Resonance theory, electronic structure molecules

The organic chemist made an important step in the understanding of chemical reactivity when he realized the importance of electronic stabilization caused by the delocalization of electron pairs (bonded and non-bonded) in organic molecules. Indeed, this concept led to the development of the resonance theory for conjugated molecules and has provided a rational for the understanding of chemical reactivity (1, 2, 3). The use of "curved arrows" developed 50 years ago is still a very convenient way to express either the electronic delocalization in resonance structures or the electronic "displacement" occurring in a particular reaction mechanism. This is shown by the following examples. 

It is into the LUMO, the energetically most accessible unfilled molecular orbital, that any further electrons will go. Hence, it may be thought of as demarking the location of positive charge in a molecule. The LUMO in planar benzyl cation is delocalized away from the formal cation center and onto the ortho and para ring carbons, in accord with classical resonance structures. On the other hand, the LUMO in perpendicular benzyl cation remains primarily localized on the benzy lie carbon. Resonance theory suggests that delocalization of the positive charge leads to stabilization. Thus, planar benzyl cation is more stable than perpendicular benzyl cation. 

Valence bond theory has two main problems (1) For molecules such as 02, valence bond theory makes an incorrect prediction about electronic structure. (2) For molecules such as O3, no single structure is adequate and the concept of resonance involving two or more structures must be added (Section 7.7). The first problem occurs rarely, but the second is much more common. To better deal with resonance, chemists often use a combination of bonding theories in which the <7 bonds in a given molecule are described by valence bond theory and it bonds in the same molecule are described by MO theory. 

Resonance was introduced when it was found that there are many molecules whose properties cannot be accounted for by means of a single electronic structure of the VB type, but rather by a combination of several structures [1], Although there is an element of arbitrariness in the resonance theory, in the sense of choosing VB structures, Wheland [50] systemized the basic principles to select the important resonance structures as well as to estimate their relative contribution to the ground state of a molecule. In fact, the qualitative resonance theory enjoyed such a great success due to its convenience and usefulness that resonance has become one of the most fundamental concepts in chemical theory. 

The Sn2 reactions are good examples to highlight the different merits of VB and MO theories in quantum chemistry [65]. In the VB method, the atomic features are preserved and the focus is the two-electron-two-center bonds, and each molecule is formed with bonds (plus the lone and core pairs). Whereas one resonance structure is not enough to describe a molecule, multi-resonance structures are adopted. In fact, the resonance theory can also be applied to illustrate the reactions in an intuitive way. For example, for the chloride-exchange reaction... 

Alternatively, the correspondence between MO theory and resonance theory in the description of electronic structures can be used to construct VB wavefimctions for the low-lying states of diatomic molecules. Test calculations... 

Examples of 1,3-dipoles include diazoalkanes, nitrones, carbonyl ylides and fulminic acid. Organic chemists typically describe 1,3-dipolar cycloaddition reactions [15] in terms of four out-of-plane 71 electrons from the dipole and two from the dipolarophile. Consequently, most of the interest in the electronic structure of 1,3-dipoles has been concentrated on the distribution of the four Jt electrons over the three heavy atom centres. Of course, a characteristic feature of this class of molecules is that it presents awkward problems for classical valence theories a conventional fashion of representing such systems invokes resonance between a number of zwitterionic and diradical structures [16-19]. Much has been written on the amount of diradical character, with widely differing estimates of the relative weights of the different bonding schemes. 

In order to apply the resonance theory of the transitional type of bond it is necessary to reconsider the electronic states of the atoms of the elements and to consider the valence bond structures of certain of their compounds, which may make an appreciable contribution to the state of the molecule. 

While both the ipsoeentrie and NICS methodologies loosely define aromaticity as the occurrenee of diatropic ring currents, in ref. 131 molecules a) and b) were examined from the VB (and chemically intuitive) viewpoint of resonance. It was shown that modern VB theory in its SC form strongly suggests that molecule (a) retains some of the resonance between electronic bonding structures observed in benzene, whilst the electronic structure of... 

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