Overlap, antibonding

Fig.4- Crystal overlap population of M0O3 (negative part antibonding overlap positive part bonding overlap)...

Destabilizing (antibonding) overlap repulsions between geminal bonds increase (i.e. IBP becomes more negative) in the order open-chain compound < four-membered ring < three-membered ring. 

The antibonding overlap in the HOMO when disrotation occurs makes the formation of the new sigma bond unfavorable. The disrotatory closure of a diene to a cyclobutene is thermally forbidden. 

An overlap of orbitals that contributes to antibonding. Overlap of lobes with opposite phases (+ phase with — phase) is generally destructive overlap, (p. 671)... 

Figure 2.7 Orbitals that don t interact because their bonding and antibonding overlap cancels.

Concerted four electron cycloaddition reactions are not thermally allowed. All this really means is that the transition state for an allowed process is much lower than that of a nonallowed process. HOMO-LUMO theory can explain why the concerted 2 + 2 cycloaddition of two ethylenes is not favored. Bonding overlap between a HOMO and a LUMO lowers the transition state energy of the allowed process, making it favorable. With the concerted 2 + 2 cycloaddition of two ethylenes, no such transition state stabilization is achieved, as shown in Figure 12.17. Any bonding overlap is exactly cancelled out by an equivalent antibonding overlap, yielding no net stabilization. 

Each MO is a linear combination of the three orbitals I, d, and p. When we examine their shapes, we find that the interactions between 1 and d, and between 1 andp, are both bonding in the lowest-energy MO (pi and both antibonding in the highest-energy MO (pi. In the orbital (pi, there is an antibonding overlap (I —d) but also a bonding overlap (I—p) (2.A3). 

However, the increased stabilization gained by distortion due to decreased 1 Pz — CaPz antibonding overlap will tend to be attenuated by the decrease in Czpz -CsPz bonding overlap. Hence, we expect that the two electron stabilization energy will vary in the order trans = gquche > cis. Note that the it —it (or ir—jt ) stabilization of the gauche is expected to be comparable to that of the trans form. 

The fact that there is at least one antibonding overlap in the cyclic array of orbitals does not reduce aromaticity. Mobius transition states are as aromatic as normal Htickel transition states. In contrast to stable Mobius annulenes, Mobius transition states in pericyclic reactions are quite frequent. They also include c aromatic... 

Figure 13.23 Ring current in the Mobius transition state of the electrocyclic reaction of octa-l,3,5,7-tetraene to cycloocta-1,3,5-triene. For details see Figure 13.21. Note that the basis orbitals (atomic orbitals that are involved in the reaction) can be written with arbitrary phase. The number of antibonding overlaps (sign inversion) will...

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