Infrared Absorption Spectra

Question 1

Will vibrational modes give an IR band?

Answer 1

Each vibrational mode will give a distinct IR band provided that the transition is allowed
(Change in dipole on vibration)

Question 2

Will CO2 give an IR band?

Answer 2

CO2 is linear and non polar - zero overall dipole
Dipoles cancel out in symmetric stretch
There is dipole in asymmetric stretching and bending so these give a band

Question 3

What difference would be on a spectra from gas phase and condensed phase?

Answer 3

In the gas phase rotational fine structure would be observed
The condensed phase would be much simpler and band positions may be shifted due to interactions

Question 4

What are skeletal vibrations?

Answer 4

They involve several or all atoms within a molecular skeleton

Give pattern in finger print region (characteristic)

Question 5

What are group vibrations?

Answer 5

These are localised on a specific bond or group

Give bands at similar wavenumber regardless of molecule- great for identifying specific groups

Question 6

What do C-C and C-X bonds display a dependence on?

Answer 6

Cc on bond order

Cx on mass

Question 7

What happens to OH stretch in condensed phase?

Answer 7

May be reduced due to H bonding

Question 8

What does IR spectrum tell us?

Answer 8

It tells us about energy gaps and frequency wave numbers
We can determine bond strength
Provide structural and analytical information