Combination out-of-phase

The plus and minus signs imply that electrons accumulate in the overlap region when the orbitals are combined in phase and out of phase, respectively. The cyclic orbital interaction gives rise to stabilization when the orbitals between a and b, between b and c, and between b and c are combined in phase and when a and b are combined out of phase. These are the orbital phase conditions for the A—>C delocalization in the trienes. When all the phase conditions are simultaneously satisfied, the orbital phase is continuous. 

Only neighboring electron-donating orbitals are combined out of phase while any other neighboring orbital pairs are combined in phase. 

A cyclic conjugate molecule composed of n bonds, A, B,..., X, and Y interacts at A with a reactant Z (Scheme 12). When the molecule is an electron donor (Scheme 12a), electrons delocalize from a to z. The resulting electron hole in a is supplied with an electron by the neighboring b. Similar delocalization sequentially follows from c to b, from d to c and so on. This is also the case with the opposite side Y, X,. It follows that the cyclic orbital interaction of a, b,.x, andy is important in the conjugated molecule. The orbitals are all electron-donating orbitals. When each neighboring pair of orbitals is combined out of phase, the interaction of the cyclic... 

Antiaromatic molecules are kinetically unstable, and aromatic molecules are kinetically stable (Scheme 13). In cyclobutadiene, the n orbitals can be combined out of phase and the n orbitals can be combined in phase. Cyclobutadiene is kinetically unstable toward electron donors and acceptors. In benzene, all neighboring pairs of n orbitals cannot be combined out of phase, and all neighboring pairs of 71 orbitals cannot be combined in phase. Benzene is kinetically stable toward donors and acceptors. 

The final molecular orbital will have three nodal planes, which must mean all the p orbitals combining out-of-phase. 

Figure 18.4. Schematic view of the formation of o s and 17js orbitals from His orbitals. Two separated Is orbitals are shown in the top panel. If the two orbitals combine in phase (add), the bonding orbital in the middle panel is obtained. If they combine out of phase (subtract), the antibonding orbital shown in the bottom panel results.

Figure 18.8. LCAO formation of aip orbital. The two p-orbitals are combined out of phase, and this combination corresponds to a bonding orbital.

Figure 18.10. LCAO formation of jt 2p orbital tronl Wo Pm or two Vv orbitals combined out of phase.

When two ls-orbitals combine out-of-phase, this produces an antibonding molecular orbital. 

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