Infrared Spectroscopy
A powerful analytical technique that exploits the interaction between infrared radiation and molecular vibrations to identify functional groups, confirm structures, and monitor environmental pollutants.
Infrared spectroscopy is based on the absorption of IR radiation by covalent bonds whose natural vibrational frequencies match the frequency of the incident radiation.
What is Infrared Radiation?
IR radiation spans wavelengths 0.7–100 µm. The "mid-IR" region (2.5–16 µm or 4000–625 cm⁻¹) is most useful for organic structural analysis, as these frequencies match molecular vibrational frequencies.
Wavenumber ν̃ (nu bar)
Wavenumber is the number of wave cycles per centimetre: ν̃ = 1/λ(cm) = 10⁴/λ(µm), units are cm⁻¹. Wavenumber is directly proportional to frequency and energy. Higher wavenumber = higher frequency = higher energy.
Ball-and-Spring Model
Atoms in a molecule are like balls of varying mass connected by springs of varying stiffness (the bonds). Each bond has a natural vibration frequency. When IR frequency equals this natural frequency, the molecule absorbs energy and vibration amplitude increases.
The Dipole Requirement
For a bond to absorb IR radiation, its vibration must cause a change in the dipole moment. Symmetric homonuclear bonds (H–H, O=O, N≡N) do not absorb IR because their vibrations produce no change in the electrical dipole.
Frequency and Bond Strength
Vibrational frequency is directly proportional to bond strength and inversely proportional to atomic mass. So: C≡C > C=C > C–C and C–H > C–C (H is lighter than C).
Energy Absorption
When νIR = νvibration, the molecule absorbs a photon. The molecule transitions from its ground vibrational state to an excited state. The energy increases the amplitude of vibration. As it returns to the ground state, heat is released.
| Bond Type | Wavenumber (cm⁻¹) | Reason |
|---|---|---|
| O–H, N–H, C–H (X–H bonds) | 2700–3700 | H is very light (mass=1) → high frequency |
| C≡C, C≡N (triple bonds) | 2100–2260 | High bond strength → high frequency |
| C=O carbonyl (double bond) | 1650–1850 | Strong bond + low combined mass |
| C=C, C=N (double bonds) | 1500–1700 | Moderate strength |
| C–C, C–O, C–N (single bonds) | 800–1300 | Weak bond + heavier atoms = low frequency |
Container and window materials must be IR-transparent — they must not absorb IR radiation. Glass is unsuitable. Common materials: NaCl, KBr, CCl₄, and CHCl₃.
🔷 KBr Discs (Solids)
Grind 1–2 mg of solid with ~100× excess dry KBr using an agate mortar and pestle. Compress in a steel die under high hydraulic pressure (~10 tonnes) to form a transparent pellet (~0.5 cm diameter). Mount in the IR beam on a disc holder.
🟡 Nujol Mulls (Solids)
Grind ~1 mg of solid with a few drops of Nujol (mineral oil/paraffin) to form a smooth paste. Spread between two NaCl plates. Note: Nujol's own C–H peaks (~2900, 1460, 1380 cm⁻¹) must be accounted for in interpretation.
💧 Liquid Samples
Place a thin film of pure liquid between two NaCl plates. The plates sit in a plate holder in the IR beam. No solvent needed for pure liquids. Path length is adjusted by changing the film thickness.
🧪 Solutions
Prepare a 1–5% (w/v) solution in an IR-transparent solvent: CHCl₃ or CCl₄. Place in a solution cell (NaCl windows, 0.1–1 mm path). Solvent absorptions can be electronically subtracted using a reference cell containing pure solvent.
💨 Gas Samples
Introduce gas into a long gas cell (~10 cm) with NaCl windows. Gas concentration at atmospheric pressure is low, so long path length is needed. Used for environmental monitoring of CO₂, CO, SO₂ in air samples.
⚡ Modern FT-IR (ATR)
Modern Fourier Transform IR (FT-IR) with Attenuated Total Reflectance (ATR) requires minimal sample prep. Place the solid or liquid directly on a diamond/ZnSe crystal. The beam undergoes total internal reflection, sampling the surface. Fast, non-destructive, and very sensitive.
⚠️ Non-polar Molecules
Symmetric homonuclear bonds (H–H, N≡N, O=O, Cl–Cl) cannot be detected by IR. Their vibrations produce no change in dipole moment, so they are IR-inactive. These require Raman spectroscopy instead.
⚠️ Ionic Compounds
Ionic substances lack covalent bonds and therefore show no characteristic molecular IR absorptions. They are unsuitable for structural analysis by IR.
⚠️ Structural Determination
IR alone cannot determine the complete molecular structure. It shows which functional groups are present, but not how the carbon skeleton is connected or the stereochemical arrangement.
⚠️ Water Interference
Water absorbs IR strongly due to its O–H stretches and H–O–H bending (~1640 cm⁻¹). This makes aqueous solutions difficult to analyse. Special techniques or dry, anhydrous samples are required.
✅ Fingerprint Matching
Overlaying two IR spectra allows exact compound identification: perfect matching of all peaks proves identical identity. The fingerprint region (1300–600 cm⁻¹) is unique to each molecule, like a molecular fingerprint.
✅ Used with Other Techniques
IR data is most powerful in conjunction with: mass spectrometry (M+, fragment ions), NMR (H and C environments), and elemental analysis. Together they allow unambiguous structure determination.
Systematic analysis uses both presence (positive evidence) and absence (negative evidence) of peaks. Key rule: always check 3600–3200 cm⁻¹ first (O–H / N–H), then 1850–1650 cm⁻¹ for C=O, then triple bond region.
| Group | Bond | Wavenumber cm⁻¹ | Appearance | Key Notes |
|---|---|---|---|---|
| Alcohol / Phenol | O–H stretch | 3200–3550 | Strong, very broad | Breadth = strong H-bonding; broadest single diagnostic peak |
| Carboxylic Acid | O–H stretch | 2500–3300 | Very broad, strong | Extends over C–H region; unmistakably broad |
| Primary Amine | N–H stretch | 3300–3500 | Two medium bands | Two bands = NH₂; one band = secondary NH |
| Terminal Alkyne | ≡C–H stretch | 3280–3330 | Sharp, strong | sp hybrid; highest C–H frequency possible |
| Alkyl C–H | C–H stretch (sp³) | 2850–2960 | Medium, sharp | Below 3000 cm⁻¹; present in virtually all organics |
| Aldehyde | C–H (Fermi doublet) | 2720 & 2820 | Two weak bands | Unique to aldehydes; Fermi resonance doublet |
| Alkyne | C≡C stretch | 2100–2260 | Medium (variable) | Absent in symmetrical internal alkynes |
| Nitrile | C≡N stretch | 2200–2260 | Medium–strong, sharp | Similar to C≡C but usually stronger; more polar |
| Acyl Halide | C=O stretch | 1780–1820 | Very strong, sharp | Highest C=O; inductive withdrawal by halogen |
| Ester | C=O stretch | 1730–1750 | Very strong, sharp | + two C–O bands; no O–H present |
| Aldehyde | C=O stretch | 1720–1740 | Very strong, sharp | + Fermi doublet 2720/2820 confirms aldehyde |
| Carboxylic Acid | C=O stretch | 1700–1725 | Strong, sharp | Combined with very broad O–H = definitive |
| Ketone | C=O stretch | 1680–1720 | Very strong, sharp | No O–H, no aldehyde C–H doublet |
| Amide | C=O (Amide I) | 1630–1690 | Strong | Lowest C=O; N lone pair resonance weakens C=O |
| Alkene | C=C stretch | 1610–1680 | Medium (variable) | Weak/absent in symmetrical alkenes |
| Primary Amide | N–H bend (Amide II) | 1550–1650 | Strong | Coupled N–H bend + C–N stretch |
| Alcohol | C–O stretch | 1000–1150 | Strong | Primary ~1050; secondary ~1120; tertiary ~1150 |
| Ester | C–O–C stretch | 1050 & 1240 | Strong, two bands | Two distinct C–O bands = diagnostic for esters |
| Alkyl Halide | C–Cl stretch | 550–800 | Strong | C–Br: 500–700; C–F: 1000–1350; heavy halogens shift lower |
CO₂ — Greenhouse Gas
CO₂ absorbs at 2349 cm⁻¹ (asymmetric stretch) and 667 cm⁻¹ (bending). Human activity (burning fossil fuels) increases atmospheric CO₂. IR spectroscopy provides continuous real-time monitoring of CO₂ concentration. CO₂ concentration has risen from ~280 ppm (pre-industrial) to over 420 ppm today.
SO₂ — Acid Rain
SO₂ absorbs at 1360 cm⁻¹ and 1151 cm⁻¹. Produced by burning S-containing fossil fuels and volcanic activity. Dissolves in rainwater to form sulfurous/sulfuric acid (acid rain). IR sensors in industrial stacks monitor emissions continuously.
CO — Toxic Pollutant
CO absorbs sharply at 2143 cm⁻¹ (C≡O stretch). Produced by incomplete combustion of fossil fuels. A colourless, odourless toxic gas that binds haemoglobin. Vehicle exhaust catalytic converters are monitored by IR sensors to check CO removal efficiency.
The Greenhouse Effect
The Sun emits ~53% IR, ~44% visible, ~3% UV. Visible light is absorbed by Earth's surface which then re-emits longer-wavelength IR upward. Greenhouse gases (CO₂, H₂O, CH₄, N₂O) absorb this outgoing IR, warming the atmosphere. Without any greenhouse effect, Earth's mean temperature would be ~−17°C instead of +17°C.
Why N₂ and O₂ Are Transparent
N₂ and O₂ make up 99% of the atmosphere but are symmetric homonuclear diatomic molecules. Their vibrations produce no change in dipole moment → they cannot absorb IR radiation → they contribute nothing to the greenhouse effect. Only polar bonds or asymmetric vibrations absorb IR.
Monitoring Method
Polluted air is drawn through a long-path gas cell. A beam of IR radiation passes simultaneously through the sample cell and a clean-air reference cell. Any decrease in transmitted intensity at a characteristic wavenumber indicates the presence of a specific pollutant. Multiple pollutants can be detected simultaneously.
The C=O stretch is the strongest, sharpest, and most reliably diagnostic absorption in IR spectroscopy. Its exact wavenumber reveals the identity of the carbonyl functional group.
| Compound Type | C=O (cm⁻¹) | Reason for Position | Other Key Peaks |
|---|---|---|---|
| Acyl Halide (–COCl) | 1780–1820 | Inductive withdrawal by Cl raises C=O frequency | C–Cl ~700–800 cm⁻¹ |
| Ester (–COOR) | 1730–1750 | Inductive effect of O (inductive > resonance) | Two C–O bands: ~1240 and ~1050 cm⁻¹ |
| Aldehyde (–CHO) | 1720–1740 | One alkyl group; less donation than ketone | Fermi doublet at 2720 and 2820 cm⁻¹ |
| Carboxylic Acid (–COOH) | 1700–1725 | H-bonded dimer; resonance weakens C=O | Very broad O–H: 2500–3300 cm⁻¹ |
| Ketone (–CO–) | 1680–1720 | Two electron-donating alkyl groups lower C=O | No O–H; no Fermi doublet |
| Amide (–CONH₂) | 1630–1690 | N lone pair donates into C=O by resonance → lowest | Amide I + II bands; N–H at 3150–3370 cm⁻¹ |
Explore IR Spectra Interactively
Select a compound, then hover over spectrum peaks to identify bonds, or click a bond in the molecular structure to highlight its absorption in the spectrum. Bidirectional bond–spectrum interaction.
Types of Molecular Vibrations
Only vibrations that change the dipole moment of the molecule are IR-active. Understand each mode to correctly predict and interpret absorption bands.
All molecular vibrations fall into two categories: stretching (change in bond length along the bond axis) and bending (change in bond angle). A non-linear molecule with n atoms has 3n − 6 vibrational modes; a linear molecule has 3n − 5. Stretching vibrations occur at higher wavenumber than bending vibrations of the same bond because stretching requires more energy.
IR Spectroscopy Quiz
12 questions covering all CAPE Chemistry learning objectives for IR Spectroscopy