Bond breaking using restricted orbitals

Using the double bonds, we conclude that the twisted configuration shown in Figure 6-17 should not be very stable. Here the p orbitals are not in position to overlap effectively in the tt manner. The favored configuration is expected to have the axes of the p-tt orbitals parallel. Because considerable energy would have to be expended to break the p-tr double bond and to permit rotation about the remaining sp2-cr bond, restricted rotation and stable cis-trans isomers are expected. Similar conclusions can be reached on the basis of the r model of the double bond. 

---