Basics of IR Spectroscopy

Basics of IR Spectroscopy (Infrared Spectroscopy) – Complete Spectroscopy Notes

Infrared (IR) Spectroscopy is one of the most important and widely used analytical techniques in chemistry. It helps us identify functional groups, understand molecular structure, and analyze chemical bonding.

In this spectroscopy notes article, we will learn IR spectroscopy from basic to advanced level in a simple and engaging way.

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🔬 What is IR Spectroscopy?

IR spectroscopy is a technique that studies the interaction of infrared radiation with molecules.

👉 When IR radiation passes through a molecule:

• Some frequencies are absorbed
• This absorption causes vibrations in chemical bonds

📌 These vibrations give a unique fingerprint of the molecule.

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💡 Key Idea

👉 IR spectroscopy works on the principle:

Molecules absorb IR radiation when the frequency of radiation matches the natural vibrational frequency of the bond.

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🌈 IR Region in Electromagnetic Spectrum

Infrared region lies between visible light and microwave region.

Region	Wavelength	Wavenumber (cm⁻¹)
Near IR	0.78 – 2.5 μm	12800 – 4000
Mid IR ⭐	2.5 – 25 μm	4000 – 400
Far IR	25 – 1000 μm	400 – 10

👉 Mid-IR region (4000–400 cm⁻¹) is most commonly used.

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⚙️ Basic Principle of IR Spectroscopy

Molecules are not static — atoms are constantly vibrating.

👉 When IR radiation hits a molecule:

• Bonds stretch or bend
• Energy is absorbed
• Transition occurs between vibrational energy levels

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🧠 Condition for IR Absorption

👉 A vibration is IR active only if:

✔ There is a change in dipole moment

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❌ Example (IR Inactive):

• O₂, N₂ (no dipole change)

✅ Example (IR Active):

• HCl, CO, H₂O

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🔁 Types of Molecular Vibrations

There are two main types:

1. Stretching Vibrations

👉 Change in bond length

Types:

• Symmetric stretching
• Asymmetric stretching

2. Bending Vibrations

👉 Change in bond angle

Types:

• Scissoring
• Rocking
• Wagging
• Twisting

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📊 Vibrational Degrees of Freedom

For a molecule with N atoms:

• Non-linear molecule → 3N − 6
• Linear molecule → 3N − 5

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Example:

👉 H₂O (Non-linear, N = 3)
= 3(3) − 6 = 3 vibrational modes

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📈 IR Spectrum

IR spectrum is a graph of:

• X-axis → Wavenumber (cm⁻¹)
• Y-axis → % Transmittance or Absorbance

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🔑 Important Regions

1. Functional Group Region (4000–1500 cm⁻¹)

👉 Helps identify functional groups

2. Fingerprint Region (1500–400 cm⁻¹)

👉 Unique for each molecule

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🧪 Characteristic IR Absorption Bands

Functional Group	Range (cm⁻¹)
O–H	3200–3600
N–H	3300–3500
C–H	2800–3000
C=O	1650–1750
C≡C	2100–2260

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🧠 Example:

👉 Ethanol (C₂H₅OH)

• O–H peak → ~3300 cm⁻¹
• C–H peak → ~2900 cm⁻¹

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⚖️ Factors Affecting IR Frequency

1. Bond Strength

👉 Stronger bond → Higher frequency

Example:

• C≡C > C=C > C–C

2. Atomic Mass

👉 Heavier atoms → Lower frequency

3. Hydrogen Bonding

👉 Causes:

• Broad peaks
• Lower frequency

4. Electronic Effects

• Inductive effect
• Resonance

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🧪 Instrumentation of IR Spectroscopy

🔧 Main Components:

1. IR Source
2. Monochromator
3. Sample holder
4. Detector
5. Recorder

⚡ Modern Technique: FTIR

👉 Fourier Transform Infrared Spectroscopy

✔ Faster
✔ More accurate
✔ Better resolution

🔬 Working of IR Spectroscopy

1. IR radiation passes through sample
2. Some frequencies are absorbed
3. Detector records remaining radiation
4. Spectrum is generated

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🧠 Interpretation of IR Spectrum

Step-by-step approach:

1. Look for strong peaks
2. Identify functional groups
3. Check fingerprint region
4. Compare with reference

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📌 Applications of IR Spectroscopy

🔬 1. Identification of Compounds

👉 Detect functional groups

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🏭 2. Pharmaceutical Industry

👉 Drug analysis and purity check

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🌱 3. Environmental Monitoring

👉 Detection of pollutants

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🧪 4. Organic Chemistry

👉 Structure determination

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🧬 5. Polymer Analysis

👉 Study of plastics and materials

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⚠️ Limitations of IR Spectroscopy

• Cannot detect symmetric molecules easily
• Complex spectra for large molecules
• Requires interpretation skills

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🧠 IR vs Raman Spectroscopy

Feature	IR	Raman
Basis	Dipole change	Polarizability
Best for	Polar bonds	Non-polar bonds
Example	HCl	O₂

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📚 Solved Example

👉 Identify functional group:

Peak at 1700 cm⁻¹

✔ Indicates → C=O (Carbonyl group)

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🎯 Exam Tips

✔ Always remember:

• IR active → dipole change
• Strong peaks → important
• 1700 cm⁻¹ → carbonyl

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📝 Quick Summary

• IR spectroscopy studies bond vibrations
• Works in mid-IR region (4000–400 cm⁻¹)
• Requires change in dipole moment
• Helps identify functional groups

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❓ MCQs for Practice

1. Which molecule is IR inactive?

A) HCl
B) CO₂
C) N₂
D) H₂O

✔ Answer: C) N₂

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2. IR absorption requires:

A) Change in mass
B) Change in dipole moment
C) Change in pressure
D) Change in volume

✔ Answer: B

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3. Carbonyl group appears near:

A) 1000 cm⁻¹
B) 1700 cm⁻¹
C) 3000 cm⁻¹
D) 400 cm⁻¹

✔ Answer: B

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IR Spectroscopy is a powerful tool that allows chemists to see invisible vibrations inside molecules. Once you understand the basics, interpreting spectra becomes much easier and highly rewarding.

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