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