2. Aromatic Theory
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Aromaticity The remarkable stability and inertness of benzene is partly derived from the “circulation” of π-electrons around the perimeter. This circulation leads to a number of measurable properties - e.g. the C-C bond lengths in benzene are all 1.39Å. In drawing M.O. diagrams for benzene, the first, most obvious factor is that while it is possible to draw a diagram where there are no nodes: However, you will note that it is not possible to draw a diagram where there is only ONE node! This restriction against having an odd numb er of nodes is due to the cyclic nature of benzene, and the high degree of s ymmetry involved. To make up for this, there are two representatives of the 2-node system: Although it is possible to draw an M.O. level for benzene with 3 nodes, we cannot use it due to symmetry considerations. We thus jump to looking at levels with 4 nodes, of which again there are two: And finally, there is but one level with 6 nodes: What does this all mean when we’ re drawing the energy levels ? The energy level diagram is shown below. Note that the HOMO is comprised of two orbitals of equal energy – these are called degenerate orbitals. There are also two degenerate (but unfilled) orbitals in the LUMO.
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