Infrared (IR) spectroscopy is an analytical technique used to determine the bonds and functional groups in the structure of chemical compounds. It destroys no sample during analysis.

Infrared spectroscopy is a type of spectroscopic technique that involves the interaction of infrared radiation with matter. It is based on the absorption, emission, or scattering of infrared radiation by a sample, and is used to identify and analyze the chemical structure of a sample. The advantage of this technique is that any sample in virtually any state can be studied. For example, powder, paste, a mixture, gas films, and fibers can be easily examined.

The principle of infrared spectroscopy is based on the vibrations of atoms and the dipole moment of the compounds. When infrared radiation passes through the sample, a fraction of incident radiation of particular energy is absorbed by the vibrating atoms. The energy of the vibratory bonds corresponds to the energy of absorption. This way infrared spectrum is obtained.

Infrared radiations are the electromagnetic radiation that lies between the visible and microwave region of electromagnetic radiation. They have a longer wavelength than UV rays and longer frequencies than visible light.

EMR wavelength chart

Molecular vibrations to Infrared radiations

There are six types of vibration usually encountered in IR spectroscopy:

  • Asymmetrical stretching
  • Symmetrical stretching
  • Bending and scissoring
  • Rocking or in-plane bending
  • Twisting
  • Wagging

molecular vibration by IR spectroscopy

Modes of vibrations in a molecule

The interactions of IR with the molecules can be explained by the change in dipole moments associated with it.
A polyatomic molecule with N atoms must have 3N degrees of freedom in total.

For example, a molecule with three atoms has nine degrees of freedom. Vibrational degrees of freedom are different for linear and non-linear molecules. For a linear molecule, these are 3N-5, while in the CO2 are of the non-linear molecule, they are 3N-6.

Carbon dioxide (CO2) and sulfur dioxide (SO2), for example, are both polyatomic molecules with three atoms. Hence, carbon dioxide has four degrees of vibration, while sulfur dioxide has three.

IR active and inactive species

As the energy difference between rotational levels and vibrational levels is so small in chemical compounds, bonds with a definite dipole moment absorb infrared rays. Such substances are considered IR-active. A nonpolar molecule generates a constant electric field, which induces a frequency to interact with incoming infrared radiation.

IR inactive species

Bonds that do not absorb IR are termed “IR inactive species.” For example, hydrogen (H2), and nitrogen (N2) are IR inactive. Symmetrically substituted alkenes and alkynes are IR-inactive species. because they have zero dipole moment.

However, ionic salts such as NaCl and KBr absorb only in the far IR region at 700 cm-1, making them suitable as sample holders.

Infrared (IR) spectrophotometer

The sample is prepared by pelleting the sample with potassium bromide (KBr). Then, 1 mg of the sample is mixed thoroughly with 100mg of anhydrous KBr powder. Now, the mixture is pressed to produce a disk shape pallet. This disc is placed in the IR beam for determination.

The instruments used for analysis based on IR spectroscopy are of two types,

  1. FT-IR spectrophotometer
  2. Dispersive IR spectrophotometer

FT-IR spectrophotometer

Fourier transform infrared spectrophotometer (FT-IR) is an analytical instrument that works on the principle of IR spectroscopy. Fourier transform is an algorithm designed to transform the data obtained from the analysis in the spectral form and plot absorbance against wave number.

instrumentation of ir spectroscopy

It gives rapid results, such that 60 seconds are enough to complete one analysis. The main components of the instrument are described below:

If the solid sample is not soluble in the KBr solvent, mineral oil or hydrocarbon oils such as nujol can be used.

Light source

Two common sources used as a light source are Nernst filament and Globar which are heated to more than 1000°C for this purpose.


The IR beam enters into the monochromator which is dispersed by grating or prism. The resulting beam is separated into single wavelengths.

The wavenumber sorting ranges of the different prism are given below:

  • Sodium chloride: 4000-650 cm-1
  • Lithium fluoride: 4000-1000 cm-1
  • Cesium iodide: 4000-400 cm-1


After the dispersion of the IR beam, it is passed through the sample. Each particular wavelength that is being transmitted is amplified. These wavelengths are then compared to the reference beam. Then the difference is represented on the digital graph.

Dispersive IR spectrophotometer

A dispersive IR spectrophotometer is the instrument used to analyze samples by analyzing the IR radiation absorbed by the sample. It uses a prism or grating to separate the different wavelengths of light and detect the intensity of the light at each wavelength using a detector. However, this technique is laborious and hence rarely used.

Spectral analysis in IR spectroscopy

When IR radiation passes through the sample analyte, some of the radiation is absorbed and the remainder is transmitted. The spectra are the graphs of transmittance versus wavenumber. The absorption energies can be observed as inverted peaks. No two produce same the infrared spectrum, so it is useful for many qualitative analyses.

Fingerprint region

The region between 140 and 900 cm-1 is a complex region that includes fundamental stretching as well as frequency sum and difference. such that this part of an analyte has characteristic peaks in a sample. For example, two similar molecules may have the same fundamental region but have marked differences in this particular region. Therefore, it is called the “fingerprint region.”

The shape of peaks in the IR spectrum

Infrared absorptions are primarily visible as broad peaks on the graph. The broadening is caused by the doppler effect. The second broadening is caused by the collision between the molecules.

Absorption occurs only when a vibration with particular energy changes a molecular’s dipole moment. The larger the change in the dipole moment, the more intense the absorption band.

Examples of IR spectrum

1. IR spectrum of paracetamol and its interpretation

IR spectrum of paracetamol

2. In epichlorohydrin, a clear peak at 3100 cm-1 and 1400 cm-1 shows the presence of a C-C bond and oxygen ring respectively.

IR spectrum of epichlorohydrin

Factors affecting the IR spectrum

1. Fermi resonance

When two vibrational bands are so close that they overlap where only one band is expected, they are said to be in phase. The phenomenon of doublet formation is called Fermi resonance. This effect occurs when an overtone or combination band has a frequency that is similar to the fundamental bands.

2. Coupling

Coupling is very common in molecules with adjacent bonds of similar frequency. For example, an IR spectrum of a molecule containing C-C, and C-N may experience this phenomenon.

3. Over tonnes

The vibrations that arise from the excitation of the ground state to the first excited state are fundamental absorptions. But in some cases, these vibrations are complicated by the other vibrations such as the excitation of zero states to the 2nd and 3rd excited states called over tonnes.
For example, in the figure print region of the IR spectrum at 500 cm-1, the main peak accompanies low-intensity peaks.

Applications of IR Spectroscopy

IR spectroscopy is a widely used analytical technique in a variety of fields, including chemistry, materials science, and biochemistry. Moreover, it is applied as follows:

  • Identification of an organic compound: Functional groups, for example, two compounds with similar overlay able spectra must have the same functional groups. Interestingly, an FT-IR spectrophotometer is used at airports to detect drug addicts.
  • Purity of chemical compound: Although thorough FTIR is not commonly used as a quantitative analysis, it is useful for determining the purity of chemical compounds.
  • Reaction progress: Organic chemical reactions are slow, so their progress is to be measured after time intervals through IR spectroscopy of the mixture
    More common applications are summarized in the table below.

Key Takeaways

Peaks of various Bonds in IR spectroscopy (5)

Related Resources

Concept Berg

What’s the frequency of infrared radiation?

The frequency of infrared radiation ranges from 300 GHz to 400,000 GHz.

What are the three types of IR vibrations?

  1. Near R
  2. Mid IR
  3. Far IR

How to read an IR spectroscopy graph?

IR spectroscopy is applicable to all compounds which have a dipole moment. Moreover, the sample in a solid, liquid, or gas state can be analyzed.

How does Raman and infrared (IR) spectroscopy differ?

Raman spectroscopy is used to investigate chemical compounds with zero or few chemical compounds, whereas IR spectroscopy is used to investigate chemical compounds with many chemical compounds. The molecules that should experience a significant change in dipole moment when exposed to an IR source.

How are IR and Raman spectroscopy complimentary?
IR and Raman’s spectroscopy are complementary in the sense that chemical species that are IR active are Raman inactive and vice versa. Furthermore, water-based solutions that cannot be studied using IR spectroscopy can be studied using Raman spectroscopy.

What are bending vibrations in IR spectroscopy?

The bending vibrations are the unsymmetrical disturbance.

How would you use IR spectroscopy to differentiate any two compounds?

IR spectroscopy is applied to differentiate the two compounds usually have organic functional groups based on the types of bonded atoms.

What is a necessary condition for IR spectroscopy?

In order to use IR spectroscopy to analyze a sample, the following conditions are necessary:

  1. The sample must be in a form that is transparent to IR radiation. This means that solid samples must be ground into a fine powder or dissolved in a solvent, while liquid or gas samples can be analyzed directly.
  2. The sample must be placed in a sample cell or cuvette, which allows the IR radiation to pass through the sample and be detected by the instrument.
  3. The instrument must be calibrated using a known reference material or standard, in order to ensure accurate results