How IR Spectroscopy Works
A complete guide to reading infrared spectra and identifying functional groups
What Is IR Spectroscopy?
Infrared spectroscopy measures how a molecule absorbs infrared light. When IR radiation passes through a sample, certain frequencies are absorbed because they match the natural vibration frequencies of specific bonds in the molecule.
The result is a spectrum - a graph showing which frequencies were absorbed and how strongly. Since different functional groups absorb at characteristic frequencies, IR spectroscopy can tell you what functional groups are present in an unknown molecule.
Why it matters
IR is usually the first spectroscopic technique taught in organic chemistry because it quickly answers the most basic structural question: what types of bonds does this molecule have? Combined with NMR and mass spectrometry, it helps determine complete molecular structures.
The spectra in Symmetria are real experimental data from the NIST Chemistry WebBook, the same reference database used by researchers and instrument manufacturers.
Molecular Vibrations
Bonds between atoms are not rigid - they behave like springs that constantly vibrate. A molecule with N atoms has 3N - 6 vibrational modes (or 3N - 5 for linear molecules). Each mode has a specific frequency.
Stretching vibrations
The bond length changes - atoms move closer together and farther apart along the bond axis. Stretching modes appear at higher frequencies (right side of the spectrum).
Symmetric stretch: both bonds stretch in phase. Asymmetric stretch: one bond stretches while the other compresses.
Bending vibrations
The bond angle changes rather than the bond length. Bending requires less energy than stretching, so bending modes appear at lower frequencies.
Types include scissoring (in-plane), rocking, wagging, and twisting (out-of-plane).
Symmetric stretch
Asymmetric stretch
Scissoring
in-plane
Rocking
in-plane
Wagging
out-of-plane
Twisting
out-of-plane
Arrows show atom displacement direction. For out-of-plane modes: ⊙ = toward viewer, ⊗ = away from viewer.
For a vibration to absorb IR light, it must cause a change in the dipole moment of the molecule. Symmetric molecules like O=C=O have a symmetric stretch that does not change the dipole moment - this mode is IR inactive. But the asymmetric stretch and bending modes do change the dipole, so they appear in the spectrum.
Reading the Axes
X-axis: Wavenumber (cm⁻¹)
Wavenumber is the reciprocal of wavelength: v̄ = 1/λ. It is directly proportional to frequency and energy. Higher wavenumber = higher energy = stronger bonds. The scale runs from 4000 (left) to 400 (right) - note that it decreases left to right, the opposite of most graphs.
Y-axis: % Transmittance
Transmittance measures how much light passes through the sample. 100% = no absorption (all light passes through). 0% = complete absorption. Dips downward represent absorptions. The deeper the dip, the stronger the absorption.
Why transmittance and not absorbance?
Most IR spectra are plotted in transmittance because peaks point downward, making it easy to scan across the baseline and spot absorptions. Some references use absorbance (where peaks point up), related by A = -log(T). The underlying data is the same; it is just flipped.
The Four Diagnostic Regions
The IR spectrum is conventionally divided into four regions. Learning these regions lets you quickly narrow down what functional groups might be present.
Ethanol IR Spectrum - Four Diagnostic Regions
CH₃CH₂OH
4000-2500 cm⁻¹: X-H Stretches
This is where bonds to hydrogen stretch. O-H appears as a broad absorption (hydrogen bonding spreads it out). N-H is medium width. C-H is sharp. The crucial 3000 cm⁻¹ boundary separates sp2 C-H (above) from sp3 C-H (below).
2500-2000 cm⁻¹: Triple Bonds
C≡C (alkynes) and C≡N (nitriles) absorb here. These are relatively rare in simple organic molecules, so this region is often empty. A peak here is very diagnostic.
2000-1500 cm⁻¹: Double Bonds
C=O is the star of this region - it produces one of the strongest, sharpest peaks in any IR spectrum. C=C (alkenes and aromatics) also absorbs here but more weakly. The exact C=O position (1715 vs 1740 vs 1660 cm⁻¹) tells you the specific carbonyl type.
1500-400 cm⁻¹: Fingerprint Region
C-O, C-N stretches and various bending modes overlap here. This region is complex and molecule-specific - hence "fingerprint." While individual peaks are hard to assign, the overall pattern can confirm a specific compound by comparison with a reference spectrum.
Key Functional Group Absorptions
These are the absorptions you should memorize. Knowing these lets you interpret most IR spectra encountered in an undergraduate course.
| Group | Range (cm⁻¹) | Appearance | Notes |
|---|---|---|---|
| O-H (alcohol) | 3200-3550 | Broad, strong | Breadth from H-bonding |
| O-H (acid) | 2500-3300 | Very broad | Overlaps C-H region |
| N-H | 3300-3500 | Medium, 1-2 peaks | 1 peak = secondary amine, 2 = primary |
| C-H (sp3) | 2850-2960 | Sharp, medium | Below 3000 cm⁻¹ |
| C-H (sp2) | 3020-3100 | Sharp, weak | Above 3000 cm⁻¹ |
| Aldehyde C-H | 2720, 2820 | Two weak peaks | Fermi resonance doublet |
| C=O (ketone) | ~1715 | Strong, sharp | The reference carbonyl |
| C=O (aldehyde) | ~1725 | Strong, sharp | Slightly higher than ketone |
| C=O (ester) | ~1740 | Strong, sharp | Highest common carbonyl |
| C=O (acid) | ~1710 | Strong, sharp | Look for broad O-H too |
| C=C (aromatic) | ~1500, ~1600 | Medium, pair | Two peaks = aromatic ring |
| C-O | 1000-1300 | Strong | Alcohols, ethers, esters |
The Carbonyl Position Rule
The exact position of the C=O stretch tells you what type of carbonyl you have. The differences are small (25-30 cm⁻¹) but consistent and diagnostic.
C=O frequencies (approximate)
The trend follows a principle: electron-donating groups lower the C=O frequency (resonance weakens the bond), while electron-withdrawing groups raise it (induction strengthens the bond). Conjugation with a C=C or aromatic ring also lowers the frequency by about 20-30 cm⁻¹.
Hydrogen Bonding and Peak Shape
Hydrogen bonding has a dramatic effect on IR spectra. When an O-H or N-H group forms hydrogen bonds with neighboring molecules, the bond strength varies continuously depending on the H-bond distance.
This spread of bond strengths means the absorption occurs over a wide range of frequencies rather than at one sharp frequency. The result is the characteristic broad absorption.
Alcohol O-H
Moderately broad (3200-3550 cm⁻¹). The hydrogen bonds are moderate strength. In dilute solution or gas phase, the peak sharpens because fewer H-bonds form.
Carboxylic acid O-H
Extremely broad (2500-3300 cm⁻¹). Carboxylic acids form very strong cyclic dimers through double hydrogen bonds, creating the widest O-H absorption in organic chemistry. Often overlaps with and obscures C-H stretches.
The 3000 cm⁻¹ Rule
One of the most useful quick rules in IR: C-H stretches above 3000 cm⁻¹ indicate sp2 or sp hybridization (aromatic, alkene, alkyne), while C-H stretches below 3000 cm⁻¹ indicate sp3 (alkane).
This works because sp2 C-H bonds have more s-character, making them shorter and stiffer, which shifts the vibration to higher frequency. A molecule like toluene shows C-H peaks on both sides of 3000 cm⁻¹ because it has both aromatic (sp2) and methyl (sp3) C-H bonds.
Reading by Absence
One of the most powerful IR interpretation strategies is noticing what is NOT there. Every absent absorption rules out entire classes of compounds:
Putting It Together: Common Patterns
With practice, you will recognize functional group "fingerprints" - combinations of peaks that reliably identify a compound class.
Alcohol
Broad O-H (3200-3550) + strong C-O (1000-1100) + no C=O. Primary at ~1050, secondary at ~1100, tertiary at ~1150 cm⁻¹.
Carboxylic acid
Very broad O-H (2500-3300) + C=O (~1710). The O-H is so broad it overlaps the C-H region. This combination is unmistakable.
Ester
C=O at ~1740 (higher than ketone) + two strong C-O stretches (~1240 and ~1040). No O-H. The high C=O + paired C-O is the ester signature.
Aldehyde
C=O at ~1725 + two weak peaks at 2720 and 2820 cm⁻¹ (Fermi resonance doublet). The doublet is unique to aldehydes - no other carbonyl shows it.
Aromatic compound
C-H above 3000 cm⁻¹ + C=C pair at ~1500 and ~1600 cm⁻¹ + strong out-of-plane C-H bending below 900 cm⁻¹. The substitution pattern determines the exact position of the out-of-plane bends.
Ether
Strong C-O-C stretch (~1100) but no O-H and no C=O. An oxygen-containing molecule with neither O-H nor C=O must be an ether.
A Strategy for Interpreting Unknown Spectra
When you encounter an unknown IR spectrum, follow this systematic approach:
- 1. Check 3200-3600 cm⁻¹: Is there a broad O-H or N-H? If broad, is it alcohol-broad or acid-broad?
- 2. Check 3000 cm⁻¹ boundary: Are there C-H peaks above 3000 (sp2) or only below (sp3)?
- 3. Check 1650-1800 cm⁻¹: Is there a C=O? If yes, note the exact position to identify the carbonyl type.
- 4. Check 1500-1600 cm⁻¹: Are there aromatic C=C ring stretches?
- 5. Check the fingerprint: Any strong C-O stretches (1000-1300)? Ester C-O pair?
- 6. Note absences: What expected peaks are missing? Each absence eliminates possibilities.
Ready to practice?
Apply what you have learned by interpreting real IR spectra with interactive 3D molecules.