Molecular Orbital Theory for Linear Pi Systems

Since the pi bonding portion of a molecule is perpendicular to the sigma bonded framework, it can generally be treated independently. In a double bond, two p orbitals 

Three p orbitals, all aligned parallel to one another, make up the allyl unit and produce three molecular orbitals. The MOs for the following systems differ from the completely carbon allylic system only in the previously discussed electronegativity effects (the bonding MOs will have a larger contrihution from the heteroatom p orbital and the antibonding MOs will have a lesser contribution. Section 2.3). 

To determine how many electrons occupy any pi system, start by counting each double bond as contributing two electrons, but do not count any pi bonds that are perpendicular to the pi system of interest For example, the second pi bond of a triple bond does not count. Because the maximum that a single atom can contribute is a single filled p orbital containing two electrons, if a heteroatom has several lone pairs it can contribute only two electrons. Only one p orbital on an atom can align itself with the pi system. If an atom is doubly bonded and contains a lone pair, the lone pair cannot be counted since it must be in an orbital that is perpendicular to the double bond. All the examples above have four electrons in a three-p orbital pi system. 

For C=C-C, the allyl cation, there would be two electrons in the pi system, filling just ipi, leaving the others empty. The shape and energy of the MOs are independent of the occupancy. The allyl anion has four electrons in the pi system, filling both ipi and t )2- The valence bond resonance forms, C-C C C C=C, describe the electron 

Chapter 12 Qualitative Molecular Orbital Theory Pericyclic Reactions 

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Section 12.2 Molecular Orbital Theory for Linear Pi Systems... 

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