Electrostatic Electronic Theory

Mechanical Theory Electrostatic (Electronic) Theory Diffusion Theory Wetting Theory Chemical Bonding 1.6.5.1 Acid-Base Theory Weak Boundary Layer Theory... 

A notable exception are chemisorbed complexes in zeolites, which have been characterized both structurally and spectroscopically, and for which the interpretation of electronic spectra has met with a considerable success. The reason for the former is the well-defined, although complex, structure of the zeolite framework in which the cations are distributed among a few types of available sites the fortunate circumstance of the latter is that the interaction between the cations, which act as selective chemisorption centers, and the zeolite framework is primarily only electrostatic. The theory that applies for this case is the ligand field theory of the ion-molecule complexes usually placed in trigonal fields of the zeolite cation sites (29). Quantum mechanical exchange interactions with the zeolite framework are justifiably neglected except for very small effects in resonance energy transfer (J30). ... 

The electronic theory is based on the assumption that the bioadhesive material and the target biological substrate have different electronic structures, and when they come in contact, electronic transfer occurs, causing the formation of a double layer of electrical charges at the bioadhesive interface [44]. The bioadhesive force is believed to be due to the attractive forces of the electrical double layer. There are discordant opinions on this item— are the electrostatic forces the causes or rather the results of the contact between the bioadhesive material and the biological substrate [44,45] ... 

The VSEPR theory predicts the three-dimensional shapes of molecules. It is based on simple electrostatics—electron pairs in a molecule will arrange themselves in such a way as to minimize their mutual repulsion. The steric number determines the geometry of the electron pairs (linear, trigonal pyramidal, tetrahedral, and so forth), whereas the molecular geometry is determined by the arrangement of the nuclei and may be less symmetric than the geometry of the electron pairs. 

Soon after the development of the quantum mechanical model of the atom, physicists such as John H. van Vleck (1928) began to investigate a wave-mechanical concept of the chemical bond. The electronic theories of valency, polarity, quantum numbers, and electron distributions in atoms were described, and the valence bond approximation, which depicts covalent bonding in molecules, was built upon these principles. In 1939, Linus Pauling s Nature of the Chemical Bond offered valence bond theory (VBT) as a plausible explanation for bonding in transition metal complexes. His application of VBT to transition metal complexes was supported by Bjerrum s work on stability that suggested electrostatics alone could not account for all bonding characteristics. 

It should be mentioned that there are, in fact, a number of covalent compounds, of which water is a well-known example, in the molecule of which there is a resultant interatomic electrostatic force. These forces are thought to be due to uneven sharing of electrons and they are responsible for the existence of dipole moments. Such forces are of considerable importance in the modern electronic theories of organic chemistry, but from our present viewpoint they need no further consideration. 

In order to derive the results in this section we have often used arguments that apply to the free-electron gas model, because electrostatic expressions then can readily be derived. In the final section we will return to more rigorous theoretical studies based on free-electron theory. 

The electronic theory is based on the assumption that the materials in contact (mucoadhesive and snbstrate) have different electronic structures. In order to balance the Fermi levels, when the materials come in contact, electron transfer occurs creating a double layer of electrical charge. In this theory, the mncoadhesive bond is believed to be maintained by the attractive forces in this electrical double layer. However, it is still not fully understood whether the electrostatic forces are a cause or the result of the contact between the two materials. ... 

The observed interbond angles of PH2 in its two valence states X (92 ) and A A (123 ) were in turn discussed in terms of the Walsh model [43 to 45], the electrostatic force theory [46], and the valence shell electron pair repulsion (VSEPR) theory [47]. 

On the basis of a simple electrostatic repulsion theory, what coordination polyhedra would you expect to find when the total number of bonded and non-bonded electron pairs equals 4 5 6 7 8 12 ... 

Relativity adds a new dimension to quantum chemistry, which is the choice of the Hamiltonian operator. While the Hamiltonian of a molecule is exactly known in nonrelativistic quantum mechanics (if one focuses on the dominating electrostatic monopole interactions to be considered as being transmitted instantaneously), this is no longer the case for the relativistic formulation. Numerical results obtained by many researchers over the past decades have shown how Hamiltonians which capture most of the (numerical) effect of relativity on physical observables can be derived. Relativistic quantum chemistry therefore comes in various flavors, which are more or less well rooted in fundamental physical theory and whose relation to one another will be described in detail in this book. The new dimension of relativistic Hamiltonians makes the presentation of the relativistic many-electron theory very complicated, and the degree of complexity is far greater than for nonrelativistic quantum chemistry. However, the relativistic theory provides the consistent approach toward the description of nature molecular structures containing heavy atoms can only be treated correctly within a relativistic framework. Prominent examples known to everyone are the color of gold and the liquid state of mercury at room temperature. Moreover, it must be understood that relativistic quantum chemistry provides universal theoretical means that are applicable to any element from the periodic table or to any molecule — not only to heavy-element compounds. 

In contrast to magnetic properties, the theory of electric-field-like properties is much easier to cast into a set of working equations. One of them has attracted particular interest, and that is the electric field gradient (EFG). This property is of decisive importance to Mossbauer spectroscopy, i.e., to the spectroscopy of excited nuclear states whose energies are modulated by the molecular structure (the chemical environment ). In order to see how this property arises, we study the electrostatic electron-nucleus interaction of extended, not spherically symmetric charge distributions. For this we apply a multipole expansion in order to generate the properties term by term. 

If the adhesive and substrate have different electronic band structures there is likely to be some electron transfer bn contact to balance Fermi levels which will result in the formation of a double layer of electrical charge at the interface. The electronic theory of adhesion is due primarily to Deryaguin and coworkers [55-57] and they have suggested that the electrostatic forces arising from such contact or junction potentials may contribute significantly to the intrinsic adhesion. The controversy this theory has caused is due to this final statement that such electrostatic forces are an important cause, rather than merely a result, of high joint strength. 

In PPP-SCF calculations, we make the Bom-Oppenheimer, a-rr separation, and single-electron approximations just as we did in Huckel theor y (see section on approximate solutions in Chapter 6) but we take into account mutual electrostatic repulsion of n electrons, which was not done in Huckel theory. We write the modified Schroedinger equation in a form similar to Eq. 6.2.6... 

In the Huckel theory of simple hydrocarbons, one assumes that the election density on a carbon atom and the order of bonds connected to it (which is an election density between atoms) are uninfluenced by election densities and bond orders elsewhere in the molecule. In PPP-SCF theory, exchange and electrostatic repulsion among electrons are specifically built into the method by including exchange and electrostatic terms in the elements of the F matrix. A simple example is the 1,3 element of the matrix for the allyl anion, which is zero in the Huckel method but is 1.44 eV due to election repulsion between the 1 and 3 carbon atoms in one implementation of the PPP-SCF method. 

M.o. theory has had limited success in dealing with electrophilic substitution in the azoles. The performances of 7r-electron densities as indices of reactivity depends very markedly on the assumptions made in calculating them. - Localisation energies have been calculated for pyrazole and pyrazolium, and also an attempt has been made to take into account the electrostatic energy involved in bringing the electrophile up to the point of attack the model predicts correctly the orientation of nitration in pyrazolium. ... 

---