Infrared spectroscopy

Atoms in molecules are joined by covalent bonds. How do these covalent bonds work?

Covalent bonds possess energy and vibrate naturally about a central point

• The amount of vibration increases with increasing temperature

How does infrared (IR) radiation affect molecules?

Each bond vibrates at its own frequency

• All molecules absorb IR radiation, making them bend or stretch more

Describe the 2 types of vibrations

Stretch

• A rhythmic movement along the line between the atoms so that the distance between the two atomic centre increases and decreases

Bend

• Results in a change in bond angle

What does the amount of vibration depend on?

• Bond strength - stronger bonds vibrate faster than weaker bonds

• Mass of each atom in the bond - heavier atoms vibrate more slowly than lighter atoms

• Bond length -

How does the Sun’s visible and IR radiation work?

The radiation passes through the atmosphere to the Earth’s surface, where most of it is absorbed

• Some is re-emitted from the Earth’s surface in the form of longer-wavelength IR radiation

Name the 3 most abundant greenhouse gases

Water vapour, carbon dioxide, methane

How do greenhouse gases contribute to climate change?

Greenhouse gases absorb the longer-wavelength IR radiation, because it has the same frequency as the natural frequency of their bonds

• The vibrating bonds in these molecules re-emit this energy as radiation which increases the temperature of the atmosphere

What is infared spectroscopy used for?

Identifying the functional groups present in organic molecules

How does infrared spectroscopy work?

1. The sample under investigation is placed inside an IR spectrometer

2. A beam of IR radiation in the range 200-400 cm^-1 is passed through the sample

3. The molecule absorbs some of the IR frequencies, and the emerging beam of radiation is analysed to identify the frequencies that have been absorbed by the sample

4. The IR spectrometer is usually connected to a computer that plots a grpah of transmittance against wavenumber

What are the ‘peaks’ on an IR spectrum?

Peaks - dips in the graph

• Each peak is observed at a wavenumber that can be related to a particular bond in the molecule

When interpreting IR spectra what do you need to remember when identifying the bond?

Identify both the bond and the functional group it belongs to

• E.g. OH bond in an alcohol

What should be remembered when looking at peaks between 2850 and 3100 cm^-1?

All organic compounds produce a characteristic peak between 2850 and 3100

• This is not to be confused with O-H peak in alcohols

State the most likely type of compound of this infrared spectrum

Alcohol

• Absorbance peak within the range 3200-3600cm^-1, indicating O-H bond in alcohol

• Peak between 1000-1300cm^-1, indicating C-O bond

State the type of compound of the infrared spectrum

Aldehyde / ketone

• Key absorbance between 1630-1820 caused by the C=O bond

Show what an infrared spectrum for a carboxylic acid would look like

Give some applications of IR spectroscopy

• Identifying functional groups present in unknown substances

• Monitoring pollution by analysing IR spectra of vehicle emissions

• IR-based breathalysers to measure alcohols

• Quality control in perfume manufacture

• Drug analysis - screening and identification of drugs

• Forensics - identification of fibres and paint chips in forensic analysis

When determining the structure of an organic molecule, what is a typical sequence for identification?

1. Elemental analysis

   • Use of percentage composition data to determine the empirical data

2. Mass spectrometry

   • Use of the molecular ion peak from a mass spectrum to determine the molecular mass; use of fragment ions to identify sections of a molecule

3. Infrared spectroscopy

   • Use of absorption peals from an infrared spectrum to identify bonds and functional groups present in a molecule

Once you have the empirical formula and the molecular mass of a compound, you can determine the molecular formula of your unknown compound. By using evidence from the infrared spectrum, it may be possible to identify an unknown compound