Core 2: Molecular Orbitals

Question 1

What is the wavefunction of a MO composed of?

Answer 1

All the electrons that interact with the MOs wavefunctions multiplied together.

Question 2

How do wavefunctions of AOs combine to form the wavefunction of MOs?

What effect does this have on their energy?

Answer 2

They are added or taken from each other for constructive and destructive intereference respectivally.

When squared, to find the electron probability, there is a ±2Φ_AΦ_B term which represents the electron density lost or gained by the overlap.

Question 3

What is the equation for bond order?

How is bond order related to 1. Bond enthalpy 2. Bond length 3. Bond stretching frequency

Answer 3

½[electrons in bonding MOs - electrons in antibonding MOs]

As bond order increases: 1. Bond enthalpy increases 2. Bond length decreases 3. Bond stretching frequency increases

Question 4

What 3 factors determine the efficiency and possibility of AO overlap?

Answer 4

1. Symmetry, they must have the correct shape to overlap
2. Efficiency depends on if the region of overlap is significant
3. Efficiency also depends on how large the energy gap is between the AOs

Question 5

Why can core interactions be ignored?

Answer 5

Overlap of core orbitals cancels out or are non bonding. They also have small overlap between the orbitals so a weak interaction.

Question 6

Define σ and π orbitals

Answer 6

σ orbitals are spherically symmetric about the internuclear axis, π bonds have a nodal plane along the internuclear axis.

Question 7

What do the labels g and u mean?

Answer 7

g means the orbital is symmetric with respect to inversion, u means the orbital is asymmetric with respect to inversion.

Question 8

What is the overlap integral?

Answer 8

S=int(φ_Aφ_B)dτ

Question 9

How can you tell which atom has lower energy orbitals?

Answer 9

The more electronegative atom will have lower energy orbitals.

Question 10

Write an equation for the wavefunction of a bonding MO formed from two different atoms

Answer 10

Ψ_σ2s=c_xψ_x+c_yψ_y where c_y>c_x

Question 11

What determines that an AO is non-bonding?

Answer 11

Poor overlap has a huge effect at making orbitals non bonding. An energy mismatch also has an effect but it is smaller.

Question 12

How can MO theory be tested?

Answer 12

Using photoelectron spectroscopy to ionise electrons and measuring their kinetic energy. This shows the energy and order of molecular orbitals.

Question 13

What diatomics show σ-π crossover? Why does it occur?

Answer 13

B₂, C₂ and N₂. It occurs because MOs of the same symmetry can mix. This moves 2p_z σ orbitals up in energy and 2s σ orbitals down in energy.

Question 14

Why doesn’t s-p mixing occur for all 1st row diatomics?

Answer 14

The 2s orbital decreases in energy faster than the 2p orbital. This means that the energy difference between the σ orbitals is too large for the σ and π orbitals to change in their order.

Question 15

When will s-p mixing be significant in hetero diatomics?

Answer 15

When the atoms would usually s-p mix or (for CO for example) the elements average to a s-p mixing diatomic.