Molecular Orbitals Flashcards
As overlap of atomic orbitals increases…
- bonding MO energy is lowered (stabilised more)
- antibonding MO energy is raised (destabilised more)
Significant overlap/formation of a MO only occurs between atomic orbitals with
- similar size
- similar energies
- correct symmetry
Aufbau Principle
Pauli’s Exclusion Principle
Hund’s Rule
Key Features: Bonding MO
- Overlap of atomic orbital wavefunctions is in phase
- Increase in the electron density between the nuclei
- E(Bonding MO) < E(AO from which it is formed)
Key Features: Antibonding MO
- Overlap of atomic orbital wavefunctions is out-of-phase
- Decrease in the electron density between the nuclei
- E(Anti-bonding MO) > E(AO from which it is formed)
Within a heteronuclear diatomic, the two atoms are from different atoms (e.g LiH):
- AO do NOT have the same energy in each atom
- Need to work out the relative energies of the AO on each atom
- Bonding MO – bigger contribution from more electronegative atom
- Anti-bonding MO – bigger contribution from less electronegative atom
Linear Combination of Atomic Orbitals (LCAO)
ψMO = c1 φAO(A) + c2 φAO(B)
( + for in-phase, - for out-of-phase)
The more electronegative the element…
The lower the AO energies
Effect of electronegativity on bonding of MO
Bonding MO: skewed towards the more electronegative atom
Anti-bonding MO: skewed towards the less electronegative atom
s-p mixing in heteronuclear diatomics
more likely to occur as the s and p orbitals are similar energies
MO Theory
- Ground & excited states
- Quantitative picture of bonding
- Delocalised MOs for electrons (can be spread over whole molecule)
- Only theory to explain behaviour of O2 (paramagnetic) & NO (radical)
VB Theory
- Ground state
- Qualitative picture of bonding & molecular structure
- Localised bond + e- pairs
- Quick “picture” of geometry
Bonding MO
- Lower in energy than the AO from which it is formed
- Increase in electron density between the nuclei
- Overlap of atomic wavefunction is in phase or constructive
Anti-bonding MO
- Higher in energy than the AO from which it is formed
- Decrease in electron density between the nuclei
- Overlap of atomic orbital wavefunction is out-of-phase or destructive
Wavefunctions interact to form Molecular Orbitals (MO):
- constructive interference (in phase)
- increase in e- density between the nuclei
- MO lower in energy than AO
Wavefunctions interact to form Molecular Orbitals (MO):
- destructive interference (out of phase)
- MO higher in energy than AO
- anti-bonding molecular orbital
Orbital label for head or end-on overlap
sigma label
Symmetry of Orbital (Labels)
Orbitals in phase:
Yes = “gerade” -> “g”
No = “ungerade” -> “u”
Orbital labels for bonding/anti-bong MO
Bonding -> no *
Anti-bonding -> *
Bond order equation
Bond order = 1/2 (No. of bonding e- - No. of anti-bonding e-)
e- density between nuclei in anti/bonding MO
If bonding = increases
If anti-bonding = decreases
