24. INFRARED SPECTROSCOPY

Introduction

Infrared (IR) Spectroscopy is a fundamental analytical technique used to identify and study molecules based on how they absorb infrared radiation. When molecules absorb IR light, their chemical bonds vibrate at specific frequencies. These vibrations produce absorption bands that act like a molecular fingerprint, allowing qualitative identification and structural interpretation. IR spectroscopy is widely used in pharmaceutical analysis, organic chemistry, polymer studies, and functional group detection.

Advantages of Using Wavenumbers Instead of Wavelength

Wavenumber (cm−1) is preferred in IR spectroscopy because it is directly proportional to energy. This provides a clearer understanding of vibrational frequencies. Using wavenumbers makes spectral comparison easier, provides linear scale representation, and simplifies interpretation of functional group regions.

Regions in the Infrared Spectrum

The IR region of the electromagnetic spectrum is divided into three main zones:

  • Near IR: 12,800–4,000 cm−1
  • Mid IR: 4,000–400 cm−1 (most commonly used)
  • Far IR: 400–10 cm−1

Mid-infrared is particularly important because most molecular vibrations occur in this range.

Theory of Infrared Absorption

Molecules absorb IR radiation when the frequency of the incident light matches the natural frequency of a vibrating chemical bond. For a vibration to be IR active, it must cause a change in the dipole moment of the molecule. Depending on atomic masses and bond strength, different bonds absorb at different frequencies.

Modes of Vibration

Molecules vibrate in numerous ways, but these vibrations fall into two broad categories: stretching and bending.

A) Stretching Vibrations

Stretching involves a change in bond length. Two forms exist:

  • Symmetric stretching: Both bonds change length together.
  • Asymmetric stretching: One bond lengthens while another shortens.

B) Bending (Deformation) Vibrations

Bending occurs when bond angles change. It includes:

  • Scissoring
  • Rocking
  • Wagging
  • Twisting

Factors Affecting Vibrations in IR

The vibrational frequency of a bond depends on:

  • Bond strength: Stronger bonds absorb at higher frequencies.
  • Atomic masses: Lighter atoms vibrate faster.
  • Hybridization: sp bonds absorb at higher wavenumbers than sp2 and sp3.
  • Substitution effects: Substituents alter dipole moment and absorption.
  • Solvent effects: Hydrogen bonding may broaden or shift bands.

Instrumentation

An IR spectrophotometer measures absorption as a function of wavelength (or wavenumber). It consists of the following components:

1) Radiation Sources

IR instruments require stable sources that emit continuous infrared radiation:

  • Globar (silicon carbide rod): Emits intense mid-IR radiation.
  • Nernst glower: Composed of rare earth oxides.
  • Incandescent wire sources: Suitable for near IR.

2) Monochromator

A monochromator isolates narrow bands of wavelengths. Common types include:

  • Prisms made of alkali halides such as NaCl or KBr
  • Diffraction gratings for high-resolution scanning

3) Optical Slits

Optical slits control the beam width entering the monochromator. They help manage resolution, signal strength, and noise levels.

4) Sampling Techniques

IR sampling varies depending on whether the sample is a solid, liquid, or gas. Common approaches include:

  • Solid samples: KBr pellet, mull technique (Nujol mull)
  • Liquids: Liquid cells with NaCl or KBr windows
  • Gases: Gas cells with long path lengths

Modern instruments often use ATR (Attenuated Total Reflectance), simplifying sample preparation.

5) Detectors

Detectors convert IR radiation into electrical signals. Common detectors include:

  • Thermocouples
  • Bolometers
  • Golay cells
  • Pyroelectric detectors (DTGS, LiTaO3)

Modern FTIR instruments primarily use high-sensitivity pyroelectric detectors for rapid scanning.

Instruments for Measurement of IR Absorption

Traditional IR instruments use dispersive optics, but most modern laboratories use Fourier Transform Infrared (FTIR) spectrometers because of:

  • High speed and sensitivity
  • Better signal-to-noise ratio
  • Simultaneous multi-wavelength measurement
  • Reduced mechanical components

FTIR instruments employ an interferometer to collect data and perform mathematical transformation to generate spectra.

Advantages of Infrared Spectroscopy

  • Rapid and accurate identification of functional groups
  • Minimal sample preparation
  • Non-destructive analysis
  • Applicable to solids, liquids, and gases
  • Highly sensitive to molecular structure differences

Disadvantages of Infrared Spectroscopy

  • Not suitable for very dilute samples
  • Complex spectra may require expert interpretation
  • Water absorbs IR strongly, interfering with measurements
  • Sample impurities can distort absorption bands

Applications of Infrared Spectroscopy

  • Identification of functional groups in organic molecules
  • Drug analysis and purity testing
  • Characterization of polymers and excipients
  • Verification of raw materials in pharmaceutical industries
  • Study of hydrogen bonding and molecular interactions
  • Monitoring chemical reactions

Features of an IR Spectrum

An IR spectrum contains:

  • Functional group region: 4,000–1,300 cm−1, used for identifying characteristic vibrations
  • Fingerprint region: 1,300–400 cm−1, unique for each molecule

The functional group region helps identify major bonds such as O–H, C=O, N–H, and C–H, while the fingerprint region differentiates structurally similar compounds.

Detailed Notes:

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