Infrared Spectroscopy: Principle & Procedure

Infrared spectroscopy refers to the infrared part of the electromagnetic spectrum, or light with a longer wavelength and a lower frequency than visible light. Infrared spectroscopy is the study of how molecules react to infrared light.

In order to observe how molecules interact and what they actually do, IR spectroscopy is frequently utilized in the domains of inorganic and organic chemistry. These particles, which are otherwise invisible to the human eye, can be seen using infrared light.

Key Terms: Infrared spectroscopy, electromagnetic, absorption, emission, frequency, radiation, wavelength, transducer. 

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What is IR Spectroscopy?

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Infrared spectroscopy is the study of a molecule's interactions with infrared light. Absorption, emission, and reflection measurements can be used to analyze this in three different ways. Most organic and inorganic chemistries use this approach. 

• It is used by chemists to locate functional groups in compounds. 
• By measuring atom vibrations using IR spectroscopy, the functional groups of molecules can be identified.
• 5 Stronger links and light atoms will often vibrate with a higher stretching frequency (wavenumber).
• A molecule can absorb certain infrared light frequencies, which can be found via IR spectroscopy. 
• Since these particular light frequencies coincide with the frequency of the bonds in molecules, molecules have a tendency to absorb them. 
• Infrared light has a frequency identical to the bond frequency found inside these molecules. 
• Infrared radiation contains the energy needed to stimulate molecular bonds and cause them to vibrate more violently. 
• However, infrared electromagnetic radiation will only interact with polar bonds. 
• The electric field component of an electromagnetic wave can drive the vibrational energy of a molecule when distinct regions of partial positive and negative charge are present. 
• The dipole moment of the particular molecule changes in tandem with the change in vibrational energy. 
• The polarity of the link affects how much energy is absorbed. 
• As they cannot interact with an electric field, symmetrical non-polar links in N=N and O=O cannot absorb radiation.

Read More: Coherent Sources

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Principle Of Infrared Spectroscopy

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The idea that certain frequencies of light that are distinctive of a certain molecule's structure have a tendency to be absorbed by that molecule is the foundation of the IR spectroscopy theory. The energies depend on the atomic masses that correspond to the molecular surfaces, the vibronic coupling that is present, and their shape. A faster spin or a more noticeable vibration might arise from the molecule absorbing the energy of the incident light, for example.

Read More: Difference Between Diffraction and Interference

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Regions of the Infrared Spectrum

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Based on how they relate to the visible spectrum, the infrared spectrum can be roughly classified into three regions: near, mid, and distant. The 0.8-2.5 m wavelengths, or 14000-4000 cm^-1, in the high energy and near IR area can cause harmonic or overtone vibration. 

• On the other hand, the mid-IR region, which has a wavelength of 2.5–25 m or 400–400 cm^-1, can be used to analyze fundamental vibrations as well as related rotational vibrational structures. 
• The microwave zone is exactly close to the far IR region, which has wavelengths between 25 and 1000 m (or 400 and 10 cm^-1). 
• This is helpful for rotational spectroscopy because it is low energy.

Read More: Polarization of Light

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Instrumentation

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Basic instrumentation for IR spectroscopy includes a radiation source, wavelength selector, sample container, transducer (detector), and signal processor. Depending on whether near-, mid-, or far-IR spectra are being measured, different sources, transducers, and beam splitters will need to be used. Most frequently, mid-IR may be utilized to identify chemical compounds. 

• The radiation source for mid-IR can be a laser, globar, or Nernst glower. 
• The IR transducers (detectors) are temperature-sensitive components whose characteristics, such as electrical resistance and temperature-dependent potential, significantly vary in response to even slight temperature changes. 
• Bolometers, thermocouples, and photo-conducting transducers are a few examples. 
• For dispersive and Fourier transform infrared (FTIR) devices, sources, and transducers are identical. 
• As the device scans the IR energy spectrum, dispersive IR instruments log energy absorption. 
• These devices work in the frequency range. 
• The time domain is where FTIR instruments operate. 
• All of the wavelengths that the source emitted are present in the infrared spectra created using Fourier transform equipment. 
• Using mirrors in a Michelson interferometer, the frequency data from the FTIR instrument is gathered to create a signal that is changed to data with a much lower frequency while still carrying the same information as the original IR signal. 
• When using background subtraction techniques, using an interferometer has a benefit.
• Two perpendicular mirrors, one fixed and one movable, are used in a Michelson interferometer by FTIR equipment to alter the incoming optical light.

Read More: Relation between phase difference and path difference

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Samples in Infrared Spectroscopy

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Samples in many physical states, such as solid, liquid, and gas, can be used in IR spectroscopy.

Solid sample: The solid Film Technique is useful for amorphous materials, this method requires melting the sample, cooling it, and then depositing it as a thin film over the KBr cell.

• Using the solid run-in solution technique, the sample must be dissolved in a concentrated solution using a solvent such as dichloromethane, carbon tetrachloride, or carbon disulfide. 
• In order to scan the solution, a thin coating is then applied to KBr plates. Bear in mind that the solvent shouldn't absorb any infrared light.

Liquid Sample: Since solvents have their own absorption spectra and may interfere with accurate results, there is no need to mix liquid samples with them. 

• The sample, which is deposited in the form of a drop and spread out like a thin film.
• It is scanned using highly polished salt plates of KBr, NaCl, or AgCl.

Gas sample: A specially built cell consisting of KBr and NaCl is employed for a gaseous sample.

• This has a route length of 5 to 10 cm. 
• The cell is filled with gaseous vapors, and it is then put in the path of infrared radiation.

Read More: Doppler Effect Formula

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Procedure for IR Spectroscopy

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In some standard-grade reference material standards, IR spectroscopy is utilized as a method of identity verification. Standard-grade reference materials must meet certain requirements in order to pass identity tests. These requirements are based on the observation of distinctive absorbances and, if available, comparison to a spectrum library. 

• A minimum of three absorbances are required for each entry of chosen standard-grade reference materials to pass the test for IR spectroscopy analysis. 
• If every absorbance recorded in the NIST spectrum is also present in the test spectrum, the reference material will pass the comparison test. 
• Many of the standard-grade reference materials referenced in this book can be found in the NIST spectrum library.

Read More: Brewsters law formula

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Things to Remember

• The infrared light frequencies that are absorbed by molecules are discovered via IR spectroscopy.
• When discrete areas of partial positive and negative charge are present in the molecule, the electric field component of an electromagnetic wave can drive the vibrational energy of the molecule.
• The majority of the bands that identify the functional group are located in the range of 4000 cm^-1 to 1300 cm^-1.
• Infrared spectroscopy can be utilized with samples that are solid, liquid, or gaseous.
• The underlying tenet of the IR spectroscopy hypothesis is that molecules prefer to absorb certain light frequencies that are distinctive to the corresponding structure of the molecules.
• Any molecule exposed to infrared light experiences energetic atom vibrations; after a certain point, the vibrations' natural frequency equals the infrared light's frequency.

Read More: Destructive Interference