Part 1 of 6Principle of Infrared Spectroscopy
Covalent bonds behave like springs that vibrate at set frequencies.
1
Infrared radiation is passed through the sample and bonds absorb radiation that matches the frequency of their vibrations.
2
Absorption occurs only if the vibration changes the dipole of the bond, so polar bonds give strong peaks.
3
The detector measures the radiation transmitted, so absorption appears as a downward peak on a plot of transmittance.
4
The horizontal axis shows wavenumber in cm⁻¹, which is proportional to the frequency of the radiation.
5
Different bonds absorb at different wavenumbers, so the peaks reveal which functional groups are present.
Part 2 of 6Absorption Data
The table below gives typical wavenumber ranges taken from standard data.
Alcohols
- A broad absorption near 3300 cm⁻¹ shows a hydrogen-bonded O–H group.
Carboxylic acids
- They show a very broad O–H band and a C=O band, so both features together point to –COOH.
Carbonyls
- A strong, sharp peak between 1680 and 1750 cm⁻¹ indicates C=O in aldehydes, ketones, acids and esters.
Absence of peaks
- The lack of a broad O–H band and of C=O helps to rule out alcohols, acids and carbonyl compounds.
| Bond | Wavenumber range / cm⁻¹ | Appearance |
|---|---|---|
| O–H (alcohol) | 3230–3550 | Broad |
| O–H (carboxylic acid) | 2500–3000 | Very broad |
| C=O | 1680–1750 | Strong, sharp |
| C=C | 1620–1680 | Medium |
| C–O | 1000–1300 | Strong |
| C≡N | 2220–2260 | Medium |
Part 3 of 6Fingerprint Region
Fingerprint Region
Location
- The region below about 1500 cm⁻¹ contains many overlapping peaks from complex vibrations of the whole molecule.
Uniqueness
- Each compound has its own pattern there, so two isomers can have the same functional group peaks but different fingerprints.
Identification
- An unknown spectrum is compared with a database spectrum, and an exact match confirms identity.
Purity
- Extra peaks that do not belong to the known spectrum can show that a sample is contaminated.
Part 4 of 6Infrared and Global Warming
Infrared and Global Warming
Greenhouse effect
- The Earth’s surface re-emits absorbed energy as infrared radiation, and some of it is absorbed by gases in the atmosphere.
Absorbing gases
- Carbon dioxide (C=O), methane (C–H) and water vapour (O–H) all have bonds that absorb infrared radiation.
Warming
- The absorbed energy is re-radiated in all directions, so less escapes to space and the lower atmosphere warms.
Human impact
- Burning fuels increases CO₂ and agriculture increases methane, strengthening the effect and contributing to global warming.
Part 5 of 6Worked Example
Question
- A compound with formula C₃H₆O₂ shows a very broad absorption from 2500 to 3000 cm⁻¹ and a strong sharp peak at 1710 cm⁻¹.
- Identify the functional group and one possible compound.
- The peak at 1710 cm⁻¹ lies in the C=O range 1680–1750 cm⁻¹, so a carbonyl group is present.
- A very broad band over 2500–3000 cm⁻¹ is typical of the O–H group in a carboxylic acid.
- Together these show a –COOH group.
- A compound with formula C₃H₆O₂ containing –COOH is propanoic acid, CH₃CH₂COOH.
Answer
- The compound contains a carboxylic acid group;
- it could be propanoic acid.
Part 6 of 6Exam Checklist & Tips
3.6.4 – Key Points for the Exam
- Wavenumber The reciprocal of wavelength, measured in cm⁻¹, used to describe infrared absorptions.
- Transmittance The fraction or percentage of radiation passing through the sample.
- Fingerprint region The part of an infrared spectrum below about 1500 cm⁻¹ that is unique to each molecule.
- Greenhouse gas A gas that absorbs infrared radiation in the atmosphere.
Common Mistakes
- Reading the broad O–H peak of an alcohol as a sharp C=O peak, or the reverse.
- Using infrared alone to name an unknown compound, when it only shows the groups present.
- Stating that the fingerprint region identifies functional groups;
- it identifies the whole molecule by matching.
- Saying greenhouse gases absorb visible light rather than infrared radiation.