IR Spectra — Lecture Notes & Visualizer
Infrared (IR) spectroscopy is an analytical technique that measures how molecules absorb infrared radiation (wavelength ~2.5–25 µm, or 400–4000 cm⁻¹). The result is an IR spectrum: a plot of %Transmittance (or Absorbance) versus wavenumber (cm⁻¹), where dips (peaks) indicate wavelengths at which the molecule absorbs IR energy.
IR absorption occurs when the frequency of the incident IR radiation matches the natural vibrational frequency of a bond, and the vibration causes a change in the molecule's dipole moment. Bonds that are symmetrical (e.g., O₂, N₂) do not absorb IR because they lack a dipole moment change.
Two fundamental types of molecular vibrations give rise to IR peaks:
| Vibration Type | Description |
|---|---|
| Stretching | Bond length changes (symmetric or asymmetric) |
| Bending | Bond angle changes (scissoring, rocking, wagging, twisting) |
| Factor | Effect on Frequency |
|---|---|
| Bond strength ↑ | Frequency ↑ (stiffer spring) |
| Atomic mass ↑ | Frequency ↓ (heavier pendulum) |
This follows Hooke's Law: ν̃ ∝ √(k / µ), where k is the force constant (bond stiffness) and µ is the reduced mass of the two bonded atoms.
IR spectroscopy is fast, non-destructive, and requires only a small sample. It is widely used for:
The spectrum is divided into two key regions:
| Region | Wavenumber (cm⁻¹) | Key Information |
|---|---|---|
| Functional Group Region | 4000 – 1500 | Characteristic stretches: O-H, N-H, C-H, C=O, C=C, C≡N, C≡C |
| Fingerprint Region | 1500 – 400 | Complex bending modes unique to each molecule; used for identity confirmation |
Modern instruments use Fourier Transform IR (FTIR) spectroscopy. A Michelson interferometer splits the IR beam, creates an interferogram (intensity vs. mirror position), and applies a Fourier transform to yield the spectrum across all frequencies simultaneously. FTIR is faster and more sensitive than older dispersive instruments.
Always start with the functional group region (4000–1500 cm⁻¹) and look for strong, characteristic absorptions before examining the fingerprint region. A systematic checklist prevents missed assignments.
1. Is there a broad O-H or N-H peak above 3000 cm⁻¹?
2. Is there a C=O peak near 1700 cm⁻¹? If yes, what type?
3. Are there C-H stretches above/below 3000 cm⁻¹ (sp² vs. sp³)?
4. Are there C≡C or C≡N triple-bond stretches near 2100–2260 cm⁻¹?
5. Use the fingerprint region to confirm identity by comparison.
| Wavenumber (cm⁻¹) | Bond / Group | Compound Class | Notes |
|---|---|---|---|
| 3200–3550 | O-H stretch | Alcohols, phenols | Broad; H-bonded. Sharp if dilute/free. |
| 2500–3300 | O-H stretch | Carboxylic acids | Very broad; overlaps C-H region |
| 3300–3500 (×2) | N-H stretch | Primary amines | Two peaks (asym. + sym.) |
| ~3310 | N-H stretch | Secondary amines | Single broad peak |
| 3000–3100 | =C-H stretch | Alkenes, aromatics | Just above 3000 cm⁻¹ |
| 2850–2960 | C-H stretch | Alkanes, most organics | Below 3000 cm⁻¹; usually 2–3 bands |
| 2700–2850 | C-H stretch (Fermi) | Aldehydes | Two bands; diagnostic for –CHO |
| 2100–2260 | C≡C / C≡N stretch | Alkynes, nitriles | Medium or absent if symmetric |
| 1700–1725 | C=O stretch | Ketones, aldehydes | Strong, sharp |
| 1700–1725 | C=O stretch | Carboxylic acids | Paired with broad O-H |
| 1730–1750 | C=O stretch | Esters | Slightly higher than ketone |
| 1630–1690 | C=O stretch | Amides | Lower due to resonance (Amide I) |
| 1620–1680 | C=C stretch | Alkenes | Medium; absent if symmetric |
| 1550–1610 | N-H bend | Primary amines | Medium intensity |
| 1475–1600 | C=C ring stretch | Aromatics | Two bands ~1500, ~1600 cm⁻¹ |
| 1000–1260 | C-O stretch | Alcohols, esters, ethers | Strong; position varies with substitution |
| 690–900 | =C-H oop bend | Alkenes, aromatics | Pattern reveals substitution type |
The carbonyl stretch is the single most informative peak in organic IR spectroscopy. Subtle shifts in its position identify the compound class:
| C=O Type | Approx. Frequency (cm⁻¹) | Explanation |
|---|---|---|
| Acid chlorides | ~1800 | Electron-withdrawal by Cl raises C=O |
| Anhydrides | ~1820 + ~1760 | Coupling gives two bands |
| Esters | 1730–1750 | Partial O→C donation lowers slightly |
| Aldehydes | 1720–1740 | Also shows Fermi doublet at 2700–2850 |
| Ketones | 1705–1725 | Benchmark reference value |
| Carboxylic acids | 1700–1725 | Paired with very broad O-H |
| Amides | 1630–1680 | Resonance delocalizes C=O → lower ν |
| Conjugated C=O | ~20–40 lower | Resonance with C=C reduces bond order |
The shape and position of X-H stretches above 3000 cm⁻¹ carry diagnostic power:
Carboxylic acid O-H: Extremely broad (2500–3300 cm⁻¹), often obscuring C-H peaks. Due to strong dimeric H-bonding.
Primary amine N-H: Two peaks (asymmetric + symmetric stretch), typically 3300–3500 cm⁻¹. Weaker than O-H.
Secondary amine N-H: Single peak around 3310 cm⁻¹.
The position of C-H stretches relative to 3000 cm⁻¹ immediately reveals hybridization:
| C-H Type | Range (cm⁻¹) | Hybridization |
|---|---|---|
| Aromatic/vinyl =C-H | 3000–3100 | sp² (above 3000) |
| Alkyne ≡C-H | ~3300 | sp (sharp) |
| Alkyl C-H (CH₃, CH₂, CH) | 2850–2960 | sp³ (below 3000) |
| Aldehyde -CHO | 2700–2850 | sp² (Fermi doublet) |
IR spectroscopy is a primary tool for structural confirmation in synthetic and analytical chemistry. A chemist who synthesizes a new compound can compare the IR spectrum with a reference spectrum (e.g., from the SDBS or NIST databases) to verify identity, or use the functional group peaks to confirm the expected structure.
IR can track the progress of a reaction in real time, particularly using in situ FTIR probes immersed in the reaction mixture. Chemists watch a carbonyl peak appear or disappear to know when conversion is complete.
The pharmaceutical industry uses IR to verify the identity and purity of raw materials, intermediates, and finished drug products. Regulatory agencies (FDA, EMA) accept IR fingerprint comparison as a standard identity test. A match between sample and reference spectra — especially in the fingerprint region — confirms the correct material.
FTIR is also used to detect polymorphic forms of drugs, since different crystal structures of the same compound can show subtle but reproducible IR differences.
IR spectroscopy characterizes polymers by identifying backbone and side-chain functional groups. It can detect oxidation (carbonyl formation in aged polyethylene), measure the degree of crosslinking, or identify unknown plastics for recycling classification.
| Polymer | Key IR Peaks (cm⁻¹) | Diagnostic Feature |
|---|---|---|
| Polyethylene (PE) | 2850, 2920, 720 | Strong C-H bands; CH₂ rocking at 720 |
| Polypropylene (PP) | 2850–2960, 1375, 1165 | CH₃ bands distinguish from PE |
| Polystyrene (PS) | 3030, 1600, 700–760 | Aromatic C-H and ring oop |
| PET | 1720, 1240, 726 | Strong ester C=O; C-O-C stretch |
| Nylon 6 | 3300, 1640, 1540 | N-H + Amide I + Amide II bands |
IR techniques are employed in environmental monitoring (detecting CO, CO₂, NO₂ and hydrocarbons in air; identifying organic pollutants in water) and in food science (analysing fatty acid composition, detecting adulteration in edible oils, measuring moisture and protein content in grain).
Forensic laboratories use FTIR-ATR to identify substances at crime scenes — from controlled substances and explosives to fibres, paints, and inks — without destroying evidence. Portable handheld FTIR devices are now deployed in field investigations.
In biochemistry, IR is used to study protein secondary structure (the Amide I band near 1650 cm⁻¹ shifts depending on α-helix vs. β-sheet content), lipid membrane composition, and DNA/RNA base-pairing. Synchrotron-based IR microspectroscopy can map chemical composition in single biological cells.
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