MO Theory for Polyatomic Molecules - Delocalization

• MOs in polyatomic molecules can be localized between two atoms, or dispersed over multiple atoms (delocalized)– Sigma bonds are often localized– Pi bonds may be delocalized, as in the case of conjugated molecules

• Polyenes have delocalized π- and π*-orbitals– Each carbon atom donates one AO and one electron to the π-system– Total number of π and π* MOs is equal to the number of carbon atoms– Only bonding orbitals are filled

• Since polyenes are described well by the PIB model, the MOs must have a similar nodal structure– As the energy of the MOs increase, so must the number of nodes– Whether an MO is bonding or antibonding is determined by the number of bonding and

antibonding interactions in the MO

MO Theory for Cyclic Conjugated Molecules

• Frost circles can be used to determine the nature of π-bonding in cyclic molecules– For hydrocarbons, each carbon atom donates one AO and one electron to the π-system– Energy levels of each MO can be drawn based on the structure of the corresponding ring

(e.g., benzene has 4 energy levels with two levels being doubly degenerate)

• Electrons are placed in each MO using the same rules as for diatomic MO energy diagrams– Two electrons per orbital, with degenerate orbitals being half-filled before they are

paired– As the energy level increases, so does the number of nodes in the MOs (degenerate MOs

have the same number of nodes)

• Aromaticity can be determined from these diagrams– In aromatic rings, all C-C bonds must be equivalent and have a bond order greater than 1– Partially filled MOs are not stable and force the system to distort their structure away

from aromaticity (e.g., cyclobutadiene and cyclopentadiene)

MO Theory of Peptide Bonds

• Peptide bonds show a strong tendency to adopt a trans configuration in folded proteins– No free rotation around C-N bond due to planarity of amide group– Rationalized through consideration of two resonance structures

• MO theory can be used to rationalize the planar structure– The O, C, and N atoms each donate 1 AO (p-orbital) to form 3 MOs– 4 electrons reside in these MOs (1 each from C and O, 2 from N lone pair)

• MO diagram explains why resonance structures work– Lowest energy MO is a delocalized π-bond between O, C, and N– HOMO is a delocalized orbital that has a node on the carbon atom, thus it looks like lone

pairs on O and N