Showing papers on "Chemical bond published in 1970"
TL;DR: The role of quantum-mechanical sum rules and spectral moments in constructing simplified models of bond and band behavior is explored in this article, where a wide range of physical properties including crystal structure, energy bands, elastic constants, ionization energies, and impurity states are discussed.
Abstract: The nature of the chemical bond in crystals is discussed. The general theories of L. Pauling based on thermochemical data and of C. A. Coulson based on valence bond concepts are compared with a recent spectroscopic theory. Particular emphasis is placed on binary crystals of formula ${\mathrm{A}}^{N}{\mathrm{B}}^{8\ensuremath{-}N}$ which includes most tetrahedrally coordinated semiconductors as well as crystals of the rocksalt (NaCl) family. A wide range of physical properties is discussed, including crystal structure, energy bands, elastic constants, ionization energies, and impurity states. The role of quantum-mechanical sum rules and spectral moments in constructing simplified models of bond and band behavior is explored. Stress is laid throughout on methods for incorporating quantum-mechanical effects into properties of chemical bonds through algebraic relations rather than through variational solutions of the wave equation.
1,394 citations
792 citations
TL;DR: In this paper, the authors examined the partitioning of the total energy from CNDO computations into mono and bicentric terms and into physical components, and showed that the two-center terms are a measure of the strength of the chemical bond.
Abstract: The partitioning of the total energy from CNDO computations into mono and bicentric terms and into physical components is examined. It is noted that the two-center terms are a measure of the strength of the chemical bond. This relationship is illustrated in the case of a nonclassical bond in a carbonium ion. The consideration of the interaction between vicinal H atoms leads to the discovery of a trans effect which is important in the interpretation of torsional barriers in hydrocarbons. The investigation of the structure of C-C bonds and the analysis of the derivatives of the total energy with respect to a bond length gives hints which are important for the parametrization of a semiempirical theory.
232 citations
128 citations
TL;DR: The total energy of hydrogen bonding is divided into electrostatic and delocalization contributions as discussed by the authors, and the total energy can be divided into two types of contributions, i.e.
Abstract: The total energy of hydrogen bonding is divided into electrostatic and delocalization contributions.
79 citations
63 citations
TL;DR: In this paper, the K β emission spectra of calcium, titanium, chromium, manganese and iron and oxidic compounds of these elements were investigated by means of the fluorescent method.
61 citations
TL;DR: In this article, the molecular structure of trifluoramine oxide has been investigated by gaseous electron diffraction at a nozzle temperature of 20°C, and the results verify the expected C3υ symmetry for the molecule.
Abstract: The molecular structure of trifluoramine oxide has been investigated by gaseous electron diffraction at a nozzle temperature of 20°C. The results verify the expected C3υ symmetry for the molecule. The N–F bond is slightly longer than the sum of the single‐bond radii corrected for electronegativity difference, but the N–O bond is very much shorter and must be regarded as essentially a double bond. The bond angles are not in themselves unusual. However, the positions of the four ligands correspond with remarkable accuracy to the corners of a regular tetrahedron, a fact which strongly emphasizes the importance of nonbonded interactions. The five covalent bonds formed by the nitrogen atom may be understood in terms of a σ system of four bonds comprising 2s–2p hybrids and a π bond obtained by utilizing an empty 3pπ or 3dπ nitrogen orbital with a filled 3pπ oxygen orbital. The electron‐diffraction analysis led to the following values for the principal parameters: r(N=O) = 1.158 A (0.0040), r(N–F) = 1.431 A (0.0030), r(O···F) = 2.214 A (0.0130), r(F···F) = 2.206 A (0.0158), r(X···X) (the average of the nonbond distances) = 2.210 A (0.0024), l(N=O) = 0.0289 A (0.0044), l(N–F) = 0.0507 A (0.0032), l(O···F) = 0.0584 A (0.0089), l(F···F) = 0.0551 A (0.0077), ∠ONF = 117.1° (0.89), and ∠FNF = 100.8° (1.12). The distances and amplitudes are ra and la values; the parenthesized quantities are 2σ. When the rotational constant B0 from microwave spectroscopy is taken as a constraint on the diffraction results, the following set of distance and angle values, which differ only very slightly from those above, is obtained: r(N=O) = 1.159 A (+ 0.0020, − 0.0025), r(N–F) = 1.432 A (+ 0.0020, − 0.0061), ∠ONF = 117.4° (+ 0.60, − 1.16), and ∠FNF = 100.5 A (+ 1.44, − 0.75).
55 citations
TL;DR: In this article, the authors defined a bond separation reaction as the process in which a polyatomic molecule is separated into the simplest molecules containing the same component bonds, and calculated the energies associated with such reactions for a series of polyatomic molecules using ab initio molecular orbital theory with a basis of contracted Gaussian Functions.
43 citations
TL;DR: A series of organotitanium compounds of structure (π-C 5 H 5 ) 2 Ti(R)Cl was prepared, where the R ligand was attached by a σ-Ti C bond as discussed by the authors.
TL;DR: Extended Huckel and CNDO/2 molecular orbital methods have been employed to examine a variety of intermolecular and intramolecular hydrogen bonds including the symmetric bonds in the bifluoride and maleate ions as discussed by the authors.
Abstract: Extended Huckel and CNDO/2 molecular orbital methods have been employed to examine a variety of intermolecular and intramolecular hydrogen bonds including the symmetric bonds in the bifluoride and maleate ions. These molecular orbital methods provide valuable information on the dissociation energies, charge densities, bond orders, and dipole moments. The CNDO/2 method gives realistic double minimum potentials for the proton in the asymmetric hydrogen bonds and a single minimum potential in the bifluoride and maleate ions.
TL;DR: In this paper, the effect of chemical bonding between phases of a glass matrix-metal composite on strength and fracture behavior was investigated, and it was shown that a chemical bond counteracts the micromechanical stress concentration and therefore increases the strength.
Abstract: The effect of chemical bonding between phases of a glass matrix-metal composite on strength and fracture behavior was investigated. When no chemical bonding occurs, strengthening can be achieved through the mechanical formation of an interface between the dispersant and matrix. Even greater strengthening can be obtained by the formation of a chemical bond. Strengthening occurs by the limitation of the Griffith flaw size and is controlled by micromechanical stress concentrations developed on loading. Internal stresses developed on cooling from the fabrication temperature control the path of fracture. A chemical bond counteracts the micromechanical stress concentration and therefore increases the strength.
TL;DR: In this article, the p-π character in nitrogen-carbon bonding is dependent on a knowledge of the trigonal nitrogen -trigonal carbon single bond length, which is known to be 1.4.
Abstract: Estimation of p-π character in nitrogen–carbon bonding is dependent on a knowledge of the trigonal nitrogen – trigonal carbon single bond length. Recent experimental results indicate a value of 1.4...
TL;DR: In this article, the trans bond-lengthening effects of ligands in linear three-center systems of the sixth main group of the Periodic Table have been investigated and the transition state in nucleophilic substitutions at divalent sulphur, selenium and tellurium is discussed.
Abstract: Linear three-centre systems of sixth-group atoms occur in the triselenocyanate ion and in a series of tellurium(ii) complexes. In the triselenocyanate ion, the Se—Se bonds are about 032 A longer than covalent single bonds. In centrosymmetric square-planar tellurium(n) complexes, the tellurium—ligand bonds are about 027 A longer than covalent single bonds. In tellurium(ii) complexes where the linear three-centre systems are not symmetrical, pronounced relative trans bond-lengthening effects of ligands are observed. The phenyl group has a particularly large trans bond-lengthening effect. The transition state in nucleophilic substitutions at divalent sulphur, selenium and tellurium is discussed. CHEMICAL bonds longer and weaker than covalent single bonds are known in various classes of compounds. Such bonds occur, for example, in the trihalide ions of the seventh Main Group of the Periodic Table. This article is concerned with similar aspects in the sixth Main Group; specifically, with linear three-atom systems centred on divalent selenium and tellurium, and also on divalent sulphur. Halogens add a halide ion to give linear trihalide ions. In the iodine molecule, in the gas phase, the iodine—iodine bond length1 is 267 A. In the triiodide ion, in the symmetrical case, the iodine—iodine bonds are 29O—293 A24, that is, &23—036 A longer than in molecular iodine. In the bromine molecule, the bromine—bromine bond length1 is 228 A; in the tribromide ion, in the symmetrical case, the bromine—bromine bonds are 254—255 A5 6, or 026—O27 A longer than in molecular bromine. Halogens and interhalogens also form linear adducts with electroneutral n donors like amines, suiphides and selenides7 In two iodine adducts where nitrogen is donor atom'° 12, the iodine—iodine bond is 2'83 A; in two adducts where sulphur is donor atom13 15 the iodine—iodine bond is 282 and 279 A; and in three adducts where selenium is donor atom 1618 the iodine—iodine bond is 287, 291 and 296 A. Thus, not only in the triiodide ion, the adduct of the halogen with its anion, but also in adducts with electroneutral n donors, the iodine—iodine bond is lengthened relative to molecular iodine.
TL;DR: In the early 1930s valence theory was confused by a conflict between molecular-orbital and valence-bond approximations as discussed by the authors. But during the l940s a reconciliation between the two methods was found; and from that time until now much the greater effort has been put into MO calculations.
Abstract: In the early 1930s valence theory was confused by a conflict between molecular-orbital and valence-bond approximations. But during the l940s a reconciliation between the two methods was found; and from that time until now much the greater effort has been put into MO calculations. These calculations have been tremendously enriched in the 1960s by the widespread use of electronic computers, and package programmes. A study of the charge distribution calculated for covalent bonds shows that the early picture of a bond as being associated with a build-up of charge between the nuclei is still valid. However, if bonding electrons are drawn into this region, other electrons tend to get forced out of it; recent studies show the importance of these two effects. The early representation of these charge-clouds in terms of hybridization was particularly fruitful in the l940s, but is now recognized as too restrictive. The development of the experimental techniques of photoelectron spectroscopy and x-ray spectroscopy now enables a direct verification to be made of the early ideas of individual molecular orbitals. In this way Mulliken's original theoretical descriptions have been triumphantly justified. Another aspect of valence theory that has developed in recent years is that of the relation between a and it electrons. No longer may the it electrons be treated as if they were independent of the a electrons, but a close relationship exists between them. The account concludes with a description of those situations where the number of valence electrons is either too few (electron-deficient molecules) or too large (electron-rich molecules) to provide the normal complement of two electrons per bond. In each case the simple concept of a bond needs to be modified. Attention is drawn to the remarkable way in which theoretical concepts have recently received experimental verification. INTRODUCTION Valence theory is much older than the fifty years referred to in the title of this Symposium. We should at the very least go back to t86 when Odling, Crum Brown and others started using the horizontal line—-which has been the symbol for a chemical bond ever since; and we should think of Kekule only a few years later trying to use the symbol to indicate some sort of geometrical relationship between the various atoms of a polyatomic molecule: or Butlerov, with his early feeling for chemical structure. Moreover, for these last hundred years, as Mulliken and Van Vleck have shown in their
TL;DR: In this paper, an X-ray study with CuKα was carried out on a fine-powder sample of GaAs and two peaks were found in Δ ρ (x, y, z ).
Abstract: An X-ray study with CuKα was carried out on a fine-powder sample of GaAs. Relative intensities of 11 lines from the 111 to the 620 were measured and the scale factor was determined from absolute intensities of the 111 and the 220. Form the three-dimensional difference Fourier synthesis on the (110) plane, two sorts of peaks were found in Δ ρ ( x , y , z ). One lies near (1/8, 1/8, 1/8), which is the midpoint of chemical bond between Ga and As. This peak seems to correspond to a covalent bond. The other lies near (1/2, 1/2, 1/2), which is the position of anions in the NaCl structure with a cation at the origin. It may be possible that this latter peak corresponds to the ionicity of the bond.
TL;DR: In this article, the K β emission spectra of various calcium, titanium, chromium, manganese and iron compounds were investigated by means of the fluorescent method, and the origin of the lines found experimentally is discussed, their energies are given and used to construct composite energy diagrams.
TL;DR: Isomer shifts of the recoilless 90 keV γ-rays of 99Ru were observed in a number of octahedrally coordinated complexes of ruthenium and reveal the influence of different ligands on the electron density at the Rn nuclei.
Abstract: Isomer shifts of the recoilless 90 keV γ-rays of99Ru were observed in a number of octahedrally coordinated complexes of ruthenium and reveal the influence of different ligands on the electron density at the ruthenium nuclei. For a given oxidation state the observed shifts are correlated with the spectrochemical series; the backbonding properties of ligands like CN−1 and NO+ cause a considerable increase of the electron density. This behaviour is largely similar to that found for compounds of iron and some 5d elements.
TL;DR: In this paper, the X-ray emission spectra of a series of lithia-aluminosilica (LAS) glasses were determined using the Xray spectrometric systems of commercially available electron microprobes.
Abstract: Silicon and aluminum Kβ band X-ray emission spectra of a series of lithia-aluminosilica (LAS) glasses were determined using the X-ray spectrometric systems of commercially available electron microprobes. Comparison with spectra of reference materials, including glassy and crystalline forms of silica, high-quartz-phase crystals of the L:A:S-1:1:2 (mol ratio) composition (high-eucryptite), and 6- and 4-coordinated aluminum oxides, led to tentative assignment of spectral peaks to specific electronic transitions. Molecular orbital theory and crystallographic structural data were used to assign bands in crystalline materials. The LAS glass spectra indicated progressive bond weakening with decreasing silica content; the Kβ peak shifts permitted approximate calculation of the decreases in Si–O and Al–O bond energies, which were as great as 30 kcal/mol relative to SiO2 (for the L:A:S-1:1:2 glass).
TL;DR: In this article, the potential energy for the migration of positive and negative defects is calculated with six valence bond structures, and the resulting height of the potential barrier for the positive defect is lower than that for the negative one.
Abstract: Potential energy curves of the proton for the formation and migration of the ionic defects in water and ice are calculated by a valence bond method. In order to explain the thermodynamical data on the dissociation of water and ice, the reaction field concept of Onsager is introduced. The potential energy for the migration of positive and negative defects is calculated with six valence bond structures. The resulting height of the potential barrier for the positive defect is lower than that for the negative one, and this agrees with the experimental fact that the mobility of the positive defect is much greater than the negative one. The experimental data on the mobility of the positive defect in ice can be explained by Gosar's theory if the O–O distance is taken to be 2.63 A
TL;DR: In this article, the effect of size on the strength of hard and soft acid base interactions was examined and it was concluded that hard-hard interactions are especially stabilized by small size that favors enhanced Madelung and covalent bonding energies.
TL;DR: In this paper, it was shown that in compounds containing an allyl-silicon group, besides the hyperconjugation effect of the CH 2 group, the bond between the d orbital of silicon and the carbon atom in β position relative to it has a π bond order of about 8%, and the long bond of about 5%.
TL;DR: In this article, the authors determined the rate of change of bond moment with bond distance, dμ/d r, for some free and H-bonded C�O bonds from infrared intensity measurements.