FMO THEORY KEY EXAM QUESTIONS AND
CORRECT ANSWERS
What is FMO theory? - CORRECT ANSWERS FMO theory concerns the application of
molecular orbitals (MOs) to describe HOMO and LUMO interactions
Give one property of the FMO theory - CORRECT ANSWERS Focuses on the highest
occupied molecular orbital and the lowest occupied molecular orbital (HOMO and LUMO)- e.g.
instead of thinking about the total energy density in a nucleophile, we should think about the
localisation of the HOMO orbital because electrons from this orbital are most free to participate
in the reaction
Name one way of approaching the analysis of an FMO interaction - CORRECT
ANSWERS Orbital shading
Define the term HOMO - CORRECT ANSWERS Highest occupied molecular orbital
Define the term LUMO - CORRECT ANSWERS Lowest unoccupied molecular orbital
Define the term SOMO - CORRECT ANSWERS Singularly occupied molecular orbital
Define the term allowed - CORRECT ANSWERS A reaction that is allowed to proceed using
FMO theory
Define the term thermal - CORRECT ANSWERS A reaction that uses heat energy
Define the term photochemical - CORRECT ANSWERS A reaction that uses light energy
, How is a SOMO orbital generated? (2) - CORRECT ANSWERS 1. Absorption of a photon of
light can lead to an excited state
2. This is by promotion of an electron from the HOMO to the LUMO, generating a SOMO
What does LCAO stand for? - CORRECT ANSWERS Linear Combination of Atomic orbitals
Summarise LCAO (linear combination of atomic orbitals) - CORRECT ANSWERS According
to LCAO, when 2 atoms come close together, two atomic orbitals (one from each atom) having
comparable energy, same symmetry and possessing large overlaps, combine to form two
molecular orbitals.
One has a higher energy than the combining atomic orbitals, giving rise to a repulsive state
called the anti-bonding molecular orbital
One has a lower energy than the combining atomic orbitals, giving rise to an attractive state
called the bonding molecular orbital
State all the 6 LCAO rules - CORRECT ANSWERS 1. n atomic orbitals combine to give n
molecular orbitals
2. The molecular orbitals are arranged symmetrically about the non-bonding energy level in the
unsubstituted parent system
3. The nth molecular orbital (ψn) has (n-1) nodes
CORRECT ANSWERS
What is FMO theory? - CORRECT ANSWERS FMO theory concerns the application of
molecular orbitals (MOs) to describe HOMO and LUMO interactions
Give one property of the FMO theory - CORRECT ANSWERS Focuses on the highest
occupied molecular orbital and the lowest occupied molecular orbital (HOMO and LUMO)- e.g.
instead of thinking about the total energy density in a nucleophile, we should think about the
localisation of the HOMO orbital because electrons from this orbital are most free to participate
in the reaction
Name one way of approaching the analysis of an FMO interaction - CORRECT
ANSWERS Orbital shading
Define the term HOMO - CORRECT ANSWERS Highest occupied molecular orbital
Define the term LUMO - CORRECT ANSWERS Lowest unoccupied molecular orbital
Define the term SOMO - CORRECT ANSWERS Singularly occupied molecular orbital
Define the term allowed - CORRECT ANSWERS A reaction that is allowed to proceed using
FMO theory
Define the term thermal - CORRECT ANSWERS A reaction that uses heat energy
Define the term photochemical - CORRECT ANSWERS A reaction that uses light energy
, How is a SOMO orbital generated? (2) - CORRECT ANSWERS 1. Absorption of a photon of
light can lead to an excited state
2. This is by promotion of an electron from the HOMO to the LUMO, generating a SOMO
What does LCAO stand for? - CORRECT ANSWERS Linear Combination of Atomic orbitals
Summarise LCAO (linear combination of atomic orbitals) - CORRECT ANSWERS According
to LCAO, when 2 atoms come close together, two atomic orbitals (one from each atom) having
comparable energy, same symmetry and possessing large overlaps, combine to form two
molecular orbitals.
One has a higher energy than the combining atomic orbitals, giving rise to a repulsive state
called the anti-bonding molecular orbital
One has a lower energy than the combining atomic orbitals, giving rise to an attractive state
called the bonding molecular orbital
State all the 6 LCAO rules - CORRECT ANSWERS 1. n atomic orbitals combine to give n
molecular orbitals
2. The molecular orbitals are arranged symmetrically about the non-bonding energy level in the
unsubstituted parent system
3. The nth molecular orbital (ψn) has (n-1) nodes
