Molecular representations resonance structures

An example of a multireference technique is the multiconfigurational SCF (MCSCF) approach, where the wave function is obtained by simultaneously optimizing both the molecular orbitals and the configuration coefficients, thereby blending the different resonance structures together. [28] Historically, the MCSCF approach has been used extensively to provide qualitatively accurate representations of surfaces however, this method still suffers two primary drawbacks (1) the ambiguous choice of configurations and (2) the lack of dynamical correlation. 

Fio. 6. Molecular orbital and resonance structure representation of cyclopentadienyl anion. 

Since the choice of the carbon toward which we pointed the first hybrid GO is arbitrary, there are five different localized MO sets. Contrary to valence bond usage, these five MO representations are not to be superimposed (as resonance structures would be). Rather, they are all equivalent and each by itself is an adequate representation of the molecular state. 

Remember that resonance forms are not isomers bnt partial molecular representations. The true structure (the resonance hybrid) is derived by their superposition, better represented by the dotted-hne drawings at the right of the classical picture. 

The resonance concept indicates that the actual molecular structure lies somewhere between these various approximations, but is not capable of objective representation. The idea can be applied to any molecule, organic or inorganic in which and electron pair bond is present. The term resonance hybrid denotes a molecule that has this property. Such molecules do not vibrate back and forth between two or more structures, nor are they isotopes or mixtures. The resonance phenomenon is rather an idealized expression of an actual molecule that cannot be accurately pictured by any graphic device. 

Whenever there are two alternative Lewi.s structures, one alone will be an inaccurate representation of the molecular itructure. A more accurate picture will be obtained by the superposition of. the two structures into a new model, which lor benzene indicated by 3. The. superposition of two or more Lewis structures into a composite picture is called resonance. 

Fig 1. (a) Molecular structures (b) Two-dimensional topological representations two resonance forms are necessary for B5H9 to preserve its molecular symmetry)... 

The right-hand column of Figure 15-11 shows the electronic structure for the allyl radical, with three pi electrons in the lowest available molecular orbitals. Two electrons are in the all-bonding MO (iri), representing the pi bond shared between the Cl—C2 bond and the C2—C3 bond. The unpaired electron goes into tt2 with zero electron density on the center carbon atom (C2). This MO representation agrees with the resonance picture showing the radical electron shared equally by Cl and C3, but not C2. Both... 

The term resonance does not mean that the molecule physically oscillates back and forth from one of these bonding structures to the other. Rather, within the limitations of the Lewis dot model of bonding, the best representation of the actual bonding is a hybrid diagram that includes features of each of the acceptable individual diagrams. This awkwardness can be avoided by using molecular orbital theory to describe chemical bonding (see Chapter 6). 

A stepwise process is used to convert a molecular formula into a Lewis structure, a two-dimensional representation of a molecule (or ion) that shows the relative placement of atoms and distribution of valence electrons among bonding and lone pairs. When two or more Lewis structures can be drawn for the same relative placement of atoms, the actual structure is a hybrid of those resonance forms. Formal charges are often useful for determining the most important contributor to the hybrid. Electron-deficient molecules (central Be or B) and odd-electron species (free radicals) have less than an octet around the central atom but often attain an octet in reactions. In a molecule (or ion) with a central atom from Period 3 or higher, the atom can hold more than eight electrons by using d orbitals to expand its valence shell. 

The idea of revolutionary progress in certain periods as developed by Kuhn [9] and very recently by McAllister [10], has some bearing on what I will discuss. It will be argued that theoretical organic chemistry has known three periods of dramatic change. The first of these periods (1850-1875) witnessed the birth of the structural formula and its development from formal representation to a reflection of physical reality. The second (1910-1935) saw the advent of quantum mechanics and the concepts of the electron pair, resonance and mesomerism, and hybridisation. In the third one (1955-1980), already mentioned, it is perhaps the succesful application of molecular orbital theory to chemical reactions, made possible by a very fruitful interplay of calculations and concepts, which is most significant. 

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