Infrared spectroscopy

Written by J.A Dobado | Last Updated on April 22, 2024

What is infrared spectroscopy?

Infrared (IR) spectroscopy uses radiation from the electromagnetic spectrum whose wavelength (λ) is between 800 and 400000 nm (0.8 and 400 μ; 1 μ = 10-4 cm).

Therefore, its effect on organic matter is to produce deformations of the bonds of the substance.

asymmetric bond deformation IR infrared spectrum IR
Asymmetric bond deformation (infrared spectrum IR).
symmetric bond deformation IR infrared spectrum IR
Symmetric bond deformation (infrared spectrum IR).

Due to its large size, it is usually divided into three zones:


Being IR the technique normally used experimentally in structure determination (2.5 – 16 μ). Thus, due to historical considerations, the most commonly used unit in IR is not the wavelength (λ) but the wavenumber (ṽ = 1/ λ cm-1). Therefore, the IR corresponds to the range between 4000 and 625 cm-1.

On the other hand, the typical appearance of an IR spectrum is as shown in the figure:

IR infrared spectrum of 4,4-dimethyl-2-pentanone AZASWMGVGQEVCS-UHFFFAOYSA-N
IR infrared spectrum of 4,4-dimethyl-2-pentanone.

Each observable absorption in the spectrum corresponds to a specific vibration of some bond within the molecule.

Bond vibrations

There are different normal modes of vibration in molecules. And these are associated with a characteristic motion of the atoms. The main ones are: bond stretching, valence angle deformations, dihedral angle deformations, out-of-plane deformations, etc.

IR infrared spectrum Bond stretching
Bond stretching.

For example, the normal modes of vibration of formaldehyde. Each of these types of vibration has a characteristic frequency associated with it, which can be calculated using Hooke’s equation for vibrational motion:

υ = (1/2π)·√k/mTo    (ec. 1)

υ = (1/2π)·√k/u    (ec. 2)

where k is the bond strength constant and u is the reduced mass of the system. Depending on the ratio of the masses of the atoms involved in the bond. Thus, we will use equation (1) when mA << mB or (2) when both masses are comparable.

In a molecule with n atoms, 3n-6 tension and bending bands should appear (3n-5 when the molecule is linear).

Thus, of all of them, only those vibrations that produce a change in the dipole moment will give a band observable in the IR (symmetric vibrations do not appear in the IR, but could be observed in Raman spectroscopy).

For example, the vibrational modes of the methylene group will be:

IR infrared spectrum: Normal modes of vibration of the methylene functional group (—CH2—); A symmetric stretching. B asymmetric stretching. C scissoring. D rocking. E wagging (out of plane). F twisting (out of plane).
Normal modes of vibration of the methylene functional group (—CH2—): A symmetric stretching. B asymmetric stretching. C scissoring. D rocking. E wagging (out of plane). F twisting (out of plane).

IR spectrum areas

When identifying functional groups with IR spectroscopy we will consider the IR spectrum divided into several areas:

IR infrared spectrum: areas
IR spectrum areas.
  • Area I from 4000 to 2500 cm-1C-H, O-H and N-H bond stretching.
  • Area II from 2500 to 2000 cm-1: Strechingt of accumulated triple and double bonds.
  • Area III from 2000 to 1500 cm-1: C=O, C=N and C=C stretching.
  • Area IV from 1500 to 600 cm-1: Fingerprint area (bending of CH,CO,CN,CC, etc. bonds).

Identification of functional groups in the IR

According to this division, various functional groups can be identified, as shown in Table 1.

Table 1: Wave number of functional groups in IR.
Nº de onda (cm-1)
OH (without hydrogen bonding)3600
OH (hydrogen bonding)3100-3200
-C ≡ C-2300-2100
-N=C=O~ 2270
-C ≡ N~ 2250
-N=C=S~ 2150
C=C=C~ 1950
α,β-Unsaturated esters1750-1715
Carboxylic acids1725-1700
α,β-Unsaturated aldehydes and ketones1715-1660
Sulfonamides and sulfonates1370-1300



As we can see, most of the most frequent functional groups in organic chemistry show a characteristic absorption in the IR spectrum.