Chemical bonds valence bond model

SIMULATION OF CHEMICAL REACTIONS IN SOLUTION USING AN AB INITIO MOLECULAR ORBITAL-VALENCE BOND MODEL... 

While more physically based models provide a picture of the underlying forces that lead to chemical bonding, the bond valence model reduces the rules of chemistry to their simplest mathematical form. In this form it is able to provide insights into the behaviour of the many complex systems found in acid-base chemistry. 

For example, students develop an elementary understanding of bonding from the Lewis model. Then they refine it through the valence bond model and finally molecular orbital theory. Some exercises challenge students to refine models further—and to develop new ones. Students will see how current chemical knowledge is based on the authority—and the fallibility—of modern experimental techniques. 

The valence bond model of covalent bonding is easy to visualize and leads to a satisfactory description for most molecules. It does, however, have some problems. Perhaps the most serious flaw in the valence bond model is that it sometimes leads to an incorrect electronic description. For this reason, another bonding description called molecular orbital (MO) theory is often used. The molecular orbital model is more complex than the valence bond model, particularly for larger molecules, but sometimes gives a more satisfactory accounting of chemical and physical properties. 

It should be emphasized that these entropy/information descriptors and the underlying probabilities depend on the selected basis set, for example, the canonical AO of the isolated atoms or the hybrid orbitals (HOs) of their promoted (valence) states, the localized MO (LMO), etc. In what follows we shall examine these IT descriptors of chemical bonds in illustrative model systems. The emphasis will be placed on the orbital decoupling in the molecular communication channels and the need for appropriate changes in their input probabilities, which weigh the contributions to the average information descriptors from each input. 

The American chemist G. N. Lewis introduced a useful model that describes the electronic structure of the atom and provides a starting point for describing chemical bonds. The Lewis model represents the valence electrons as dots arranged around the chemical symbol for the atom the core electrons are not shown. The first four dots are displayed singly around the four sides of the elemental symbol. If the atom has more than four valence electrons, their dots are then paired with those already present. The result is a Lewis dot symbol for that atom. The Lewis notation for the elements of the first two periods is... 

Chang, Y.-T. and Miller, W.H. (1990) An Empirical Valence Bond Model for Constructing Global Potential Energy Surfaces for Chemical Reactions of Polyatomic Molecular Systems, J. Phys. Chem. 94, 5884-5888. 

The valence-bond model for covalent bonds, described below, has its limitations, but it is still extremely useful. For example, you will see in Chapter 14 that it helps us understand the attractions between molecules and predict relative melting points and boiling points of substances. The model is also extremely useful in describing the mechanisms of chemical changes. Therefore, even though it strays a bit from what scientists think is the most accurate description of real molecules, the valence-bond model is the most popular model for explaining covalent bonding. 

Although it is possible to obtain good estimates of the electron correlation energy by either density functional or configuration interaction methods both of these methods (for different reasons) suffer from the same defect it is not possible to obtain a clear physical and chemical interpretation of the results of the calculation. The valence bond model, in principle and in practice, puts the physical interpretation as a top priority but pays a price in the complexity of its implementation. 

However the valence bond model seems inextricably bound up with the idea of electron spin indeed the chemical bond is often qualitatively described as due to the pairing of electron spins. What is certainly true is that the largest contributions to a description of electron-pair bonds are functions which have a singlet spin function for each bond what is not so obvious is that the bonding is due to this spin pairing. Ideas of electron spin have become entangled with the Pauli principle and antisymmetry. 

In using the determinantal method, the explicit use of a spin coordinate and spin-orbitals for each electron has provided the simplest mathematically elementary technique for ensuring that this permutational symmetry is obeyed and retained. No new algebraic or analytical tools were required the definition of spin integration is all that is needed. But the price which had to be paid for this mathematical simplicity was the entanglement of space emd spin variables. It would be physically and chemically attractive to be able to remove any explicit appearance of electron spin from the valence bond model and concentrate on the main physical features of the model which are ... 

Valence bond model (7.6) A description of chemical bonding in which all bonds result from overlap between atomic orbitals of the two atoms forming the bond. 

Fig. 10.2 The electron density in Hj accordingto the valence-bond model of the chemical bond and the electron densities corresponding to the contributing atomic orbitals.

Quantum Chemical Descriptions of Lewis Acid/Base Complexes 1.5.1 Valence-Bond Model... 

However these debates resolve, it is safe to say that chemists justifications for using highly idealized models are not merely pragmatic. Simple molecular orbital and valence bond models are not in widespread use simply because of uncertainties about the fundamental theory, lack of data, or computational difficulties. Rather, the use of such models is tied to the explanatory practices of chemistry and the use of such models is likely to be an enduring part of chemical theorizing. 

We refer to models where we write the total potential energy in terms of chemical endties such as bond lengths, bond angles, dihedral angles and so on as valence force field models. 

Lewis s interest in chemical bonding and structure dated from 1902. In attempting to explain "valence" to a class at Harvard, he devised an atomic model to rationalize the octet rule. His model was deficient in many respects for one thing, Lewis visualized cubic atoms with electrons located at the corners. Perhaps this explains why his ideas of atomic structure were not published until 1916. In that year, Lewis conceived of the... 

Valence band spectra provide information about the electronic and chemical structure of the system, since many of the valence electrons participate directly in chemical bonding. One way to evaluate experimental UPS spectra is by using a fingerprint method, i.e., a comparison with known standards. Another important approach is to utilize comparison with the results of appropriate model quantum-chemical calculations 4. The combination with quantum-chcmica) calculations allow for an assignment of the different features in the electronic structure in terms of atomic or molecular orbitals or in terms of band structure. The experimental valence band spectra in some of the examples included in this chapter arc inteqneted with the help of quantum-chemical calculations. A brief outline and some basic considerations on theoretical approaches are outlined in the next section. 

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