Spectroscopic Techniques (2) Infrared (IR) Spectroscopy

Infra–red spectroscopy was one of the first spectroscopic techniques developed to analyse samples of molecules non–invasively.

The essential principle of infrared spectroscopy is that polar covalent bonds resonate at energies and frequencies in the infra–red region of the electromagnetic spectrum. 

And basically that’s it. 

But note that the molecule visible to this spectroscopy must contain polar covalent bonds.

So molecules like chlorine do not interact with infra red radiation because the Cl—Cl bond is not polar.

Whereas molecules with covalent bonds in which the two atoms have a difference in electronegativity do resonate with infrared radiation and vibrate like those in water H2O.

So what happens in molecules when their bonds resonate with infra-red radiation?

Essentially, the bond in question vibrates.

Infrared energy lifts the bond into a higher vibrational energy state. 

The quanta of energy needed for this process is related the bond’s strength or its force constant. 

You will notice from this equation that the force constant f is proportional to 1/λ the inverse of the wavelength of infra-red resonance energy absorbed. 

This property 1/λ is called the wave number and is also proportional to the energy ∆E absorbed since

     c  =  ν λ   and therefore  c/λ= ν

and from Planck’s equation: ∆E =  h ν

therefore combining these two equations we have ∆E  =  hc/ λ

therefore ∆ E  ∝ 1/λ

Wavenumber has units of cm^–1

So what sort of vibrational modes occur when infra-red energy is absorbed?

Bonds can either stretch or bend.

In polar diatomic molecules like CO NO and HCl the polar covalent bond stretches

So     H — Cl

         ↔︎

H and Cl atoms pulsate in and out along the bond axis and the interatomic distances change.

It is as if the bond were a spring joining the two atoms together and one end of the spring is pulled allowing the atoms to move together then move apart. 

In polar non-linear molecules the bonds have different vibrational modes.

Not only can the bonds stretch either symmetrically or asymmetrically they can also bend

A good example to show these modes is the water molecule and you can find many illustrations of water’s molecular vibrational modes on the Internet e.g.

When we examine organic molecules we find that they also behave in similar ways because most of their bonds are polar. 

Certain functional groups have particular absorption bands in the infra-red region.

If you are studying chemistry at school or college level you will know that your institution or examination authority provides tables of these absorption bands related to functional groups in organic molecules.

Here is typical example from the UK’s Royal Society of Chemistry:

All you need to do is then take you infra-red spectrum and examine it in the light of these absorption bands.

At school and college level you will have fairly straightforward examples to deconstruct and they will usually be part of a question in which you have other information about the example molecule as well as its IR spectrum. 

For example here is a simple example that of methanol (formaldehyde)

Here is a typical ketone showing the carbonyl absorption

here is  carboxylic acid

and here is an alcohol