Infrared spectroscopy (IR Spectroscopy)

What is Infrared Spectroscopy?

Infrared spectroscopyInfrared Spectroscopy

IR spectroscopy which is short for infrared spectroscopy deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. Infrared Spectroscopy is an analysis of infrared light interacting with a molecule.

The IR spectroscopy can be analyzed in three ways: by measuring absorption, emission, and reflection. The major use of this technique is in organic and inorganic chemistry to determine functional groups of molecules. A basic IR spectrum is essentially a graph of infrared light absorbed on the vertical axis vs. frequency or wavelength on the horizontal axis.

What does IR Spectroscopy do?

Infrared Spectroscopy detects frequencies of infrared light that are absorbed by a molecule. Molecules absorb these frequencies of light because they correspond to frequencies of vibrations of bonds in the molecule.

The energy needed to excite the bonds in a molecule to make them vibrate with greater amplitude occurs in the IR region. A bond will only interact with the electromagnetic infrared radiation, however, if it is polar. The presence of separate areas of partial positive and negative charge in a molecule allows the electric field component of the electromagnetic wave to excite the vibrational energy of the molecule. The change in the vibrational energy produces a corresponding change in the dipole moment of the molecule. The intensity of the absorption depends on the polarity of the bond. Symmetrical non-polar bonds in N≡N and O=O do not absorb radiation, as they cannot interact with an electric field.

Regions of the IR spectrum

  • 4000 cm1 – 1300 cm1 1300 cm1 – 400 cm1 

Most of the bands that indicate what functional group is present are found in the mentioned region, from 4000 cm1 to 1300 cm1. Their bands can be identified and used to determine the functional group of an unknown compound.

Bands that are unique to each molecule, like a fingerprint, and are found in the fingerprint region, from 1300 cm1 to 400 cm1. These bands are only used to compare the spectra of one compound to another.

IR Spectroscopy

IR Spectrum

Types of samples which can be used on an IR

Liquid samples: Thin film (neat) – a few drops of the compound are placed on a transparent crystal or between two salt plates

Solid samples: Melt – low melting point, melted between two salt plates KBr pellet – ground with KBr, put into a press – KBr becomes transparent Nujol mull – mush compound in a hydrocarbon oil

Principle Of Infrared Spectroscopy

The IR spectroscopy theory exploits that the fact that molecules absorb specific frequencies that are characteristic of their structure. The energies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms and the associated vibronic coupling.

For instance, the molecule can absorb the energy contained in the incident light and the result is a faster rotation or a more pronounced vibration.

IR Spectroscopy instrumentation

IR Spectroscopy

Graph of IR spectrum

Given below is a sample of typical Infrared Absorption Frequencies.

IR Spectroscopy Graph

Thus, IR spectroscopy involves collecting absorption information and analyzing it in the form of a spectrum.


Practise This Question

Tincture of iodine is a/an