Homonuclear Bonds Research Articles

Atomic‐orbital‐symmetry based σ‐, π‐, and δ‐decomposition analysis of bond orders

AbstractThe atomic‐orbital‐symmetry based (AOSB) scheme for the decomposition of Mayer and Wiberg bond orders into σ‐, π‐, and δ‐components is used to investigate different types of covalent bonds. Four series of compounds are studied: simple molecules with homonuclear bonds, inorganic molecules with polar heteronuclear bonds, [Ru(CN)5(XY)]q transition metal complexes with π‐acceptor ligands, and dimetal complexes with multiple metal–metal bonds. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Unique homonuclear multiple bonding in main group compounds

Significant progress in the chemistry of main group compounds (group 13, 14, and 15) containing homonuclear multiple bonds has been made over the past three decades. This feature article addresses the unique structural and bonding motifs of these compounds, with a particular emphasis on both iconic molecules and recent novel discoveries.

Effects of quench rates on the short- and medium-range orders of amorphous silicon carbide: A molecular-dynamics study

Amorphous silicon carbide (a-SiC) networks generated from melted SiC at various quench rates (from 1014 to 5×1011 K/s) are studied with Tersoff potential based molecular-dynamics simulations. With the decreasing quench rates, dramatic changes are observed in chemical order, as well as in its topological orders over both short and medium ranges. The corresponding modification of topological short-range order is manifested not only by improvement of the characteristic tetrahedral configuration, but also by variation in the spatial distributions of the homonuclear bonds. On the other hand, the corresponding development over medium range gives rise to a more compact and more homogeneous structure. The essential mechanisms determining the atomic arrangements on both length scales are further explored. It is reasonable to argue that chemical order, as a function of the quench rate, should be mainly responsible for the topological features of a-SiC.

First-principles density-functional calculations on the field emission properties of BN nanocones

The first-principles density-functional theory is used to study the geometrical structures and field emission properties of different boron nitride nanocones with 240 ∘ disclination. It is found that the nanocones can be stable under applied electric field and the emission current is sensitively dependent on the tips of nanocones. The nanocones with homonuclear bonds at the tip can introduce additional energy states near Fermi level, which can reduce the ionization potential and increase the emission current of these boron nitride nanocones. This investigation indicates that the boron nitride nanocone can be a promising candidate as a field emission electron source.

Atomistic modeling of amorphous silicon carbide using a bond-order potential

Molecular dynamics simulations were performed with a Brenner-type bond-order potential to study the melting of silicon carbide (SiC), the structure of amorphous SiC produced by quenching from the melt, and the evolution of the amorphous state after isochronal annealing at elevated temperatures. The simulations reveal that SiC melts above 3700 K with an enthalpy of fusion of about 0.6 eV/atom. The density of the quenched liquid is about 2820 kg/m 3, in excellent agreement with the experimental value for SiC amorphized by neutron irradiation. In addition to the loss of long-range order, the quenched liquid shows short-range disorder as measured by the C homonuclear bond ratio. Upon annealing, there is partial recovery of short-range order.

The physical origin of large covalent–ionic resonance energies in some two-electron bonds

This study uses valence bond (VB) theory to analyze in detail the previously established finding that alongside the two classical bond families of covalent and ionic bonds, which describe the electron-pair bond, there exists a distinct class of charge-shift bonds (CS-bonds) in which the fluctuation of the electron pair density plays a dominant role. Such bonds are characterized by weak binding, or even a repulsive, covalent component, and by a large covalent-ionic resonance energy RE(cs) that is responsible for the major part, or even for the totality, of the bonding energy. In the present work, the nature of CS-bonding and its fundamental mechanisms are analyzed in detail by means of a VB study of some typical homonuclear bonds (H-H, H3C-CH3, H2N-NH2, HO-OH, F-F, and Cl-Cl), ranging from classical-covalent to fully charge-shift bonds. It is shown that CS-bonding is characterized by a covalent dissociation curve with a shallow minimum situated at long interatomic distances, or even a fully repulsive covalent curve. As the atoms that are involved in the bond are taken from left to right or from bottom to top of the periodic table, the weakening effect of the adjacent bonds or lone pairs increases, while at the same time the reduced resonance integral, that couples the covalent and ionic forms, increases. As a consequence, the weakening of the covalent interaction is gradually compensated by a strengthening of CS-bonding. The large RE(cs) quantity of CS-bonds is shown to be an outcome of the mechanism necessary to establish equilibrium and optimum bonding during bond formation. It is shown that the shrinkage of the orbitals in the covalent structure lowers the potential energy, V, but excessively raises the kinetic energy, T, thereby tipping the virial ratio off-balance. Subsequent addition of the ionic structures lowers T while having a lesser effect on V, thus restoring the requisite virial ratio (T/-V = 1/2). Generalizing to typically classical covalent bonds, like H-H or C-C bonds, the mechanism by which the virial ratio is obeyed during bond formation is primarily orbital shrinkage, and therefore the charge-shift resonance energy has only a small corrective effect. On the other hand, for bonds bearing adjacent lone pairs and/or involving electronegative atoms, like F-F or Cl-Cl, the formation of the bond corresponds to a large increase of kinetic energy, which must be compensated for by a large participation or covalent-ionic mixing.

A new approach to the relationship between bond energy and electronegativity

The familiar equation whereby Pauling related heteronuclear bond energies D A–B to the electronegativity difference Δ χ (=∣ χ A − χ B∣) and the homonuclear bond energies D A–A and D B–B has been the subject of critical scrutiny for at least half a century. A modification to this equation that combines the concepts of electronegativity and hardness/softness can be rewritten in terms of two quantities x and y, both having absolute significance. Both homo- and heteronuclear bond energies can be rationalised from these new equations. The quantities x are linearly related to Pauling electronegativities, while y appears to be a measure of an atom’s intrinsic bonding potential, related to size and availability of valence orbitals.

Interpretation of Hydrogen Bonding in the Weak and Strong Regions Using Conceptual DFT Descriptors

Hydrogen bonding is among the most fundamental interactions in biology and chemistry, providing an extra stabilization of 1-40 kcal/mol to the molecular systems involved. This wide range of stabilization energy underlines the need for a general and comprehensive theory that will explain the formation of hydrogen bonds. While a simple electrostatic model is adequate to describe the bonding patterns in the weak and moderate hydrogen bond regimes, strong hydrogen bonds, on the other hand, require a more complete theory due to the appearance of covalent interactions. In this study, conceptual DFT tools such as local hardness, eta(r) and local softness, s(r), have been used in order to get an alternative view on solving this hydrogen-bonding puzzle as described by Gilli et al. [J. Mol. Struct. 2000, 552, 1]. A series of both homonuclear and heteronuclear resonance-assisted hydrogen bonds of the types O-H...N, N-H...O, N-H...N, and O-H...O with strength varying from weak to very strong have been studied. First of all, DeltaPA and DeltapK(a) values were calculated and correlated to the hydrogen bond energy. Then the electrostatic effects were examined as hard-hard interactions accessible through molecular electrostatic potential, natural population analysis (NPA) charge, and local hardness calculations. Finally, secondary soft-soft interaction effects were entered into the picture described by the local softness values, providing insight into the covalent character of the strong hydrogen bonds.

2-Anisyl-5-sulfonyldiphenylphosphine Oxide, Solid-State Structure Bearing Covalent Homonuclear Negative and Positive Charge-Assisted Hydrogen Bonds

The X-ray diffraction structure of 2-anisyl-5-sulfonyldiphenylphosphine oxide is reported. The molecule presents two different types of strong covalent homonuclear charge-assisted hydrogen bonds, negative and positive, giving a monodimensional polymeric arrangement. Charges and electron density were calculated for the molecule conformation found by the X-ray diffraction studies.

Comparison of the AIM Delocalization Index and the Mayer and Fuzzy Atom Bond Orders

In this paper the behavior of three well-known electron-sharing indexes, namely, the AIM delocalization index and the Mayer and fuzzy atom bond orders are studied at the Hartree-Fock level. A large number of five-membered ring molecules, containing several types of bonding, constitute the training set chosen for such purpose. A detailed analysis of the results obtained shows that the three indexes studied exhibit strong correlations, especially for homonuclear bonds. The correlation is somewhat poorer but still significant for polar bonds. In this case, the bond orders obtained with the Mayer and fuzzy atom approaches are normally closer to the formally predicted bond orders than those given by the AIM delocalization indexes, which are usually smaller than those expected from chemical intuition. In some particular cases, the use of diffuse functions in the calculation of Mayer bond orders leads to unrealistic results. In particular, noticeable trends are found for C-C bonds, encouraging the substitution of the delocalization index by the cheaper fuzzy atom or even the Mayer bond orders in the calculation of aromaticity indexes based on the delocalization index such as the para-delocalization index and the aromatic fluctuation index.

Chemical short-range order in ion-beam-induced amorphous SiC: Irradiation temperature dependence

Chemical short-range order of ion-beam-induced amorphous SiC (a-SiC) has been examined by transmission electron microscopy in combination with imaging plate techniques. Single crystals of 6H-SiC were irradiated with 300keV xenon ions to a fluence of 1015cm−2 at cryogenic and elevated temperatures. Atomic pair-distribution functions showed that not only heteronuclear (Si–C) bonds but also homonuclear (Si–Si and C–C) bonds exist within the first coordination shell of a-SiC networks. It was found that chemical short-range order of a-SiC depends on the ion irradiation conditions: the amorphous networks became more chemically disorder with decreasing the irradiation temperature.

SULFUR'S TURN FOR MULTIPLE BONDS

THE SYNTHESIS OF COMPOUNDS with multiple bonds between main-group elements in the third row or lower of the periodic table has brought much excitement to inorganic and theoretical chemists. But Canadian chemist Jack Passmore, whose group has prepared some of the compounds, has felt a bit like a spectator. That's because the spotlight mainly has been on alkene and alkyne analogs of group 14 elements silicon, germanium, tin, and lead, while Passmore and his colleagues have focused on group 16 and 17 elements, particularly sulfur. After years of work, Passmore, a professor at the University of New Brunswick, Frederic ton, now believes he has definitive experimental evidence that the S 2 I 4 2+ cation in S 2 I 4 (MF 6 ) 2 , where M = As or Sb, contains a sulfur-sulfur bond with the highest bond order for a homonuclear multiple bond involving a heavy main-group element ( Inorg Chem . 2005 , 44 , 1660). The research was carried out by graduate student Scott Brownridge, postdoc Carsten ...

Organometallic Compounds with Homonuclear Bonds between Bismuth Atoms, 70 Years after Paneth' Report on the Violet Dimethyl Bismuth Compound

AbstractThe chemistry of organometallic compounds of the types (R2Bi)2, (RBi)n (n = 2—5) and R6Bi8 is the objective of this report. Common features and differences between these bismuthanes and analogous compounds of lighter group 15 elements are discussed.

The influence of the tip structure and the electric field on BN nanocones

Different configurations for BN nanocones with240° disclination are analysed through first-principles calculations based on the density-functionaltheory. The studied cone tips involve either four pentagons with two homonuclear bonds ortwo squares and two hexagons with no homonuclear bonds. The structures are bothterminated by two three-coordinated atoms. Cohesive energy results show that the coneterminated in pentagons is slightly energetically preferred. The influence of a1.7 V Å−1 external electric field, applied along the axis, on the structural changes and chargedistributions is analysed. The responses of the charge distributions to the external electricfield demonstrate the main role played by the B atoms for the applications of BN cones asprobes in electronic microscopy as well as field emitters.

Amorphous alloys of C 0.5Si 0.5, Si 0.5Ge 0.5 and In 0.5Se 0.5: atomic topology

Using a recently developed ab initio molecular dynamics method based on the Harris functional to amorphize structures of semiconducting elements starting from 64 atom, periodic, diamond-like supercells, we have generated the amorphous structures corresponding to the C 0.5Si 0.5, Si 0.5Ge 0.5 and In 0.5Se 0.5 alloys. We report the atomic topology of the three samples and show that for the C 0.5Si 0.5 a few homonuclear bonds are observed; for Si 0.5Ge 0.5 a large number of homonuclear bonds appear, whereas for In 0.5Se 0.5 each Se is bonded to three In and each In is tetrahedrally bonded to at least three Se.

From a Cyclotrisilane to a Cyclotriplumbane: Low Coordination and Multiple Bonding in Group 14 Chemistry

AbstractFor Abstract see ChemInform Abstract in Full Text.

Theoretical study of segregation of Zn and Pd in Pd–Zn alloys

Surface segregation of Zn and Pd in PdZn alloys was investigated based on density functional calculations using periodic slab models. The segregation propensity of Pd–Zn films was found to be support-dependent: Zn showed a stronger propensity to segregate on Zn support than on Pd support; likewise, for Pd the propensity was stronger on Pd support than on Zn support. In self-supported 1:1 alloy films, segregation of neither Zn nor Pd was favorable. Segregation was suppressed due to the fact that Pd–Zn heteronuclear bonds are stronger than the corresponding homonuclear bonds Pd–Pd and Zn–Zn. However, in films rich in Zn or Pd, segregation of the dominant component was possible. For instance, when a monolayer of Pd is deposited on a Zn(0 0 0 1) film, segregation of Zn is calculated energetically favorable, in agreement with experimental observations. With increasing Zn concentration in the top surface layer, the corrugation of alloy films was reduced.

From a Cyclotrisilane to a Cyclotriplumbane: Low Coordination and Multiple Bonding in Group 14 Chemistry

In less than three decades the chemistry of low-coordinated compounds of the heavier elements of group 14 has made unforeseen and unprecedented advances. In the meantime not only have homonuclear double bonds been realized between all elements of this group but also the first examples of compounds analogous to the alkynes have been isolated. Similarly, syntheses of molecules containing heteronuclear bonds have been achieved. Analogues of the allenes as well as the first example of a spiropentasiladiene are of particular interest. The present short review covers mainly work done in the author's laboratories together with other relevant reports from the literature. It begins with the synthesis of hexa-tert-butylcyclotrisilane, which is cleaved photochemically into a short-lived silylene and a marginally stable disilene; in subsequent reactions both of these species can undergo cycloadditions to furnish a broad palette of mostly novel ring systems. Furthermore, the first molecules with conjugated SiSi, GeC, ...

Ion-beam-induced amorphous structures in silicon carbide

Atomistic structure of ion-beam-induced amorphous silicon carbide (a-SiC) has been investigated by cross-sectional transmission electron microscopy. The electron intensities of halo patterns recorded on imaging plates were digitized quantitatively to extract reduced interference functions. We demonstrated the relationship between maximum scattering vector ( Q max) measured in scattering experiments and the resolution of the corresponding pair-distribution function by changing Q max values from 160 to 230 nm −1. The results revealed that the C–C peak becomes broadened and eventually a shoulder as the Q max value becomes shorter, indicating that Q max values of <160 nm −1 measured in previous studies are not enough to detect C–C homonuclear bonds in a-SiC. We are the first to reveal the existence of C–C and Si–Si homonuclear bonds in a-SiC using a diffraction technique.

Electron diffraction structure analysis for amorphous materials

Amorphous structures of liquid-quenched Fe90Zr7B3 and ion-bombarded SiC were studied using modern electron diffraction structure analysis techniques. From a reverse Monte-Carlo (RMC) structural simulation on the experimental atomic pair distribution function (PDF) of amorphous Fe90Zr7B3, “nano-scale phase separation” was found to occur in this structure with spatial distributions of body-centered cubic-like, icosahedral and trigonal prism clusters. In amorphous SiC, the network includes not only heteronuclear (Si–C) bonds but also homonuclear (Si–Si and C–C) bonds. The ratio of homonuclear to heteronuclear bonds changes due to structural relaxation.