Molecules with delocalized bonding

Lewis electron-dot formulas are simple representations of the valence-shell electrons of atoms in molecules and ions. You can apply simple rules to draw these formulas. In molecules with delocalized bonding, it is not possible to describe accurately the electron distribution with a single Lewis formula. For these molecules, you must use resonance. Although the atoms in Lewis formulas often satisfy the octet rule, exceptions to the octet rule are not uncommon. You can obtain the Lewis formulas for these exceptions by following the rales for writing Lewis formulas. The concept of formal charge will often help you decide which of several Lewis formulas gives the best description of a molecule or ion. 

Writing resonance formulas Given a simple molecule with delocalized bonding, write the resonance description. (EXAMPLE 9.9)... 

One of the advantages of using molecular orbital theory is the simple way in which it describes molecules with delocalized bonding. Whereas valence bond theory requires two or more resonance formulas, molecular orbital theory describes the bonding in terms of a single electron configuration. 

Bonding Theories and Descriptions of Molecules with Delocalized Bonding... 

We begin our exploration of delocalized bonds with ozone, O3. As described in Chapter 7, ozone in the upper stratosphere protects plants and animals from hazardous ultraviolet radiation. Ozone has 18 valence electrons and a Lewis stmcture that appears in Figure 10-36a. Experimental measurements show that ozone is a bent molecule with a bond angle of 118°. 

A molecule with delocalized 7 bonds involving oxygen... 

Resonance Molecules with two or more valid structures are said to be resonant. The actual structure is neither of the alternatives but a lower-energy molecule with delocalized valence electrons. Benzene with its alternating double and single bonds is an example of a resonant structure. Benzene actually has no single... 

Further analysis is based on the idea that the characteristic experimental behavior of different classes of compounds and the suitability of those or other models used to describe this behavior is ultimately related to the extent to which the chromophores or electron groups physically present in the molecular system are reflected in these models. It is easy to notice, that the MM methods work well in case of molecules with local bonds designated in Table 1 as valence bonds the QC methods apply both to the valence bonded systems, and for the systems with delocalized bonds (referred as orbital bonds in Table 1). The TMCs of interest, however, not covered either by MM or by standard QC techniques can be physically characterized as those bearing the d-shell chromophore. The magnetic and optical properties characteristic for TMCs are related to d- or /-states of metal ions. The basic features in the electronic structure of TMCs of interest, distinguishing these compounds from others are the following ... 

If a molecule with no-bond homoaromaticity is investigated, the system in question possesses a non-classical structure with an interaction distance typical of a transition state rather than a closed-shell equilibrium structure. One can consider no-bond homoconjugative interactions as a result of extreme bond stretching and the formation of a singlet biradical, i.e. a low-spin open-shell system. Normally such a situation can only be handled by a multi-determinant description, but in the case of a homoaromatic compound the two single electrons interact with adjacent rc-electrons and form together a delocalized electron system, which can be described by a single determinant ab initio method provided sufficient dynamic electron correlation is covered by the method. 

A valid alternative for systems of delocalized electrons is to represent the considered molecules by their canonical forms according to VB-theory. However, this approach does not reflect chemical reality and it is certainly not adequate for molecules with multicenter bonds and many organometallic systems. 

In chemical usage [23, Section 14.11] an electron is said to be delocalized if its molecular orbital cannot be ascribed to a two-center bond otherwise it is localized. It is, however, always possible, but perhaps rarely convenient, to describe the electron distribution in a molecule with delocalized orbitals only. The situation in a covalent insulator such as diamond is similar to the molecular case. There are four valence electrons per atom, and four neighbors. Therefore, it is possible to describe the structure with four two-center, two-electron bonds, and localized Wannier orbitals. But keep in mind that the only physical reality is the resulting charge distribution. This reality can also be described by freely moving Bloch electrons. 

Molecules with double bonds next to each other and aromatic molecules based on benzene contain more than two tt orbitals on adjacent atoms. The bonds, as well as the entire molecules, are described as being conjugated. Electrons in these molecules are free to move from one bond to the next on the same molecule and so are delocalized. In 0014, we saw electron delocalization extending throughout the entire substance in materials with metallic bonds. Benzene (CeHe) has the following resonance forms ... 

We should note that molecules with delocalized molecular orbitals are generally more stable than those containing molecular orbitals extending over only two atoms. For example, the benzene molecule, which contains delocalized molecular orbitals, is chemically less reactive (and hence more stable) than molecules containing localized C=C bonds, such as ethylene. 

In this chapter we have seen a number of new concepts, including the delocalization of tt systems of molecules and the molecular orbital description of molecular bonding. A connection between these concepts is provided by the field of organic dyes, molecules with delocalized tt systems that have color. The color is due to the... 

Molecules with delocalized molecular orbitals are generally more stable than those containing molecular orbitals locahzed on only two atoms. The benzene molecule, for example, which contains delocalized molecular orbitals, is chanically less reactive (and hence more stable) than molecules containing localized C=C bonds, such as ethylene. Benzene is so stable because the energy of the pi electrons is lower when the electrons are delocalized over the entire molecule than when they are localized in individual bonds, much as the energy of the particle in a one-dimensional box is lowered when the length of the box is increased (see Section 1.3). 

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