the latter two processes are exergonic, they are kinetically protected by a high free energy barrier of 26.5-39.5 kcal mol-1. The experimental characterization of M2(PhDipp)2 despite similar exergonic channels reveals such kinetic stability to be enough for the viability of the M2(NHBMe)2 complexes. Furthermore, the ligand exchange reaction considering M2(PhMe)2 as the starting material also turned out to be feasible. Therefore, the M2(NHBMe)2 complexes are the first cases that feature a neutral M2 moiety with a single M0-M0 covalent bond, where M is a Group 12 metal.

Half-Projection of the Strongly Orthogonal Unrestricted Geminals’ Product Wave Function

Half-projection of a wave function built as a product of singlet-triplet mixed two-electron fragments (geminals) is explored. The condition of strong orthogonality is imposed between geminals, the initial Ansatz accommodating the unrestricted Hartree-Fock (UHF) wave function as a special case. The here explored geminal product is more general than UHF, it allowing for single covalent bond breaking in a spin pure manner. The strong correlation present in two geminals simultaneously is accompanied by spin contamination at the geminal product level. Variation after half-projection leads to an eigenvalue problem of an effective two-electron Hamiltonian, conserving the computational economy of the initial model. While the resulting expressions are shown to violate size consistency, quantification reveals a suppressed error with half-projection as compared to the full spin projected then varied scheme. Spin contamination is also found to be diminished compared to the initial, unprojected wave function.

The Change in the Nature of the Chemical Bond through Hydrogen Tunneling Process in BN‐Ethylamine

AbstractThe change process in the nature of the chemical bond between BN single covalent (…H2B‐NH2) and coordinate covalent (…H2B‐NH3) bonds was studied considering the BN‐ethylamine molecule (NH2BH2NH3). This molecule is an inorganic ethylamine in which the C atoms were substituted by BN pairs and the B atom is in the center of molecule.

Acetylene polymerization in plasma of direct current

Plasma polymerization is a technique that allows obtaining polymer of any type of hydrocarbon, including the non-polymerizable by standard methods. This because of in the plasma state the activation energy can be reduced for the realization of reactions difficult to be presented under standard experimental conditions. For plasma polymerization only the implementation of different types of discharges in the radio frequency regime has been reported, leaving aside the application of direct current discharges for this process. The acetylene polymer consists of carbon chains with alternation of single and double covalent bonds. The formation of this polymer has been reported using radio frequency discharges in the abnormal regime. The paper reports the formation of polymer from acetylene using the abnormal glow discharge in the direct current regime, a novel process not yet reported in the literature. The deposition of the polymer is carried out at different times on a polycrystalline copper substrate previously treated in a glow discharge of argon and hydrogen. For the generation of this polymer an atmosphere of 60% Ar+35% H2+5% C2H2 at 2 torr pressure and a temperature of 600 °C is used. The structural and morphological analysis of the deposits is carried out by infrared spectroscopy and scanning electron microscopy, respectively.

Nature of interlayer carbon–carbon covalent bonding in graphene-based materials

This study provides a fundamental understanding of the nature of C–C interlayer covalent bonding in graphene-based materials by using the dispersion-corrected density functional theory B3LYP-D3/6-31+G(2df,p)//B3LYP-D3/6-31G(d,p) (single-point energies at the larger 6-31+G(2df,p) basis set using fully optimized geometries at the B3LYP-D3/6-31G(d,p) level). With a bilayer hydrogenated graphene model, a partial covalent bonding was found upon dehydrogenation from opposite layers between interior carbon atoms even at the distance of greater than 4.00 A. To facilitate such bonding carbon atoms must transform from its sp3 to sp2 hybridization upon dehydrogenation so that pz orbitals can extend farther for better overlap at a large distance. The structure containing a single partial covalent bond was found to be less stable compared to its nonbonding triplet state. However, adding normal interlayer covalent bonds at the edge helps to stabilize such partial bond. In addition, forming a partial interlayer covalent bond greatly reduces band gap to 1 eV, whereas the formation of a normal covalent bond causes only a slight change in the band gap. Thus, controlling the population of the two types of interlayer bonds can, therefore, open up a promising way to control the band gap for organic semiconductor materials.

Formal structure of periodic system of elements.

For more than 150 years, the structure of the periodic system of the chemical elements has intensively motivated research in different areas of chemistry and physics. However, there is still no unified picture of what a periodic system is. Herein, based on the relations of order and similarity, we report a formal mathematical structure for the periodic system, which corresponds to an ordered hypergraph. It is shown that the current periodic system of chemical elements is an instance of the general structure. The definition is used to devise a tailored periodic system of polarizability of single covalent bonds, where order relationships are quantified within subsets of similar bonds and among these classes. The generalized periodic system allows envisioning periodic systems in other disciplines of science and humanities.

Determination of Energy-Level Alignment in Molecular Tunnel Junctions by Transport and Spectroscopy: Self-Consistency for the Case of Oligophenylene Thiols and Dithiols on Ag, Au, and Pt Electrodes.

We report detailed measurements of transport and electronic properties of molecular tunnel junctions based on self-assembled monolayers (SAMs) of oligophenylene monothiols (OPT n, n = 1-3) and dithiols (OPD n, n = 1-3) on Ag, Au, and Pt electrodes. The junctions were fabricated with the conducting probe atomic force microscope (CP-AFM) platform. Fitting of the current-voltage ( I-V) characteristics for OPT n and OPD n junctions to the analytical single-level tunneling model allows extraction of both the HOMO-to-Fermi-level offset (εh) and the average molecule-electrode coupling (Γ) as a function of molecular length ( n) and electrode work function (Φ). Significantly, direct measurements of εhUPS by ultraviolet photoelectron spectroscopy (UPS) for OPT n and OPD n SAMs on Ag, Au, and Pt agree remarkably well with the transport estimates εhtrans, providing strong support-beyond the high quality I-V simulations-for the relevance of the analytical single-level model to simple molecular tunnel junctions. Because the UPS measurements involve SAMs bonded to only one metal contact, the correspondence of εhUPS and εhtrans also indicates that the top contact has a weak effect on the HOMO energy. Corroborating ab initio calculations definitively rule out a dominant contribution of image charge effects to the magnitude of εh. Thus, the effective molecular tunnel barrier εh is determined, and essentially pinned, by the formation of a single metal-S covalent bond per OPT n or OPD n molecule.

Thermoset Polymer Matrix Structure and Properties: Coarse-Grained Simulations.

The formation of a thermoset polymer network is a complex process with great variability. In this study, we used dissipative particle dynamics and graph theory tools to investigate the curing process and network topology of a phthalonitrile thermoset to reveal the influence of initiator and plasticizer concentration on its properties. We also propose a novel way to characterize the network topology on the basis of two independent characteristics: simple cycle length (which is mainly affected by the initiator amount) and the number of simple cycles passing through a single covalent bond (which is determined primarily by plasticizer concentration). These values can be treated in the more familiar terms of network “mesh size” and “sponginess”, correspondingly. The combination of these two topological parameters allows one to characterize any given network in an implicit but precise way and predict the resulting network properties, including the mechanical modulus. We believe that the same approach could be useful for other polymer networks as well, including rubbers and gels.