Benzene interaction diagram

The Z-substituted benzene (benzaldehyde, Figure 11.2) is not activated toward electrophilic attack since the HOMO of benzene is scarcely affected. No preferred site for attack of the electrophile can be deduced from inspection of the HOMOs. The interaction diagram for a Z-substituted pentadienyl cation, substituted in the 1-, 2-, and 3-positions, as models of the transition states for the ortho, meta, and para channels are too complex to draw simple conclusions. The HOMO and LUMO of the three pentadienyl cations with a formyl substituent are shown in Figure 11.4. The stabilities of the transition states should be in the order of the Hiickel n energies. These are 6a — 9.204 / , 6a — 9.2031/ , and 6a -9.1291/ , respectively. Thus, by SHMO, the ortho and meta channels are favored over the para channel, with no distinction between the ortho and meta pathways. Experimentally, meta substitution products are usually the major ones, contrary to the SHMO predictions. Either the SHMO method fails in this case or the predominance of meta products may be attributed to steric effects. 

Figure Bll.l. (a) Bonding interactions and structure of benzene-NO+ complex with Cev symmetry. b) Bonding interactions and structure of benzene-NO+ complex with Cs symmetry, (c) Interaction diagram for the Cs complex. The dashed lines correspond to possible assignments for the two lowest transitions.

Density functional theory, 21, 31, 245-246 B3LYP functional, 246 Hartree-Fock-Slater exchange, 246 Kohn-Sham equations, 245 local density approximation, 246 nonlocal corrections, 246 Density matrix, 232 Determinantal wave function, 23 Dewar benzene, 290 from acetylene + cyclobutadiene, 290 interaction diagram, 297 rearrangement to benzene, 290, 296-297 DFT, see Density functional theory... 

Use orbital interaction diagrams to explain why methoxy benzene (anisole) prefers a conformation in which the methyl group lies in the plane of the aromatic ring. 

We can reach a similar conclusion from an interaction diagram, by looking at the effect of changing butadiene 1.24 into cyclobutadiene 1.25 (Fig. 1.47). This time there is one drop in n energy and one rise, and no net stabilisation from the cyclic conjugation. As with benzene, we can see that the drop is actually less (from overlap of orbitals with a small coefficient) than the rise (from overlap of orbitals with a large coefficient). Thus cyclobutadiene is less stabilised than butadiene. 

Figure 4. Part of the interaction diagram between one benzene ligand and Ru(CO)6(/r-CO)3 (left-hand side) or Ru3(CO)9 (right-hand side).

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