Understanding infrared spectroscopy

Atoms, molecules and ions can absorb (or emit) electromagnetic radiation of specific frequencies, and this can be used to identify them.

Electromagnetic radiation absorbed

What the energy is used for

Spectroscopy technique

Ultra-violet/visibleMovement of electrons to higher energy levels

Ultra-violet / visible spectroscopy

Infrared To vibrate bondsInfrared

spectroscopy

Microwaves To rotate moleculesMicrowave

spectroscopy

Radio waves To change nuclear spin NMR spectroscopy

Spectroscopy

All chemical bonds vibrate at a characteristic frequency.

There are different types of vibration.

The frequency depends on the mass of the atoms in the bond, the bond strength and the type of vibration.

The frequencies at which they vibrate are in the infrared region of the electromagnetic spectrum.

Only molecules that change their polarity as they vibrate absorb infrared radiation.

Symmetric stretch

Asymmetric stretch

Bending

Infrared spectroscopy

In infrared spectrometry, infrared radiation is passed through the sample, which absorbs some or all of the radiation at the frequencies at which its bonds vibrate.

The rest of the radiation is transmitted and arrives at the detector.

The detector displays the percentage transmittance of each frequency.

The frequencies of radiation involved in bond vibration are 12–120 THz, which corresponds to 2.5–25 µm in wavelength.

However, rather than use either THz or µm when displaying infrared spectra, chemists use a unit called wavenumber.

A wavelength of 2.5 µm is equal to 4000 cm–1, while 25 µm is equal to 400 cm–1.

Notice that wavenumber goes down as wavelength increases. IR spectra are displayed with wavenumber on the horizontal axis running from right to left.

1

(cm)Wavenumber

wavelength

Scale and units

The horizontal axis shows wavenumber from right to left.

The vertical axis shows percentage transmittance.

This spectrum shows a strong peak at 3000–2800 cm–

1, indicating that almost all radiation of this wavenumber was absorbed, with very little left to transmit.

Reading the spectrum

Strong

Medium

The deepest peaks are described as strong,

those in the middle band are called medium,

and those whose transmittance is below 35% are described as weak.

Weak

The region to the right of 1500 cm–1 is called the fingerprint region because its complex pattern of peaks is unique for each compound.

Chemists can confirm the identity of a compound by comparing its IR spectrum with that of a known sample of the compound.

The most important region of an IR spectrum is the region 4000–1500 cm–1, where peaks characteristic of important functional groups appear.

C–H stretch

C=O stretch

This spectrum shows three strong peaks:

3000–2800 cm–1 is from C–H stretch.

1700–1800 cm–1 is from C=O stretch.

~1200 cm–1 is from C–O stretch.

This compound is an ester.

C–O stretch

A broad peak at 3500–3300 cm–1 is from O–H stretch.

It is broad because the molecular motion is modified by varying amounts of hydrogen bonding, causing different O–H bonds in the sample to absorb slightly different frequencies of radiation.

What type of compound must this be? An alcohol.

C–H stretch

O–H stretch

You will learn to recognise the characteristic peaks from the other important functional groups when you work through the practical for this topic.

In your exam you will be given a table of molecular motions and wavenumbers, but it is helpful to recognise the shapes of the peaks as well as their wavenumbers.