Planar molecules with delocalized orbitals

Criterion Three Use planar molecules with delocalized tc molecular orbitals... 

The stability of a molecule based on its filled and empty MOs, and its delocalization energy or resonance energy, based on a comparison of its total n-energy with that of a reference system. The pattern of filled and empty MOs led to Hiickel s rule (the An I 2 rule) which says that planar molecules with completely conjugated p orbitals containing An I 2 electrons should be aromatic. 

Characteristics Planar molecule with hexagonal C framework trigonal-planar geometry and sp hybridization at C atoms a framework has C-C and C-H single bonds 2p-orbitals (one from each C atom) overlap side-by-side form three rr-bonds six 7t-electrons) delocalized over the entire six C atom ring... 

One of the most popular of the semi-empirical LCAO methods is that of Hfickel. It is applicable to planar molecules which have n -electron systems. The delocalization of these systems, as treated by this method, has particular chemical significance. The traditional application is to the benzene molecule. Historically, different structures of this molecule were suggested by K6kuI6+ and by Dewar that are described in virtually all textbooks of organic chemistry. These structures represented the first efforts to represent the delocalization of the 3r orbitals in such systems. In the present context the delocalization can be better specified with the use of the method of Hilckel. 

Planar molecules D or A with delocalized n molecular orbitals are best. Molecular components should be of appropriate or compatible size. Onedimensional metals need segregated stacks of radicals, and not mixed stacks. 

Delocalizing electrons stabilizes a molecule. The electron pair on the carbon atom adjacent to the C=O can only be delocalized, though, if it has a p orbital that can overlap with two other p orbitals on two adjacent atoms. Thus, the terminal carbon atom is sp hybridized with trigonal planar geometry. Three adjacentp orbitals make the anion conjugated. 

Benzene is a planar molecule. Each of its six carbon atoms is sp hybridized, with bond angles of 120°. A p orbital of each carbon overlaps the p orbitals of both adjacent carbons. The six tt electrons are shared by all six carbons. Compounds such as benzene that are unusually stable because of large delocalization energies are called aromatic compounds. 

The Hiickel method is a simple semi-empirical method for determining approximate LCAO molecular orbitals to represent delocalized bonding in planar molecules. It treats only pi electrons and assumes that the framework of sigma bonds has been treated separately. As an example we consider the allyl radical, CH2 = CH - CH2.. If the plane of the molecule is the xy plane, each carbon atom has an unhybridized 2pz orbital that is not involved in the sigma bonds, which are made from the Isp hybrids in the xy plane with the appropriate rotation of the coordinate system at each atom to provide maximum overlap. We construct linear combinations from the three orbitals, as in Eq. (21.6-2). 

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