The bond order—bond length relationship

The difference in length between two bond orders was reported by Pauling to be essentially the same, regardless of the atoms that make up the bond. To a first approximation these differences hold not only for bond orders 1, 2 and 3 but also for six membered aromatic rings containing all carbon, carbon-nitrogen, nitrogen-nitrogen, carbon-phosphorous, carbon-arsenic, and carbon-antimony bonds. An equation was developed (based upon these differences) that relates bond order and bond length. The output of this equation was compared with those of Gordy and Pauling. Our equation as well as the Gordy equation (with revised constants) return a bond length of 1.4 Å for bond order 1.67 which is consistent with theory. (This bond order was not used in developing either the equation or the revised Gordy constants.)     

Carbon-carbon and carbon-hydrogen bond energies and bond distances

Two types of mathematical relations are discussed, which represent the connection between carbon-carbon bond energies and carbon-carbon bond distances. They similarly describe the relation between carbon-hydrogen bond energies and the corresponding carbon-hydrogen bond distances.     

Defining the bond energy of a strong hydrogen bond

Based on a recent definition of hydrogen-bond energy the hydrogen bond in [HCOO…H…F]^? is weaker than that in [F…H…F]^?, although the former still ranks as a very strong hydrogen bond.     

Hydrogen bond and halogen bond inside the carbon nanotube

The hydrogen bond and halogen bond inside the open-ended single-walled carbon nanotubes have been investigated theoretically employing the newly developed density functional M06 with the suitable basis set and the natural bond orbital analysis. Comparing with the hydrogen or halogen bond in the gas phase, we find that the strength of the hydrogen or halogen bond inside the carbon nanotube will become weaker if there is a larger intramolecular electron-density transfer from the electron-rich region of the hydrogen or halogen atom donor to the antibonding orbital of the X-H or X-Hal bond involved in the formation of the hydrogen or halogen bond and will become stronger if there is a larger intermolecular electron-density transfer from the electron-rich region of the hydrogen or halogen atom acceptor to the antibonding orbital of the X-H or X-Hal bond. According to the analysis of the molecular electrostatic potential of the carbon nanotube, the driving force for the electron-density transfer is found to be the negative electric field formed in the carbon nanotube inner phase. Our results also show that the X-H bond involved in the formation of the hydrogen bond and the X-Hal bond involved in the formation of the halogen bond are all elongated when encapsulating the hydrogen bond and halogen bond within the carbon nanotube, so the carbon nanotube confinement may change the blue-shifting hydrogen bond and the blue-shifting halogen bond into the red-shifting hydrogen bond and the red-shifting halogen bond. The possibility to replace the all electron nanotube-confined calculation by the simple polarizable continuum model is also evaluated.     

The carbon-carbon triple bond and the nitrogen-nitrogen triple bond

The unshared-pair orbital of a nitrogen atom in :NN: is estimated to have 21 per cent p character and 79 per cent s character. The nature of this orbital is such that the energy of repulsion between unshared pairs for the nitrogen-nitrogen triple bond is expected to be very small, whereas it is large, about 40 kcal/mole, for N=N and N=N; in consequence the triple bond is especially stable. For---CC---, on the other hand, there is significant repulsion energy of the electrons involved in the adjacent single bonds, causing instability of the triple bond.     

The use of bond paths for quantitatively characterizing bond strain

The concept of the bond path as a means for characterizing individual strained bonds is developed in detail and applied to a variety of molecules. A reference path is defined in terms of the superposed electronic densities of the free atoms, and a “bond deviation index” is introduced, for expressing quantitatively the difference between the actual and the reference bond paths. This difference is taken as indicating the degree of strain in the bond. It is shown that bond paths can be significantly affected by not only neighboring bonding electrons, but by lone pairs as well, as in the H₂O and NH₃ molecules.     

The muonium bond

Muonium is a light isotope of hydrogen, so light that in strong hydrogen bonds its zero-point energy is close to or above the energy barrier; this favours the formation of symmetrical muonium bonds. In water, muons will exist as Mu(H₂O)₆⁺ in contrast to H(H₂O)₄⁺; the stability of this species will slow the exchange of Mu and H. With alkenes and alkynes, muons will preferentially form non-classical complexes because of their lower zero-point energy in weak bonds: in liquids, a muon will attach two molecules via a symmetrical muonium bond.     

The hydrogen bond

The concept of the hydrogen bond is a century old but remains youthful because of its vital role in so many branches of science and because of continued advances in experiment, theory and simulation. We discuss the structural and energetic characteristics of normal hydrogen bonds X–H···Y as well as some exceptions to the normal, including proton-shared and ion-pair bonds. We consider the harmonic and anharmonic vibration of X–H in a variety of complexes, and demonstrate that there is no fundamental difference between blue-shifting and red-shifting bonds. We discuss water clusters and liquid water and indicate possible directions of future progress.     

Generalized bond orders

We introduce generalized bond orders defined in terms of weighted Kekule valence structures. The weights were determined by the contributions of linearly independent and minimal conjugated circuits in individual Kekule valence structure. When special values for the contributions of conjugated circuits of different size are assumed, one obtains quantities that show considerable similarity to the Pauling and the Clar's bond orders. Pauling bond orders are obtained when one assumes that all conjugated circuits make equal contribution to bond orders. © 1994 John Wiley & Sons, Inc.     

Hydrogen bond studies

The oxonium ion, H₃O⁺, has been studied with the MO-LCAO method in order to determine its equilibrium geometry. The main purpose has been to study effects of the external electrostatic forces exerted on the ion situated in crystals whose structures are experimentally known. Calculations have also been performed on the free ion, where the energy minimum is found for a non-planar conformation with H-O-H angles of 116.6° and O-H distances of 0.96 Å. The effect of an external field is essentially to lengthen the O-H distances and decrease the H-O-H angles in order to form approximately linear hydrogen bonds.

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Zusammenfassung Die Gleichgewichtsgeometrie des Oxoniumions H₃O⁺ wurde mit Hilfe der MO-LCAO Methode bestimmt. In erster Linie sind die Effekte, die von externen elektrostatischen Kräften auf das Ion in Kristallen mit bekannter Struktur ausgeübt werden, untersucht worden. Es wurden aber auch Rechnungen für das freie Ion durchgeführt, wobei die minimale Energie im Falle der nicht planaren Konformation mit H-O-H-Winkeln von 116,6° und einem O-H-Abstand von 0,96 Å erhalten wurde. Der Effekt eines externen Feldes besteht hauptsächlich in einer Verlängerung der O-H-Bindung und einer Verringerung des H-O-H-Winkels, so daß näherungsweise lineare Wasserstoffbindungen gebildet werden.

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Part 65 will appear in Acta Cryst. in the near future.     

Trigger bond analysis of nitroaromatic energetic materials using wiberg bond indices

The identification of trigger bonds, bonds that break to initiate explosive decomposition, using computational methods could help direct the development of novel, “green” and efficient high energy density materials (HEDMs). Comparing bond densities in energetic materials to reference molecules using Wiberg bond indices (WBIs) provides a relative scale for bond activation (%ΔWBIs) to assign trigger bonds in a set of 63 nitroaromatic conventional energetic molecules. Intramolecular hydrogen bonding interactions enhance contributions of resonance structures that strengthen, or deactivate, the C NO₂ trigger bonds and reduce the sensitivity of nitroaniline‐based HEDMs. In contrast, unidirectional hydrogen bonding in nitrophenols strengthens the bond to the hydrogen bond acceptor, but the phenol lone pairs repel and activate an adjacent nitro group. Steric effects, electron withdrawing groups and greater nitro dihedral angles also activate the C NO₂ trigger bonds. %ΔWBIs indicate that nitro groups within an energetic molecule are not all necessarily equally activated to contribute to initiation. %ΔWBIs generally correlate well with impact sensitivity, especially for HEDMs with intramolecular hydrogen bonding, and are a better measure of trigger bond strength than bond dissociation energies (BDEs). However, the method is less effective for HEDMs with significant secondary effects in the solid state. Assignment of trigger bonds using %ΔWBIs could contribute to understanding the effect of intramolecular interactions on energetic properties. © 2018 Wiley Periodicals, Inc.     

Mechanism of C[bond]H bond activation/C[bond]C bond formation reaction between diazo compound and alkane catalyzed by dirhodium tetracarboxylate

The B3LYP density functional studies on the dirhodium tetracarboxylate-catalyzed C-H bond activation/C-C bond formation reaction of a diazo compound with an alkane revealed the energetics and the geometry of important intermediates and transition states in the catalytic cycle. The reaction is initiated by complexation between the rhodium catalyst and the diazo compound. Driven by the back-donation from the Rh 4d(xz) orbital to the C[bond]N sigma*-orbital, nitrogen extrusion takes place to afford a rhodium[bond]carbene complex. The carbene carbon of the complex is strongly electrophilic because of its vacant 2p orbital. The C[bond]H activation/C[bond]C formation proceeds in a single step through a three-centered hydride transfer-like transition state with a small activation energy. Only one of the two rhodium atoms works as a carbene binding site throughout the reaction, and the other rhodium atom assists the C[bond]H insertion reaction. The second Rh atom acts as a mobile ligand for the first one to enhance the electrophilicity of the carbene moiety and to facilitate the cleavage of the rhodium[bond]carbon bond. The calculations reproduce experimental data including the activation enthalpy of the nitrogen extrusion, the kinetic isotope effect of the C[bond]H insertion, and the reactivity order of the C[bond]H bond.     

Self‐healing polyurethane based on disulfide bond and hydrogen bond

Tough and transparent polyurethane networks with self‐healing capability at mild temperature conditions were successfully prepared in a 1‐pot procedure. The self‐healing ability of synthesized polyurethane comes from the covalent disulfide metathesis and non‐covalent H‐bonding. The mechanical testing indicates that disulfide metathesis reforms the covalent bonds on a longer time scale, while H‐bonding gives rise to a healing efficiency of around 46% in the early healing processing. The compromise between mechanical performance and healing capability is reached by tailoring the concentration of disulfide. The tensile strength of the sample with 100% self‐heal efficiency can get to 5.01 MPa, which can be explained by higher mobility of polymer chain under ambient temperature from creep testing.     

Sulfoxide carbon-sulfur bond activation

The alkynylsulfoxide, TMSCCSO(p-tolyl) (TMS = trimethylsilyl, tolyl = C6H4Me), undergoes reaction with (eta5-C5H5)Co(PPh3)2 at room temperature to give the cobaltosulfoxide complex, (C5H5)Co(PPh3)(eta1-CCTMS)[eta1-(S)-SO(p-tolyl)], which was characterized by X-ray crystallography. Exposure of this cobaltosulfoxide complex to oxygen gas leads to the formation of the corresponding metallosulfone complex, (C5H5)Co(PPh3)(eta1-CCTMS)[eta1-(S)-SO2(p-tolyl)], which was characterized by X-ray crystallography. Alternatively, in solution at room temperature, the metallosulfoxide is converted to a 1:4 mixture of the equatorial-equatorial and equatorial-axial bridging cobalt-thiolato dimers, {(C5H5)Co[mu-S(p-tolyl)]}2, respectively. The equatorial-equatorial isomer was characterized by X-ray crystallography.     

Polarity of P-N bond

Conclusions The polarity of the P-N bond in the amides of phosphorus acids depends on the valence state of the phosphorus atom and the nature of the substituents attached to both atoms.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 178–181, January, 1977.     

Cobalt-catalyzed aryl-sulfur bond formation

A new cobalt-catalyzed coupling of aryl halides with thiophenols and alkanethiols is reported. A variety of aryl sulfides can be prepared in excellent yields under mild reaction conditions using 1-2 mol % of CoI2(dppe) and Zn. This new cobalt-catalyzed coupling represents an interesting addition to previously known methods to synthesize thioethers. [reaction: see text].     

Gold-mediated C-H bond functionalisation

The transition metal-catalysed direct functionalisation of C-H bonds is an increasingly viable alternative to the multi-step strategies traditionally adopted. The use of powerful and environmentally benign gold(I) and gold(III) catalysts in such transformations has highlighted their remarkable reactivity and led to a significant increase in their utilisation. This tutorial review provides an overview of gold-catalysed C-H functionalisation, looking at transformations which rely on the ability of gold to perform C-H activation, as well as those exploiting its potent π-acidity.     

Intermolecular copper-catalyzed carbon [bond] hydrogen bond activation via carbene insertion

A series of catalysts of general formula TpXCu (TpX = homoscorpionate ligand) promote the insertion of :CHCO2Et (ethyl diazoacetate as the carbene source) into the C-H bonds of cycloalkanes and cyclic ethers in moderate to high yield. A correlation between the steric hindrance of these catalysts and the yield of the transformation has been observed.