Infrared spectroscopy, Chemistry tutorial


Infrared spectrophotometry is a branch of spectroscopy that is concerned by the measurement of absorption of electromagnetic radiation, through molecules due to their vibrational ability. It is extremely significant here to note that atoms in a molecule generally vibrate inside the links that link them mutually. The vibrational energy in a molecule is attained within infrared region of the electromagnetic spectrum. Therefore, when ever an infrared radiation of a precise frequency interacts through a molecule, the energy is absorbed leading to an enhanced vibrational energy of that bond. This can even lead to transform in the dipole moment of the bond concerned. Therefore, the infrared instrument (spectrophotometer) is planned to calculate the energy absorbed through the bond in the molecule at dissimilar wavelengths (nm) or wave number (cm-1) to generate a chart termed infrared spectrum.


Infrared spectroscopy is a process that involves the study of the absorption of electromagnetic radiation in the range of 0.78 to 300um.

Basic principles of infrared spectrometry

Every molecule possesses kinetic energy (E) that is responsible for its vibration or rotation. The total energy (Et) of a molecule is due to vibrational, rotational and translational energy inside the molecule. Therefore, the total kinetic energy (Et) is specified via the expression:


Ev is vibrational energy, Er is rotational energy and Etr is the rotational energy

A molecule enclosing n atoms will have 3n mode degrees of freedom of motion. Such are made of 3 rotational, 3 translational and 3n - 6 vibrational motions. Therefore, for a non linear poly atomic molecule, the fundamental mode of vibration is 3n - 6 while a linear molecule has 3n - 5. Through this, it is possible to expect theoretically the number of infrared bands that can be attained from a specified molecule.

Instance: Ethylmethylketone, CH3-CH2-CO-CH3 that is made up of 13 atoms, has 33 theoretical mode of vibration. This is computed as below:

Then, 3n -6 = (3 x 13) - 6 = 33

But, it isn't possible to detect all of such vibrations in the IR spectrum since many of such bands might overlap, others might be symmetrical vibration which might not suck up any radiation. In a complex molecule, stretching and bending of bond is possible and these depend on the bond strength and masses of the corresponding atoms that form such bond.

Types of molecular vibrations

In an organic molecule there are 2 major kinds of fundamental vibrations. Such are:

  • Stretching of bond
  • Bending of bond

2387_Stretching vibrations.jpg

Fig: Stretching vibrations 

The stretching vibration could either be symmetrical or asymmetrical. These are represented as above

But bending vibration is of 4 different types. Such are:

  • Scissoring
  • Rocking
  • Wagging
  • Twisting

These are represented as follows:

1429_Bending vibrations.jpg

Fig: Bending vibrations

The energy required to bend a bond is not great and falls within the range of 400-1300cm-1. This region is called the finger print region. Thus, this region is used to establish the identity of the chemical compounds. The energy required to stretch a bond is a little bit higher. This falls within the region of 1300 - 4000cm-1. This signal is caused via groups such as OH, NH, C= O, C = C, CHO, and so on. Such group frequencies are self-governing of other parts of the molecule and are utilized to detect the functional groups in molecules.

Group frequencies

Group frequencies are the absorption bands or signals that occur at certain frequencies due to stretching or bending vibration within a molecule. For instance, the bands at 3300cm-1 and 1050cm-1 are characteristics of the OH group in alcohols. Group frequencies can transform depending on the nature of the molecule and the solvent.

Example of several group frequencies is given below:

447_Example of some group frequencies are given below.jpg


The instrument utilized is termed infrared spectrophotometer. This instrument is essentially alike to those employed for the UV/ Visible measurements, but only differ from the energy or radiation sources, optical substances and the detection device.

1134_The schematic diagram of infra-red spectrophotometer.jpg

Fig: The schematic diagram of infra-red spectrophotometer  

Radiation source

The radiation source may be:

  • Nernst glower which is a mixture of oxides of zirconium or thorium
  • Globar unit that is a small rod of silicon carbide


This might be a prism or grating. The prism monochromator is generally made of sodium chloride crystals that disperse electromagnetic radiation between 4000 and 650cm-1. The grating systems monochromator contain a better resolving power and disperse consistently in all regions of electromagnetic radiation.


The detecting machine is either a thermocouple or a bolometer. A thermocouple is made of 2 different metals attached mutually to which 2 sensitive galvanometers are attached from the other end. The IR radiation impinges on the junction of the metals to produce a thermo electromotive force that enables the current to flow. The current generated is proportional to the quantity of the radiation impinged on the metal.


  • Utilized for the identification of the identity or non identity of 2 samples. This is completed mainly in the finger print region of the spectrum. It is employed for the detection of impurities. This is applied only whenever the impurities absorb strongly in the region where the main component is transparent (for example not absorbing any radiation)
  • Identification of functional groups. I. R. Is extremely helpful for the recognition of several functional groups these as OH, CO, CHO, C = C, NH2 e.t.c.

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