An In-Depth Introduction to Infrared Spectroscopy

An In-Depth Introduction to Infrared Spectroscopy


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  • Source: Microbioz India

  • Date: 14 Jan,2024

IR spectroscopy is a powerful technique used for identifying and characterizing substances through their absorption of infrared radiation. The purpose of this book is to give a step-by-step account of the fundamental principles and uses of IR spectroscopy in simple language that can be understood by beginners.

What Is Infrared Spectroscopy?

Principle of Operation:

  1. IR spectroscopy is based on the fact that individual molecules resonate at individual frequencies and absorb selective IR radiation in the course of their vibrational transitions.
  2. Molecules have vibratory frequency corresponding to chemical bonding observed in the IR spectrum.

Key Concepts:

Types of Vibrations:

  1. Stretching: The change of bond lengths
  2. Bending: Variation in bond angles occurs.
  3. There are various vibrational modes exhibited by different functional groups.


  1. An IR spectrometer consists of an infrared radiation source, sample holder, detector and a means to analyze either transmitted or absorbed radiation.
  2. Commonly used sources include Nernst glowers or globars while detectors can be thermal (thermocouples) or photoelectric.

Transmittance And Absorbance:

  1. Transmittance (T): Ratio of transmitted intensity to incident intensity.
  2. Absorbance (A): Antilogarithm of the reciprocal value, transmittance (-log(T)).

IR Spectrum:

  1. The x-axis on an IR spectrum represents wavenumber (the reciprocal value of wavelength) measured in cm⁻¹ mostly.
  2. Peaks are observed in this spectrum when there is absorption of infrared light and they correspond to definite types of molecular vibrations taking place therein.

Functional Groups And Characteristic Absorptions


  1. Alkanes: C-H stretching 2850-2960 cm⁻¹
  2. Alkenes: C-H stretching and bending 3020-3100 cm⁻¹

Alcohols And Phenols:

  1. O-H stretching normally takes place somewhere around 3200-3600 cm⁻¹.
  2. C-O stretching at about 1000-1300 cm⁻¹.

Carbonyl Compounds:

  1. Aldehydes: C=O stretching around 1700-1750 cm⁻¹.
  2. Ketones: C=O stretching around 1650-1750 cm⁻¹.


  1. N-H stretching close to 3300-3500 cm⁻¹.
  2. C-N stretching between 1000-1300 cm⁻¹.


  1. Functional Group Identification
  2. An example of the utility in IR spectra in analyzing a molecule’s functionality is provided by the HCl peak at ~3000cm-1.

Quality Control

This technology is used to ensure uniformity and purity in chemical substances manufacture sectors.

Forensic Analysis

IR analysis helps identify unknown substances during investigations in forensic science laboratories.

Environmental Analysis

It is applied to detect air and water pollutant and contaminants respectively .

Interpreting IR Spectra – A Few Tips:

Fingerprint Region

The area below 1500cm–1 is frequently referred to as the ‘fingerprint’ region where each molecule has unique patterns.

Peak Intensity

The intensity of peaks indicate that absorption more significant absorption has taken place if they are strong.

Broad vs Sharp Peaks

Hydrogen bonding may be present in broad peaks.

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