Applications and Importance of IR Spectroscopy: Shedding Light on Molecular Structures |ChemFam #68|

Greetings to everyone! In the world of science, there exists a vast array of analytical techniques, each serving a unique purpose and offering valuable insights into the composition and structure of matter. Among these techniques, infrared spectroscopy stands out as a powerful tool used extensively in chemistry, biology, and various other scientific disciplines. In this article, we will explore the principles, applications, and the compelling need and importance of infrared (IR) spectroscopy.

Infrared spectroscopy deals with the recording of the absorption of radiations in the infrared region of the electromagnetic spectrum. The position of a given infrared absorption is expressed in terms of wavelength in micron μ or more commonly in terms of wavenumber ṽ (cm^-1) since it is directly proportional to energy.

The ordinary infrared region 2.5-15 μ (4000-667 cm^- ) is of greatest practical use to organic chemists. The region 0.8-2.5 μ (12,500- 4000 cm-1) is called the near infrared and the region 15-200 μ (667-50 cm^-) is called the far infrared. The absorption of infrared radiation by a molecule occurs due to quantized vibrational and rotational energy changes when it is subjected to infrared irradiation. Thus, IR spectra are often called vibrational-rotational spectra.

Theory of Infrared Spectroscopy

IR absorption spectra originate from transitions in vibrational and rotational energy Ievels within a molecule. On absorption of IR radiation, vibrational and rotational energies of the molecule are increased.

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When a molecule absorbs IRradiation in the range 100-10,000 cm-1, the absorbed radiation causes transitions in its vibrational energy Ievels. These energy Ievels are also quantized, but vibratonal spectra appear as bands rather than discrete lines. The energy differences between various rotational energy Ievels of a molecule are far less than that between its vibrational energy Ievels. Thus, a single transition in vibrational energy Ievels is accompanied by a large number of transitions in rotational energy Ievels and so the vibrational spectra appear as vibrational-rotational bands instead of discrete lines. Organic chemists are mainly concemed with these vibrational-rotational bands, especially with those occurring in the region 4000-667 cm^-.

Modes of Molecular Vibrations

Various atoms in a molecule may be regarded as balls of different masses and the covalent bonds between them as weightless tiny springs holding such balls together. Atoms in a molecule arenot still but they vibrate. The two types (modes) of fundamental molecular vibrations known are: (a) stretching and (b) bending vibrations (deformations).

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(i) Stretching Vibrations

In stretching vibrations, the distance between two atoms increases ordecreases, but the atoms remain in the same bond axis. Stretching vibrations are of two types:

• (a) Symmetrical stretching: In this mode of vibration, the movement of atoms with respect to the common (or central) atom is simultaneously in the same direction along the same bond axis.
• (b) Asymmetrical Stretching: In this vibration, one atom approaches the common atom while the other departs from it.

(ii) Bending Vibrations (Deformations)

In such vibrations, the positions of the atoms change with respect to their original bond axes. Bending vibrations are of four types:

• (a) Scissoring: In this mode of vibration, the movement of atoms is in the opposite direction with change in their bond axes as well as in the bond angle they form with the central atom.
• (b) Rocking: In this vibration, the movement of atoms takes place in the same direction with change in their bond axes.
  Scissoring and rocking are in-plane bendings.
• (c) Wagging: In this vibration, two atoms simultaneously move above and below the plane with respect to the common atom.
• (d) Twisting: In this mode of vibration, one of the atom moves up and the other moves down the plane with respect to the common atom.

The Importance of IR Spectroscopy

• 1. Structural Analysis

One of the primary applications of IR spectroscopy is determining the structure of organic and inorganic compounds. It's an indispensable tool for chemists, enabling them to identify unknown substances and confirm the presence of specific functional groups within a molecule. For example, in the field of drug development, IR spectroscopy plays a critical role in characterizing and confirming the structure of newly synthesized compounds.

• 2. Quality Control

Industries such as pharmaceuticals, food, and petrochemicals rely heavily on IR spectroscopy for quality control and assurance. By comparing the spectra of raw materials and finished products, manufacturers can ensure the consistency and purity of their products. Any deviations from the expected spectra can signal impurities or undesired chemical changes.

• 3. Environmental Analysis

Environmental scientists and regulators use IR spectroscopy to monitor and analyze pollutants in the atmosphere and water. By identifying the specific compounds contributing to pollution, steps can be taken to mitigate the environmental impact and safeguard human health.

Scientific Reports (Sci Rep) ISSN 2045-2322 (online)

(A) Averaged FTIR spectra and (B) averaged second derivative of the FTIR spectra of the COVID-19 positive and negative groups of the LIQUID 1 dataset.

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• 4. Forensic Science

IR spectroscopy has found its place in forensic science as well. Investigators use it to analyze trace evidence, such as fibers, paints, and drugs. This aids in solving crimes, identifying substances, and linking evidence to specific individuals or locations.

• 5. Material Science

In the field of material science, researchers use IR spectroscopy to study the properties of materials like polymers, ceramics, and composites. Understanding the molecular structure and behavior of these materials is essential for developing new and improved products for various applications.

The Need for IR Spectroscopy

The need for IR spectroscopy is driven by its unique capabilities and the information it provides. Here are some reasons why this technique is essential in the scientific world:

• 1. Non-Destructive Analysis

Unlike some other analytical methods, IR spectroscopy is non-destructive. It doesn't alter or consume the sample being analyzed. This is especially valuable when working with precious or limited quantities of a substance.

• 2. Versatility

IR spectroscopy is highly versatile, accommodating a wide range of samples, from liquids and gases to solids. It can be used in various fields, from chemistry to biology to materials science, making it a versatile and widely applicable technique.

• 3. Speed and Precision

The speed at which IR spectra can be obtained is impressive, with results available in seconds. This rapid analysis is crucial in industries where quick decisions are necessary, such as pharmaceutical manufacturing.

• 4. Quantitative Analysis

In addition to qualitative analysis, IR spectroscopy can be used for quantitative analysis. It can determine the concentration of specific compounds within a mixture, making it a powerful tool for quality control and process monitoring.

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• 5. Minimal Sample Preparation

Sample preparation for IR spectroscopy is relatively simple, often requiring minimal or no preparation at all. This simplifies the analytical process and reduces the chances of introducing errors.

Conclusive thoughts

Infrared spectroscopy is more than just a scientific technique; it's a cornerstone of modern research and industry. Its ability to reveal the hidden molecular architecture of matter has made it indispensable in a wide range of applications, from chemistry and materials science to environmental monitoring and forensic investigations. The need for IR spectroscopy arises from its unparalleled versatility, speed, precision and ability to provide valuable information without damaging precious samples. As science continues to advance, IR spectroscopy remains a shining beacon of insight, shedding light on the intricacies of the molecular world.

Until we meet again :)

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B I B L I O G R A P H Y

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IR Spectroscopy Principle and Instrumentation

IR Spectroscopy | ACS Reagent Chemicals

IR Spectroscopy an Overview

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