Cyclic conjugated molecules, molecular orbitals

Benzene is described by molecular orbital theory as a planar, cyclic, conjugated molecule with six it electrons. According to the Htickel rule, a molecule must have 4n + 2 77 electrons, where n - 0, 1, 2, 3, and so on, to be aromatic. Pianar, cyclic, conjugated molecules with other numbers of tt electrons are antiaromatic. 

MOLECULAR ORBITAL MODEL FOR CYCLIC CONJUGATED MOLECULES... 

Draw resonance structures and molecular orbital diagrams for benzene and other cyclic conjugated molecules. 

Non-cyclic interactions of two and three orbitals are described in the preceding chapters of this volume. We describe here cyclic interactions of three or more orbitals (Scheme 1). In 1982, cyclic orbital interaction was found in non-cyclic conjugation [15]. Interactions of bonds in molecules contain cyclic interactions of bond (bonding and antibonding) orbitals even if the molecular geometry is non-cyclic. The cyclic... 

In the previous section, we discussed the construction of the a molecular orbitals in AH systems. In this section, we confine our treatment to the jz molecular orbitals in cyclic conjugated polyenes. In most molecules, there are a bonds as well as n bonds, and these systems can be treated by the methods introduced in these two sections. 

Pattern of molecular orbitals in a cyclic conjugated system. In a cyclic conjugated system, the lowest-lying MO is filled with two electrons. Each of the additional shells consists of two degenerate MOs, with space for four electrons. If a molecule has AN+2) pi electrons, it will have a filled shell. If it has (47V) electrons, there will be two unpaired electrons in two degenerate orbitals. 

Having just seen a resonance description of benzene, let s now look at the alternative molecular orbital description. An orbital view of benzene makes clear the cyclic conjugation of the benzene molecule and the equivalence of the. six carbon-carbon bonds. Benzene is a planar molecule with the shape of a regular hexagon. All C-C-C bond angles are 120 , all six carbon atoms are sp -hybridized, and each carbon has a p orbital perpendicular to the plane of the six-membered ring. 

The theory of topolo cal resonance energy (TR ) represents an important area for chemical applications of matching pdynomials. As this theory will be discussed in more detail elsewhere in this book, we offer here only few brief remarks. As already explained in 4.S.3.5, TRE is the measure of the effect of cyclic conjugation [100,101] on total x-electron energy. TRE therefore also measures the effect of cyclic conjugation on the thermodynamic stability of a conjugated molecule (within the Huckd molecular orbital approximation). This interpretation of TRE is beyond dispute. 

Both thermochemical and molecular orbital approaches agree that benzene is an especially stable molecule and are reasonably consistent with one another in the stabilization energy that is assigned. It is very significant that MO calculations also show a destabilization of certain conjugated cyclic polyenes, cyclobutadiene in particular. The instability of cyclobutadiene has precluded any thermochemical evaluation of the extent of destabilization. Compounds that are destabilized relative to conjugated but noncyclic polyene models are called antiaromatic. ... 

Planar, cyclic, fully conjugated molecules containing 4m -I- 2 7t electrons are especially stable and are called aromatic. The 4m -h 2 rule works because such molecules have a set of filled degenerate molecular orbitals, in a sense a closed shell. 

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