Definition electronic structure

The Ionic Bond and Other Electrostatic Bonds.—In case that there can be assigned to each of two atoms or groups of atoms a definite electronic structure essentially independent of the presence of the other atom or group and such that electrostatic interactions are set up that lead to strong at ruction and.the formation of a chemical bond, we say that the bond is an electrostatic bond. 

Interpretation of magnetic effects in metals and alloys is difficult because a definite electronic structure cannot be easily assigned to individual atoms, and there may be some unpaired 4i-spin density in the conduction band. The... 

While compounds with a formal MM quadruple bond have been isolated and studied since the mid-1960s, the first report of a compound with a formal MM quintuple bond was as recent as 2005. Bond order is a concept that is difficult to distinguish experimentally. For example, there are dichromium compounds with formal quadruple bonds that have shorter Cr—Cr bond lengths than those with quintuple bonds. Mixing of the metal and ligand molecular orbitals complicates trying to assign a definitive electronic structure in theoretical studies and also reduces the calculated bond order between the metals. Therefore, we use the term quintuple bond in this section to simply illustrate that five electron pairs are involved in the metal-metal interaction [82]. 

In this chapter, we will consider the other half of a model chemistry definition the theoretical method used to model the molecular system. This chapter will serve as an introductory survey of the major classes of electronic structure calculations. The examples and exercises will compare the strengths and weaknesses of various specific methods in more detail. The final section of the chapter considers the CPU, memory and disk resource requirements of the various methods. 

The modem theory of valency is not simple—it is not possible to assign in an unambiguous way definite valencies to the various atoms in a molecule or crystal. It is instead necessary to dissociate the concept of valency into several new concepts—ionic valency, covalency, metallic valency, oxidation number—that are capable of more precise treatment and even these more precise concepts in general involve an approximation, the complete description of the bonds between the atoms in a molecule or crystal being given only by a detailed discussion of its electronic structure. Nevertheless, these concepts, of ionic valency, covalency, etc., have been found to be so useful as to justify our considering them as constituting the modern theory of valency. 

Model A cannot be eliminated definitely by the photographs there are, however, some points which make this model improbable. From the curve for this model the first minimum would be expected to be at least as well pronounced as the second minimum, whereas on the photographs the first minimum is not very well defined. That the qualitative appearance of the photographs supports model C rather than model A is further shown by the fact that the photographs resemble those of methyl nitrate more closely than those of carbon tetrafluoride. Some evidence is also provided by the radial distribution curve (Fig. 1), the first peak being displaced by 0.15 A. from the position expected for it for model A. For these reasons and the additional reason that it is difficult to correlate the tetrahedral configuration with an electronic structure involving only completed octets, we consider model A not to be satisfactory.7... 

We shall show both from experimental evidence about gas-phase complexes and, to a lesser extent, from the results of electronic structure calculations that a parallel definition of the intermolecular halogen bond is appropriate The halogen bond is an attractive interaction between a halogen atom X and an atom or a group of atoms in different molecule(s), when there is evidence of bond formation. ... 

The first plausible theory of the electronic structure of the atom was proposed in 1914 by Niels Bohr (1885-1962), a Danish physicist. In order to explain the hydrogen spectrum (Fig. 17-1), he suggested that in each hydrogen atom, the electron revolves about the nucleus in one of several possible circular orbits, each having a definite radius corresponding to a definite energy for the electron. An electron in the orbit closest to the nucleus should have the lowest energy. With the... 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

As an example, Figure 2.5 shows both a simple (plane) and a more advanced (tetrahedral) picture of the electronic structure around the oxygen atom. In addition to the existing six electrons in the outer orbits, oxygen requires another two electrons to be stable, i.e., according to the definition of OX and the text OX0 = -2. 

In words, s describes the interaction of the solute charge distribution component p, with the arbitrary solvent orientational polarization mediated by the cavity surface. The arbitrary weights p,, previously defined by (2.11), enter accordingly the definition of the solvent coordinates, and reduce, in the equilibrium solvation regime, to the weights tv,, such that the solvent coordinates are no longer arbitrary, but instead depend on the solute nuclear geometry and assume the form se<> = lor. weq. In equilibrium, the solvent coordinates are correlated to the actual electronic structure of the solute, while out of equilibrium they are not. 

In this chapter, we use the definitions of bond order and valence indices provided by Mayer [4-6] (for a historical account, see Ref. [6a] and for other types of bond indices, see Ref. [6b]). In terms of electronic structure theory, they represent an extension to Mulliken s population analysis. The bond order is defined as... 

Since rigorous theoretical treatments of molecular structure have become more and more common in recent years, there exists a definite need for simple connections between such treatments and traditional chemical concepts. One approach to this problem which has proved useful is the method of localized orbitals. It yields a clear picture of a molecule in terms of bonds and lone pairs and is particularly well suited for comparing the electronic structures of different molecules. So far, it has been applied mainly within the closed-shell Hartree-Fock approximation, but it is our feeling that, in the future, localized representations will find more and more widespread use, including applications to wavefunctions other than the closed-shell Hartree-Fock functions. 

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