Principles of IR Spectroscopy
IR spectroscopy is based on the principle that molecules absorb specific frequencies of infrared light, which are characteristic of their chemical bonds. Each chemical bond has a specific vibrational frequency, and the absorbance of infrared light causes these bonds to vibrate. The amount of light absorbed at a particular frequency is related to the strength of a specific chemical bond. Therefore, when a molecule is exposed to infrared radiation, it absorbs specific frequencies of light, resulting in an IR spectrum, which can be used to identify the different functional groups present in the molecule.
For a vibrational mode to be IR active, there must be a net change in the dipole moment of the molecule during that vibration.
- IR Inactive: Homonuclear diatomics like O2 and N2, and the symmetric stretch of non-polar molecules like CO2, do not change the dipole moment and are thus IR inactive.
- IR Active: The asymmetric stretch and bending modes of CO2 are IR active as they cause a change in the dipole moment.
Number of Vibrational Modes: A molecule with N atoms has
- 3N - 6 fundamental vibrational modes for non-linear molecules.
- 3N - 5 fundamental vibrational modes for linear molecules.
Factors Affecting Frequency (ṽ): The frequency of absorption is primarily determined by
- Bond Strength (Force Constant, k): Triple > Double > Single bonds absorb at higher frequencies (ṽ ∝ √k).
- Reduced Mass (μ): Bonds involving lighter atoms absorb at higher frequencies (ṽ ∝ 1/√μ).
Instrumentation
The basic components required for an IR spectroscopy experiment are a source of infrared radiation, a sample holder, and a detector. The most commonly used sources are a Globar (a silicon carbide rod) or a Nernst Glower (a mixture of oxides of yttrium, zirconium, and other rare earth elements). These sources emit a broad spectrum of IR radiation, covering the entire mid-infrared range (4000–400 cm-1).
The sample holder used in IR spectroscopy varies depending on the nature of the sample. For liquids, a salt plate or a liquid cell is used, while for solids, a KBr pellet is used. For gases, a gas cell is used, where the sample is placed at reduced pressure. The detector used in IR spectrometers are mainly of two types: Thermal detectors, such as a thermocouple or a thermopile, and photon detectors, such as a photomultiplier tube or a charge-coupled device (CCD) camera.
Modern high-level analysis relies almost exclusively on Fourier Transform Infrared (FT-IR) spectroscopy, which provides significant advantages over older dispersive instruments. Key Component: Michelson Interferometer: Instead of a monochromator, FT-IR uses a Michelson Interferometer to measure all IR frequencies simultaneously. This converts the absorption information into an interferogram (intensity vs. mirror position) which is then converted into the final spectrum (intensity vs. frequency) using a Fourier Transform algorithm.
FT-IR Advantages:
- Fellgett's Advantage (Multiplexing): Measuring all frequencies simultaneously significantly reduces the scan time and improves the signal-to-noise ratio.
- Jacquinot's Advantage (Throughput): The use of a circular aperture rather than a slit allows for more energy (light) to reach the detector, improving sensitivity.
- Conne's Advantage (Wavelength Accuracy): The use of a laser (often He-Ne) as an internal reference provides extremely high wavelength (frequency) accuracy.
Detectors:
- DTGS (Deuterated Triglycine Sulfate): A common, sensitive thermal detector.
- MCT (Mercury Cadmium Telluride): A high-speed, high-sensitivity semiconductor (photon) detector, essential for fast processes like gas chromatography hyphenation.
Sample Holders: Stress the limitations: Alkali halide salt plates (NaCl) or KBr) must be used for liquids/mulls because glass and quartz absorb heavily in the IR range.
Applications
IR spectroscopy has a wide range of applications in various fields, such as pharmaceuticals, forensics, environmental science, and materials science. It is used in the identification and quantification of drug compounds and their metabolites in pharmaceutical analysis. IR spectroscopy is also widely used in forensic analysis to determine the composition and origin of trace evidence found at crime scenes.
In environmental science, IR spectroscopy is used to analyze air and water samples for contaminants. It helps in the determination of the types and quantities of pollutants present in the sample, aiding in the monitoring and regulation of environmental pollution. In materials science, IR spectroscopy is used to determine the chemical composition and structure of materials, such as polymers, ceramics, and minerals.
Advantages and Disadvantages
One of the main advantages of IR spectroscopy is its non-destructive nature, as the sample does not undergo any chemical or physical changes during the analysis. It is also relatively fast and requires minimal sample preparation, making it a popular choice for routine analysis. However, one limitation of IR spectroscopy is that it cannot provide information about the spatial distribution of the functional groups in a sample. Hence, it is often coupled with other techniques such as microscopy or chromatography to overcome this limitation.
P Q R Bands
Fermi Resonance
IR Spectra of Fe2(CO)9
Infrared (IR) Spectroscopy
Complete Mastery Guide for JEE Advanced • NEET • IIT-JAM • CSIR-NET • GATE Chemistry
1. Fundamentals (Must Know)
Range: 4000 – 400 cm⁻¹ (2.5 – 25 μm)
Absorption occurs when IR frequency = natural vibrational frequency of bond.
where k = force constant, μ = reduced mass
→ Stronger bond & lighter atoms → higher wavenumber
Factors Affecting Frequency
| Factor | Effect on ν̄ |
|---|---|
| Bond strength ↑ | ν̄ ↑ (triple > double > single) |
| Reduced mass ↓ (H vs heavier) | ν̄ ↑ (O–H > O–D) |
| H-bonding | ν̄ ↓ and broadens (especially O–H, N–H) |
| Conjugation / Resonance | C=O ↓ by 30–40 cm⁻¹ |
2. IR Regions (High Yield)
1500 – 400 cm⁻¹ → Fingerprint Region (unique to molecule)
3. Master IR Absorption Table (Memorize This!)
| Bond / Group | Wavenumber (cm⁻¹) | Intensity & Shape | Notes (JEE/NEET Favorite) |
|---|---|---|---|
| O–H stretch (alcohols) | 3200–3600 | Strong, very broad | H-bonded → broader & lower frequency |
| O–H stretch (carboxylic acids) | 2500–3300 | Very broad (dimer) | Classic “mountain” shape |
| N–H stretch | 3300–3500 | Medium, sharp (1°: 2 peaks, 2°: 1 peak) | Primary amine → two bands |
| ≡C–H stretch | 3300 | Strong, sharp | Terminal alkyne diagnostic |
| C–H stretch (alkanes) | 2850–3000 | Strong | >3000 = sp², <3000 = sp³ |
| C–H stretch (alkenes, aromatics) | 3000–3100 | Medium | |
| C≡C stretch | 2100–2260 | Weak to medium | Often absent if symmetrical |
| C≡N stretch | 2200–2260 | Medium, sharp | Nitrile diagnostic |
| C=O stretch | 1650–1750 | VERY STRONG | Most important peak! |
| – Aldehyde | 1720–1740 | Two peaks with C–H at ~2700, 2800 | Classic doublet |
| – Ketone | 1705–1725 | Strong | Conjugation → ~1710 |
| – Ester | 1730–1750 | Strong | Highest C=O |
| – Acid | 1700–1725 | Strong, broad (dimer) | |
| – Amide | 1640–1690 | Strong | Lowest C=O |
| C=C stretch | 1620–1680 | Variable (weak if symmetrical) | Conjugation ↓ frequency |
| Aromatic C=C | 1450–1600 | Multiple bands | Typically 1500 & 1600 |
| C–O stretch | 1000–1300 | Strong | Alcohols, esters, ethers |
4. Fingerprint Region (1500–400 cm⁻¹)
JEE/NEET rarely ask exact peaks here, but know:
• Aromatic out-of-plane bending: 690–900 cm⁻¹
• C–Cl: ~700 cm⁻¹ C–Br: ~600 cm⁻¹
5. Selection Rules & Intensity
→ Homodiatomics (N₂, O₂, Cl₂) are IR INACTIVE
→ CO, HCl, NO → strong IR absorption
→ Symmetrical alkenes (trans) → weak C=C
6. Real PYQs & Exam Tricks (JEE Adv + NEET)
1. First look for O–H / N–H (3200–3600)
2. Then C=O (1650–1750) → if present, highest priority
3. Then C≡C / C≡N (2100–2260)
4. Then C=C / aromatic (1450–1600)
5. Below 1500 → fingerprint
Quick Identification Flowchart
- ~3300 sharp → ≡C–H (terminal alkyne)
- Two peaks ~2700 & 2800 → –CHO (aldehyde)
- C=O at 1680–1700 → conjugated ketone/amide
- C=O at 1750 → ester or anhydride
- Very broad O–H + C=O → carboxylic acid