Properties of Sulfur-Sulfur Bonds

SS bonds are extraordinarily flexible and have properties that are observed only on isolated occasions for other homonuclear bonds: the bond lengths very between 1.8 and 3.0Å, the bond angles between 90 and 180° and the dihedral angles between 0 and 180°; the bond energies amount to up to 430 kJ/mol. The SS stretching frequencies can appear over the range 177–820 cm^?1 and force constants of 1.4 to 6.3 mdyne/Å have been calculated. This variability is illustrated with examples containing isolated and cumulated SS bonds.     

Non-Equivalent Hybrid Atomic Orbitals and Covalent Bonds in a2 Molecules

In addition to having a noninteger value of λ for non-equivalent hybrid atomic orbitals sp ^λ the introduction of a noninteger occupation number to a valence configuration for atoms in a molecule renders a precise account of the covalent bond in homonuclear diatomic molecules. For instance, the following bond orders 0.05, 1.42 and 2.62 are obtained for the molecules Be², B² and C² according to the proposed method.     

Covalent versus Dative Bonds to Main Group Metals,a Useful Distinction

Textbooks of inorganic chemistry describe the formation of adducts by coordination of an electron donor to an electron acceptor, often using the amine-boranes, X₃N → BY₃, as examples. In the Lewis (electron dot) formulas of the compounds, the dative bond in H 3 N → BH₃ and the covalent bond in H₃C?CH₃ are both represented by a shared electron pair. In the simple molecular orbital or valence bond models the wave functions of both electron pairs would be constructed in the same manner from the appropriate sp³ type atomic orbitals on the bonded atoms; the difference between the covalent and the dative bond becomes apparent only after the orbital coefficients have been analyzed. This may be the reason why many structural chemists seem reluctant to distinguish between the two types of bonds. The object of this article is to remind the reader that the physiocochemical properties of covalent and dative bonds may be – and often are – quite different, and to show that a distinction between the two provides a basis for understanding the structures of a wide range of main group metal compounds.     

An Exchange Charge Model for Covalent Bonds in Simple Homonuclear Diatomic Molecules

The point charge in Parr's simple bond charge model is replaced by the exchange charge, which can be evaluated according to a simple ab initio method. The calculated exchange charge correlate well with the experimental values of force constants and dissociation energies for homonuclear diatomic molecules H₂, Li₂, F₂, Na₂ and Cl₂.     

Force Constants and Bond Orders of Nitrogen Bonds

The force constants and the corresponding bond orders of nitrogen bonds have been calculated from the vibrational spectra (infrared and Raman spectra) of a great number of nitrogen compounds. Plotting the maximum bond order of stable nitrogen bonds against the sum of Pauling's electronegativities of the bonding partners (Σx) leads to one continuous curve for the N? X bonds where X represents elements of the first and the second short period of the periodic table. Furthermore, when the bonds formed between these elements are arranged in a coordinate system in such a way that the position of each bond is determined by the difference between the electronegativities of the bonding partners (Δx along the ordinate) and the sum of the electronegativities of the bonding partners (Σx along the abscissa), the bonding partners capable of forming multiple bonds all lie within a closed domain, where their position can be correlated with their polymerizability and other reactivities of the multiple bonds. Also discussed are the orders of bonds between nitrogen and some transition elements. In an appendix, the present methods used to calculate force constants and bond orders are surveyed.     

Spectroscopic Investigation of Various Phosphorus-Sulfur Bonds

The results of spectroscopic investigations on phosphorus-sulfur compounds, and particularly the force constants, reveal several factors that decisively influence the bonding. The effects of hybridization, the other bonding partners and their electronegativities, the π-bonding components, and the ionic charge are discussed. It is shown that the P? S bond, unlike the P? O bond, never attains full double bond character, despite its great variability.     

Nature of Alkali- and Coinage-Metal Bonds versus Hydrogen Bonds

We have quantum chemically studied the structure and nature of alkali- and coinage-metal bonds (M-bonds) versus that of hydrogen bonds between A−M and B⁻ in archetypal [A−M⋅⋅⋅B]⁻ model systems (A, B=F, Cl and M=H, Li, Na, Cu, Ag, Au), using relativistic density functional theory at ZORA-BP86-D3/TZ2P. We find that coinage-metal bonds are stronger than alkali-metal bonds which are stronger than the corresponding hydrogen bonds. Our main purpose is to understand how and why the structure, stability and nature of such bonds are affected if the monovalent central atom H of hydrogen bonds is replaced by an isoelectronic alkali- or coinage-metal atom. To this end, we have analyzed the bonds between A−M and B⁻ using the activation strain model, quantitative Kohn-Sham molecular orbital (MO) theory, energy decomposition analysis (EDA), and Voronoi deformation density (VDD) analysis of the charge distribution.     

Polarity of Binary Liquid Mixtures

In contrast to the thoroughly studied plarity properties of pure liquids, only little is known about the polarity of mixtures of liquids, although the majority of machanistic and preparative work is not carried out in pure phases. Using a widely applicable two-parameter equation, polar behavior of binary liquid mixtues can be described quantitatively as a function of their composition. Based on this equation, satisfactory explanations are found for deviations observed for binary solvent mixtures from the linear correaltion of polarity scales, as well as for the unusual activation parameters estimated by Winstein for solvolysis of tert-butyl chloride. Applications of the equation range from a rapid test for determining water contents of solvents, the study of reaction mechanisms, to polymer chemistry.     

High-Resolution X-ray Crystallography—An Experimental Method for the Description of Chemical Bonds

By increasing the accuracy of an X-ray analysis, results are obtained which allow one to beyond the location of hydrogen atoms and say something about the distribution of electrons in molecules. Using several examples from organic and organometallic chemistry, the methods, problems, and results of this approach are described.     

Multiple Heteroatom-Hydrogen Bonds Bridging Electron Transport in Covalent Organic Framework-Based Supramolecular System for Photoreduction of CO2

Supramolecular systems consisting of covalent organic frameworks (COFs) and Ni complex are designed for robust photocatalytic reduction of CO₂. Multiple heteroatom-hydrogen bonding between the COF and Ni complex is identified to play a decisive role in the photoexcited electron transfer across the liquid-solid interface. The diminution of steric groups on COF or metal complex can optimize catalytic performance, which is more attributable to the enhanced hydrogen-bond interaction rather than their intrinsic activity. The photosystem with relatively strong strength of hydrogen bonds exhibits remarkable photocatalytic CO₂-to-CO conversion, far superior to photosystems with supported atomic Ni or metal complex alone in the absence of hydrogen-bond effect. Such heteroatom-hydrogen bonds bridging electron transport pathway confers supramolecular system with high photocatalytic performance, providing an avenue to rationally design efficient and steadily available photosystems.     

DFT Study on Homolytic Dissociation Enthalpies of C-I Bonds

The C-I bond dissociation enthalpies （BDE） of various organic iodides were calculated using high-level theoretical methods including MP2 and CCSD（T） with extrapolated basis set as well as a number of density functional theory methods. After systematic evaluation of the theoretical results against available experimental C-I BDEs, it was found that the MPW LYPIM method gave the lowest root mean square error. We, therefore, used this method to examine the substituent effects on different categories of C（sp3）-I and C（sp2）-I bonds. Fur thermore, the remote substituent effects on the C-I BDEs of substituted iodobenzenes and substituted （iodomethyl）benzenes were also investigated at the same level. The C-I BDEs of typical heteroaromatic iodides including five-membered and six-membered heterocyclic iodides were also examined.     

共价键极性的讨论

根据元素原子在不同化学环境下吸引电子能力的差异性,指出了化学环境对极性共价键和非极性共价键的影响。     

The Ionization (Heterolysis) of Covalent Compounds as a Coordination-Chemical Phenomenon

The ionization of a covalent compound into a cation and an anion is a coordination-chemical phenomenon. The ions are formed with coordination either by nucleophilic attack of an electron pair donor or by electrophilic attack of an electron pair acceptor. The dielectrically controlled component of the ionization affects only the degree to which it occurs. It is shown that the ionization generally increases with increasing donor strength of the donor or with increasing acceptor strength of the acceptor. For a given M, the ease of ionization of the M? X bond increases in the order X = fluoride < chloride < bromide < iodide. These general aspects are discussed with the aid of examples.     

Isomeric Oligo(Phenylenevinylene)-Based Covalent Organic Frameworks with Different Orientation of Imine Bonds and Distinct Photocatalytic Activities

Imine-linked covalent organic frameworks (COFs) have been extensively studied in photocatalysis because of their easy synthesis and excellent crystallinity. The effect of imine-bond orientation on the photocatalytic properties of COFs, however, is still rarely studied. Herein, we report two novel COFs with different orientations of imine bonds using oligo(phenylenevinylene) moieties. The COFs showed similar structures but great differences in their photoelectric properties. COF-932 demonstrated a superior hydrogen evolution performance compared to COF-923 when triethanolamine was used as the sacrificial agent. Interestingly, the use of ascorbic acid led to the protonation of the COFs, further altering the direction of electron transfer. The photocatalytic performances were increased to 23.4 and 0.73 mmol g⁻¹ h⁻¹ for protonated COF-923 and COF-932, respectively. This study provides a clear strategy for the design of imine-linked COF-based photocatalysts and advances the development of COFs.     

介质极性敏感膜的制备和性能研究

以交联壳聚糖膜为载体,芘为介质极性探针,制备并表征了一种介质极性敏感膜,研究了该膜对醇-水混合溶剂极性的响应行为.基于极性测定,对十二烷基硫酸钠(SDS)和十二烷基磺酸钠(SLS)水溶液中胶束的形成进行了检测.结果表明,该膜具有较好的可逆性和重现性,且制膜方法简单、快速.     

Dynamic Covalent Bonds in Polymeric Materials

Dynamic covalent bonds (DCBs) have received significant attention over the past decade. These are covalent bonds that are capable of exchanging or switching between several molecules. Particular focus has recently been on utilizing these DCBs in polymeric materials. Introduction of DCBs into a polymer material provides it with powerful properties including self‐healing, shape‐memory properties, increased toughness, and ability to relax stresses as well as to change from one macromolecular architecture to another. This Minireview summarizes commonly used powerful DCBs formed by simple, often “click” reactions, and highlights the powerful materials that can result. Challenges and potential future developments are also discussed.     

Polarity Effects of Propylene Carbonate on Breakdown Strength in Microsecond Range

We investigate the polarity effects of the propylene carbonate on the breakdown voltage using the needle-plate electrodes with gaps of 0.5, 1.0, and 2.0 mm. The devices used in this study involve a compact capacitive-energy-storage pulse power source with charging time varying from 5 ms to 20 ms and a test cell with the needle-plate electrodes. The breakdown voltage is recorded by a digital oscilloscope for each gap. The results of these three groups indicate that the positive breakdown voltage is higher than the negative one and the breakdown voltage of the PC increases with the ascending electrode gap. In addition, a simulation isconducted to support this experiment. Some explanations about the polarity effect of the PC are also given.     

气相色谱固定液极性和选择性评价

对非同族化合物溶质在两种固定相上保留值的关系进行了理论推导，并将该方程应用于８９种常见固定相的极性和选择性的评价。固定相的极性可用Ｂ值来评价，而选择性则用Ｂ、Ｔ３＾ａ，Ｃ３β值来评价。用该方程评价固定相具有两个优点，一是基于分子间作用力来评价固定相；二是探针化合物的选择不再受限制。     

Templated Chromophore Assembly by Dynamic Covalent Bonds

Through the simultaneous use of three orthogonal dynamic covalent reactions, namely disulfide, boronate, and acyl hydrazone formation, we conceived a facile and versatile protocol to spatially organize tailored chromophores, which absorb in the blue, red, and yellow regions, on a preprogrammed α‐helix peptide. This approach allowed the assembly of the dyes in the desired ratio and spacing, as dictated by both the relative positioning and distribution of the recognition units on the peptide scaffold. Steady‐state UV/Vis absorption and emission studies suggest an energy transfer from the yellow and red donors to the blue acceptor. A molecular dynamics simulation supports the experimental findings that the helical structure is maintained after the assembly and the three dyes are confined in defined conformational spaces.