Infrared spectroscopy is a technique used to identify chemical bonds by measuring which infrared frequencies a sample absorbs. It matters because different bonds vibrate at characteristic frequencies, so an IR spectrum acts like a molecular clue sheet. Chemists use IR spectra to recognize functional groups, compare unknowns with known compounds, and monitor reactions.
The graph is usually plotted as percent transmittance versus wavenumber, with wavenumber decreasing from 4000 to 400 cm^-1 from left to right.
Understanding Chemistry: Infrared (IR) Spectroscopy
A molecule does not stay perfectly still. Its atoms move in repeated patterns, rather like balls joined by tiny springs. Some motions pull atoms farther apart and push them back together.
These are stretching vibrations. Other motions change bond angles. These are bending vibrations.
A vibration absorbs infrared light only if the motion changes how electrical charge is distributed across the molecule. This rule explains why some bonds give clear signals while others are weak or missing. For example, a bond between two identical atoms has no permanent charge imbalance, so its simplest vibration is often invisible in an ordinary infrared spectrum.
The position of a band depends on more than the name of a bond. A strong, stiff bond needs more energy to stretch than a weaker bond. Atoms with greater mass move more slowly, which shifts vibrations to lower values.
Nearby atoms can affect each other too. A double bond next to another double bond may vibrate differently from an isolated double bond. The shape of a band gives clues as well.
An oxygen-hydrogen stretch is often broad because hydrogen bonding creates a range of slightly different bond strengths. A carbonyl stretch is commonly narrow and strong. Band intensity depends on how much the charge distribution changes during the vibration, so a large peak does not simply mean there is more of that bond.
Reading a spectrum is a process of collecting evidence rather than matching one peak to one compound. Students usually begin with the most distinctive high value bands, then check whether expected bands are present or absent. A compound thought to be an alcohol should show evidence for an oxygen-hydrogen bond.
A compound thought to contain a carbonyl group should show its characteristic strong absorption. The lower part of the spectrum contains many overlapping motions from the whole molecular framework.
It can be hard to interpret alone, but it is very useful when comparing an unknown sample with a reference. In many classroom plots, absorption bands point downward because less light passes through the sample at those positions.
Infrared spectroscopy appears in practical work whenever chemists need a quick check on a material. A student might use it after a reaction to see whether a starting group has disappeared or a new group has formed. In industry, it can help check polymers, fuels, medicines, paints, or food ingredients.
Samples may be examined as solids, liquids, films, or gases. Water vapour and carbon dioxide in air can add unwanted bands, so clean handling matters.
Modern instruments often use a crystal pressed against the sample, which makes testing easier, but the result still needs careful judgment. Infrared evidence is strongest when it is combined with the sample history, simple chemical tests, and other methods such as mass spectrometry or nuclear magnetic resonance.
Key Facts
- IR absorption occurs when infrared radiation matches a molecular vibration that changes the molecule's dipole moment.
- Wavenumber is proportional to energy: E = hcν~, where ν~ is wavenumber in cm^-1.
- Frequency and wavenumber are related by ν = cν~, using consistent units.
- Stronger bonds vibrate at higher wavenumber, and heavier atoms vibrate at lower wavenumber.
- The functional group region is about 4000 to 1500 cm^-1 and contains many diagnostic bond stretches.
- The fingerprint region is about 1500 to 400 cm^-1 and is useful for matching an unknown spectrum to a known compound.
Vocabulary
- Infrared spectroscopy
- A method that measures absorption of infrared light to identify molecular vibrations and functional groups.
- Wavenumber
- The number of wave cycles per centimeter, commonly used as the x-axis unit in IR spectra.
- Transmittance
- The percentage of infrared light that passes through a sample without being absorbed.
- Functional group region
- The part of an IR spectrum from about 4000 to 1500 cm^-1 where many important bond stretches appear.
- Fingerprint region
- The complex part of an IR spectrum from about 1500 to 400 cm^-1 that helps distinguish one compound from another.
Common Mistakes to Avoid
- Reading the IR x-axis from left to right as increasing wavenumber is wrong because standard IR spectra decrease from about 4000 to 400 cm^-1.
- Assuming every bond vibration gives a strong IR peak is wrong because a vibration must change dipole moment to absorb IR strongly.
- Identifying a compound from one peak alone is unreliable because many functional groups overlap and the full pattern should be considered.
- Confusing transmittance peaks with absorption peaks is wrong because in a transmittance spectrum, strong absorption appears as a downward dip.
Practice Questions
- 1 An IR absorption occurs at 1700 cm^-1. Using E = hcν~, calculate the energy of one photon in joules. Use h = 6.63 x 10^-34 J s and c = 3.00 x 10^10 cm/s.
- 2 A spectrum shows a broad band near 3300 cm^-1 and a strong sharp band near 1710 cm^-1. Which two functional groups are suggested by these absorptions?
- 3 Two compounds have similar peaks above 1500 cm^-1 but very different patterns between 1500 and 400 cm^-1. Explain why the fingerprint region can help tell them apart.