Orbital diagram, showing conjugation

These diagrams show the overlap of a series of five conjugated p orbitals in this cation. The hydrogens have been omitted for clarity in the drawing at the left. 

Q Explain how to construct the molecular orbitals of a conjugated cyclic system similar to benzene and cyclobutadiene. Use the polygon rule to draw the energy diagram, and fill in the electrons to show whether a given compound or ion is aromatic or antiaromatic. 

Electron delocalization also causes a conjugated diene to be more stable than an isolated diene. The tt electrons in each of the double bonds of an isolated diene are localized between two carbons. In contrast, the rr electrons in a conjugated diene are delocalized. As you discovered in Section 7.6, electron delocalization stabilizes a molecule. Both the resonance hybrid and the molecular orbital diagram of 1,3-butadiene in Figure 7.9 show that the single bond in 1,3-butadiene is not a pure single bond, but has partial double-bond character as a result of electron delocalization. 

Figure 9.10 Molecular orbital energy diagrams for conjugated molecules in the neutral state (N) and in the oxidized polaron (P) and bipolaron (BP) forms. The dotted arrows show the allowed electronic transitions...

Figure 6.5 Frontier moiecuiar orbital energy diagram showing the interactions of the oxalate anion and the two M2-8 combinations that have the correct symmetry to electronically couple the two Mj centers via M2-5 to bridge-x conjugation.

The third compound is naphthalene. The structure drawn is the best curly arrow diagrams do not do this molecule justice and other versions are less convincing. In particular, they do not show that the middle bond common to both rings is the shortest bond. There are no charges anywhere and all the p orbitals are conjugated. This is the basic ten-electron aromatic system. 

The photochemistry of alkenes, dienes, and conjugated polyenes in relation to orbital symmetry relationships has been the subject of extensive experimental and theoretical studyThe analysis of concerted pericyclic reactions by the principles of orbital symmetry leads to a complementary relationship between photochemical and thermal reactions. A process that is forbidden thermally is allowed photochemically and vice versa. The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the reaction types discussed in Chapter 10 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2Tr- -2Tr] cycloaddition of two alkenes, which was classified as a forbidden thermal reaction (see Section 10.1), can serve as an example. The correlation diagram (Figure 12.17) shows that the ground state molecules would lead to a doubly excited state of cyclobutane, and would therefore involve a prohibitive thermal activation energy. 

F. 13. (a) The two 2p atomic orbitals (A.O.) of ethylene combine to give two pir molecular orbitals (M.O.). (b) The similar result found in non-conjugating dienes, such as cyclo-octa-1,5-diene, (c) Shows the greater variety of energy levels formed by the butadiene pz orbitals (see Figure 17). (d) The energy diagram for ( clobutadiene M.O.s... 

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