Diatomic Molecules

Question 1

Atomic orbitals must be … to give Molecular orbitals

Answer 1

Same Symmetry

Question 2

What 3 factors increases the strength of the interaction between atomic orbitals

Answer 2

• AOs that are closer in energy interact more strongly
• AOs that are close in energy to the MO contribute more than those that are further away
• AOs with compatible sizes interact more strongly

Question 3

The number of MOs is equal to

Answer 3

The number of interacting AOs

Question 4

As the energy seperation increases…

Answer 4

The lower energy atomic orbital contributes more to the bonding Molecular orbital, while the high energy atomic orbital contributes ,ore to the antibonding molecular orbtial

Question 5

Why does no interaction occur between atomic orbitals when the energy seperation is very large

Answer 5

When the energy seperating is very large, the bonding MO is essentially the same as the lower energy AO, and the antibonding MO is essentially the same as the higher energy AO
In other words there is no longer any interaction

Question 6

What type of symmetry do 2s orbitals have

Answer 6

2s orbitals have the same spherical symmetry as 1s orbitals and overlap in the same way to form MOs with cylindrical (sigma) symmetry
The bonding MO has σg symmetry and the antibonding MO has σu symmetry

Question 7

How does the overlap between the 2pz atomic orbitals occur

Answer 7

The 2pz orbitals (point along the internuclear axis) overlap in a head-to-head fashion to form a bonding σg MO (with electron density between the atomic nuclei) and a head-to-head to form an anti-bonding σu MO

Question 8

How does the overlap between the 2px and 2py atomic orbitals occur

Answer 8

The 2px and 2py orbitals (point perpendicular to the internuclear axis) overlap sideways on to give bonding an antibonding MOs
The resulting Mos do not have cylindrical symmetry, but have one nodal plane which contains the internuclear axis.
It has the symmetry lable π, where the bonding orbital has πu and the antibonding has πg inversion symmetry

Question 9

Overlap between the 2s and 2p AOs can be negleted for

Answer 9

Elements on the right of the second period

Question 10

When constructing a MO diagram, an element has unpaired electrons, this means it is

Answer 10

Paramagnetic
Attracted to a magnet

Question 11

When constructing a MO diagram, an element has paired electrons, this means it is…

Answer 11

Diamagnetic
Repelled by a magnetic

Question 12

How is bond order calculated

Answer 12

1/2 (Bonding electrons - Antibonding electrons)

Question 13

The energy separation between the 2s and 2p AOs decreases towards the left of the second period
This causes

Answer 13

s-p-mixing
Results in a decrease in energy for the 2σ MOs and an increase in energy for the 3σ MOs
An example of large s-p-mixing is shown in the diagram

Question 14

What wavelength are required for electronic transition

Answer 14

UV or visible eletromagnetic radiaition

Question 15

How could you measure electronic transition from the HOMO to LUMO

Answer 15

The lowest energy (longest wavelength) transition in any molecule involves promoting an electron from the HOMO to the LUMO
However not very accurate due to electrostatic repulsion

Question 16

What affect does conjugation have in the energy needed for HOMO to LUMO transition

Answer 16

Increasing the number of conjugated double bonds, increases the wavelength (decreases the energy) of the electronic transition

Question 17

What is the name of the method used to measure the energies of MOs of molecules
How does it work

Answer 17

Ultraviolet photoelectron spectroscopy
This approach relies on the photoelectric effect which involves the emission of electrons when electromagnetic radiation (UV) hits a material
Koopman’s theory: ionisation energy may be identified with the energy of an occupied MO

Question 18

How the the kinetic energy of an emitted photoeletron is given by

Answer 18

Ek = hv - i
h = planks constant
v = frequency
i = ionisation energy