Aromatic molecular orbital treatment

Study of the polynuclear aromatic hydrocarbons offers a point for the merger of the fully empirical pa relationship with the more theoretical molecular orbital treatment. A detailed review of recent progress in this area is presented by Streitwieser (1961). The success of the simplest molecular orbital treatment is illustrated in Fig. 35. This figure presents a correlation of the relative rates of nitration (Table 6) for a series of alternant polynuclear hydrocarbons. The correlation parameters are the localization energies, NT, devised by Dewar (1952). A more sophisticated procedure, the molecular orbital w-technique, which permits the... 

Parallel with this work went the development of quantum theory, with the valence bond and molecular orbital treatment of aromatic molecules. Theoretical predictions were verified by accurate bond length measurements and excellent agreement was finally obtained, particularly in the much studied naphthalene-anthracene series. At this stage it could be claimed that the structure of planar aromatic systems was well understood. 

There are several simple introductions to the molecular orbital treatment of aromatic molecules [2], but a brief account is included here, mainly to outline the special problems associated with heteroaromatic molecules. 

The effects of different types of electron distribution are of much interest. The most effective bridges, i.e., those with the smallest values of f) (0.14—0.56 A ), are those in which the bridging group contains mainly tt-orbitals. Correspondingly, theoretical molecular-orbital treatments by extended Hiickel methods indicate that aromatic or polyene bridges should be better conductors than purely aliphatic ones (cf. Section 9.2.2.5 below) [27], Peptide bridges, made up of a combination of saturated and unsaturated units, provide a fairly good medium (P 0.7 A ) and likewise proteins = 0.7—1.0 A ). A weak dependence of p on the ionization potential of D has been noted [28]. All this evidence links conduction properties with other indications of electronic mobility. 

Pople, J. A. The electronic spectra of aromatic molecules. II A theoretical treatment of excited states of alternant hydrocarbon molecules based on self-consistent molecular orbitals. Proc. Phys. Soc. (London) A 68, 81—89 (1955). 

A quantitative scale of reactivity for aromatic substrates (fused, heterocyclic, and substituted rings) has been devised, based on the hard-soft concept (p. 261).71 From molecular orbital theory, a quantity, called activation hardness, can be calculated for each position of an aromatic ring. The smaller the activation hardness, the faster the attack at that position hence the treatment predicts the most likely orientations for incoming groups. 

Dewar s perturbation molecular orbital (PMO) method analyzes the interactions that take place on assembling p orbitals in various ways into chains and rings.44 It is similar to the methods we have used in Section 10.4 in considering aromaticity, but lends itself better to a semiquantitative treatment. We shall nevertheless be concerned here only with the qualitative aspects of the theory as it applies to pericyclic transition states. 

Up-to-Date Treatment In addition to the classical reactions, this book covers many techniques and reactions that have more recently gained wide use among practicing chemists. Molecular-orbital theory is included early and used to explain electronic effects in conjugated and aromatic systems, pericyclic reactions, and ultraviolet spectroscopy. Carbon-13 NMR spectroscopy is treated as the routine tool it has become in most research laboratories, and the DEPT technique is included in this edition. Many of the newer... 

Three levels of explanation have been advanced to account for the patterns of reactivity encompassed by the Woodward-Hoffmann rules. The first draws attention to the frequency with which pericyclic reactions have a transition structure with (An + 2) electrons in a cyclic conjugated system, which can be seen as being aromatic. The second makes the point that the interaction of the appropriate frontier orbitals matches the observed stereochemistry. The third is to use orbital and state correlation diagrams in a compellingly satisfying treatment for those cases with identifiable elements of symmetry. Molecular orbital theory is the basis for all these related explanations. 

More recently, molecular orbital theory has provided a basis for explaining many other aspects of chemical reactivity besides the allowedness or otherwise of pericyclic reactions. The new work is based on the perturbation treatment of molecular orbital theory, introduced by Coulson and Longuet-Higgins,2 and is most familiar to organic chemists as the frontier orbital theory of Fukui.3 Earlier molecular orbital theories of reactivity concentrated on the product-like character of transition states the concept of localization energy in aromatic substitution is a well-known example. The perturbation theory concentrates instead on the other side of the reaction coordinate. It looks at how the interaction of the molecular orbitals of the starting materials influences the transition state. Both influences on the transition state are obviously important, and it is therefore important to know about both of them, not just the one, if we want a better understanding of transition states, and hence of chemical reactivity. 

This chapter describes the structures of aromatic heterocycles and gives a brief summary of some physical properties. The treatment we use is the valence-bond description, which we believe is appropriate for the understanding of all heterocyclic reactivity, perhaps save some very subtle effects, and is certainly sufficient for a general textbook on the subject. The more fundamental, molecular-orbital description of aromatic systems is less relevant to the day-to-day interpretation of heterocyclic reactivity, though it is necessary in some cases to utilise frontier orbital considerations, however such situations do not fall within the scope of this book. 

Theoretical quantitative treatments of cycloadditions mainly concern the Diels-Alder reaction. A possible approach is that of calculating the para-localisation energy, that is the variation of Tr-electron energy of the conjugated diene system, when two Tr-electrons are localised upon the atoms, in 1,4-relation to each other, which must form the new tr-bonds. This has been done by Brown , using the molecular orbital method some successful predictions of reactivity and of the positions of addition for polycyclic aromatic hydrocarbons and polyenes were made. This method has also been used by other authors - . 

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