Huckel molecular orbital method benzene

We will consider the application of the Huckel molecular orbital method to the benzene molecule and we will first see what happens when we do not make use of symmetry. The benzene molecule has a framework of six carbon atoms at the comers of a hexagon and each carbon atom contributes one w-electron. The -electron MOs will be constructed from six 2pf atomic orbitals, each located at one of the carbon atoms, thus, e... 

Various aromatic and conjugated polyunsaturated hydrocarbons undergo one-electron reduction by alkali metals." Benzene and naphthalene are examples. The EPR spectrum of the benzene radical anion was shown in Fig. 12.2a (p. 657). These reductions must be carried out in aprotic solvents, and ethers are usually used. The ease of formation of the radical anion increases as the number of fused rings increases. The electrochemical reduction potentials of some representitive compounds are given in Table 12.1. The potentials correlate with the energy of the LUMO as calculated by simple Huckel MO theory." A correlation that includes a more extensive series of compounds can be observed with the use of somewhat more sophisticated molecular orbital methods." ... 

In recent years the fundamental ideas of Huckel molecular orbital theory, the Huckel rule, and other aspects of aromaticity have been extended to polyhedral three-dimensional inorganic structures regarded as aromatic like the two-dimensional aromatic hydrocarbons. Such an extension of Huckel molecular orbital theory requires recognition of its topological foundations so that they can be applied to three-dimensional structures as well as two-dimensional structures. In this connection graph theoretical methods can be used to demonstrate the close analogy between the delocalized bonding in two-dimensional planar aromatic systems such as benzene and that in three-dimensional deltahedral boranes, and carboranes. Related ideas can be shown to be applicable for metal carbonyl clusters, bare post-transition metal clusters, and polyoxometallates. ... 

The Huckel method has been used to obtain n molecular orbitals, and their associated enei gies, for butadiene and benzene. The n electron energy is found to be lower than it would be for localized n bonding, and this difference is known as the delocalization energy. 

Abstract. Guided by an intuitive choice of approximations which shows remarkable chemical insight into the topic of aromaticity, Huckel mastered the difficult mathematical treatment of a complex molecule like benzene at a very early stage of quantum theory using method 1 (now valence bond theory) and method 2 (now molecular orbital theory). He concluded that methoci 2 is clearly superior to method 1 because the results of this method explain directly the peculiar behaviour of planar molecules with 6 n electrons. 

In 1931 Hiickel published the first in a series of four papers in which he applied the new quantum mechanics to the benzene problem using for the first time the VB method for aromatic compounds. Little reference is now made to this part of Hiickel s paper. In the second part of this same paper, he applied to aromatics a method that Bloch had used for crystal lattices. In this method electrons are placed into orbitals that extend over the entire molecule instead of being localized on a single atom. These molecular orbitals may in turn be constructed as linear combinations of atomic orbitals, and such an approach is usually called the Hund—Mulliken—Huckel (HMH) or molecular orbital (MO) method. Huckel s work in this second part of the paper leads to the famous 4/7 -f 2 rule for monocyclic conjugated hydrocarbons, though we do not find it explicitly stated here. 

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