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.

deformación de enlace asimétrica espectro infrarrojo IR
Deformación de enlace asimétrica (espectro infrarrojo)
Deformación de enlace simétrica espectro infrarrojo IR
Deformación de enlace asimétrica (espectro infrarrojo)

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

 

Siendo el IR medio el normalmente utilizado experimentalmente en determinación estructural (2.5 – 16 μ). Así, debido a consideraciones de tipo histórico, la unidad más usada en IR no es la longitud de onda (λ) sino el número de onda (υ,υ = 1/ λ cm-1). Por consiguiente, al IR medio le corresponde la zona comprendida entre 4000 y 625 cm-1.

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

espectro infrarrojo IR 4,4-dimetil-2-pentanona

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 deformations, valence angles, dihedral angles, out-of-plane deformations, etc.

fig-3

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. Según sea la relación entre las masas de los átomos que intervienen en el enlace. 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:

fig-4

IR spectrum zones

When identifying functional groups with IR spectroscopy we will consider the IR spectrum divided into several zones as shown in the following figures:

fig-5

  • From 4000 to 2900 cm-1: C-H, O-H and N-H stretching.
  • From 2500 to 2000 cm-1: Accumulated triple and double bonds stretching.
  • From 2000 to 1500 cm-1: C=O, C=N and C=C stretching.
  • De 1500 a 600 cm-1: Fingerprint area (CH,CO,CN,CC, etc.. bending)

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.
Functional
group
Nº de onda (cm-1)
OH (without hydrogen bonding)3600
NH3500-3300
OH (hydrogen bonding)3100-3200
-C ≡ C-2300-2100
-N=C=O~ 2270
-C ≡ N~ 2250
-N=C=S~ 2150
C=C=C~ 1950
Anhydrides1850-1740(2)
-COCl1815-1785
Cyclobutanones1780-1760
γ-lactonas1780-1760
Cyclopentanones1750-1740
Esters1750-1735
α,β-Unsaturated esters1750-1715
δ-Lactones1750-1735
Aldehydes1740-1720
Ketones1725-1700
Carboxylic acids1725-1700
α,β-Unsaturated aldehydes and ketones1715-1660
Amides1690-1630
C=N-1690-1480
NO21650-1500
1400-1250
C-F1400-1000
Sulfonamides and sulfonates1370-1300

1180-1140

sulfones1350-1300
1150-1100
S=O1070-1010
C-Br800-560
C-Cl780-580
C-I600-500

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

fig-6