18 Jan 2021

Interpreting Infra-red Spectroscopy

Introduction
 
Molecules have covalent bonds and have characteristic frequencies, these bonds rotate, bend, and stretch continuously. If these bonds are given some energy i.e. infra-red radiation they will bend, rotate, or stretch more vigorously and radiation of that frequency will be absorbed. Each bond in a molecule has its own frequency which it absorbs, when infra-red radiation is passed through a sample of an organic compound, some frequencies are absorbed, and some pass through without being absorbed. An infrared spectrum shows which frequencies were absorbed and which passed through, giving a unique “fingerprint” that can be used to identify the functional groups (fragments of molecules) of the molecules present. Below shows a typical infra-red spectrum.

Spec.jpg

The above graph shows a spectrum of Butan-2-ol, the y (vertical) axis shows the transmittance as a percentage and the x (horizontal) axis shows the wavenumber (in cm‑1). If the transmittance is 100%, it means that all the light was transmitted through the sample and nothing was absorbed. So, for that frequency was not absorbed by your compound. However, if the transmittance is less than 100%, it means that some of the light was absorbed. This is known as a peak. Peaks are typically described by their intensity (strong, medium, or weak) and by their peak width (sharp and broad).
 
 
The table below shows sharp and broad peaks present from the infra-red spectrum above.
 
Sharp Peaks cm-1 Broad Peaks cm-1
830 500-800
950 3100-3500
1140  
1180  
1300  
1380  
1500  
2970  
 
Different bonds with different functional groups absorb at different wavenumbers, the peaks shown in an infra-red spectrum is used to determine the different functional groups present in the molecule.

All organic compounds contain C-C and C-H bonds. Some peaks are very easy to recognise such as C=O and O-H bonds.
 
Since many compounds contain C-H and C-C bonds, these peaks are almost always present in an infra-red spectrum: Any other bonds present, however, will give more distinctive peaks:
 
  Bond Frequencies absorbed/ cm-1  
  C-H 2650 – 3310 (sharp)  
  C-C 720 – 1175 (sharp)  
  C-O 1120 - 1310 (sharp)  
  C=C 1620 - 1690 (sharp)  
  C=O 1630 - 1815 (sharp)  
  O-H (alcohols) 3230 - 3350 (broad)  
  O-H (acids) 2500 - 3000 (broad)  
  C-O 1000 - 1300 (sharp)  
 
 
Identifying functional groups
 
Functional groups can be determined by the absorbed frequencies. Four examples of these characteristic absorptions are shown below.

1. Carbonyls (C=O) – Acetone

Spec-1-1.jpg
 
Assignment Intensity Frequency in cm-1
C=O Strong 1680-1750
 
 


2. Alcohols (C-O and O-H): Ethyl Alcohol

Spec-2.jpg

 
Assignment Intensity Frequency in cm-1
O-H Broad, Strong 3200-3550
O-H Bend, Strong 1087-1205





3. Carboxylic acids (C=O, C-O and O-H): Acetic Acid

Spec-3.jpg
 
Assignment Intensity Frequency in cm-1
O-H Broad, Strong 2500-3300
C-O Bend, Medium 1395-1440
C=O Stretch, Strong 1680-1769







Spec-4.jpg
Assignment Intensity Frequency in cm-1
C-O Stretch, Strong 1163-1210
C=O Stretch, Strong 1735-1750
 





5. Hydrocarbons (C-C, C=C, C≡C)

Hydrocarbon compounds contain only C-H and C-C bonds, but there is plenty of information to be obtained from the infrared spectra arising from C-H stretching and C-H bending, depends on whether the C-C is single, double or triple bond as well as how long the molecular chain is.

Spec-5.jpg

 
  n-heptane) n-hept-1-ene n-hept-1-yne
  Frequency cm-1
C–H strong 2800–3000 (strong) 3000-3100 (medium) 3270-3330 (medium)
C=C stretch   1640-1680 (strong)  
C≡C stretch     2100-2260 (weak)
C–H bend 1400-1470 (medium) 650-1000 (medium) 610-700 (medium)
C(H)2-H 1350-1380 (medium)    

6. Aromatic (C=C, C-H): Benzene

Spec-6.jpg
 
Assignment Intensity Frequency in cm-1
C-H Bend, Strong 680-860
C=C stretch, Medium 1550-1700
=C-H Stretch, Variable 3330-3335






Spec-7.jpg

8. Primary Amine (N-H, C-N): Propyl amine

Spec-8.jpg

9. Nitrile (C≡N): Propionitrile

Spec-9.jpg

Inorganic Compounds
Generally, the infrared bands for inorganic materials are broader, fewer in number and appear at lower wavenumbers than those observed for organic materials. If an inorganic compound forms covalent bonds within an ion, it can produce a characteristic infrared spectrum.
Main infrared bands of some common inorganic ions:
 
Bond Frequencies in cm-1
CO32- 1450-1410, 880-800cm-1
SO42- 1130-1080, 680-610cm-1
NO3-       1410-1340, 860-800cm-1
PO43- 1100-950cm-1
SiO42- 1100-900cm-1
NH4+ 3335-3030, 1485-1390cm-1
MnO4- 920-890, 850-840cm-1 
 
 
Diatomic molecules produce one vibration along the chemical bond. Monatomic ligand, where metal s coordinate with atoms such as halogens, H, N or O, produce characteristic bands. These bands are summarized in below.
Characteristic infrared bands of diatomic inorganic molecules: M(metal), X(halogen)
 
Bonds Intensity Frequencies in cm-1
M-H          Stretching 1700-2250
M-H            Bending  600-800-600
M-X          Stretching 750-100cm-1
M=O         Stretching 850-1010-850
M=N          Stretching 875-1020
 
The fingerprint region
 
The region of the spectrum which is mostly used to identify functional groups is between 1500 and 3500 cm-1 as most functional groups give characteristic absorptions in this region.
 
The region between 500 – 1500 cm-1 of the spectrum is more complex and typically has a lot of peaks which are very close together and thus could be difficult to identify. These peaks are not from specific bonds but a result of the structure of the molecule as a whole. This region is completely differentfor each molecule structure even those with the same functional group. This region is identified as the fingerprint region.
 
One of the most common application of infrared spectroscopy is to the identification of organic compounds. The major classes of organic molecules are shown in this article.
 
Identification  
The following is a suggested strategy for determination of a molecular structure.
  1. Concentrate on the 1500 and 3500 cm-1 regions first and on the typically functional groups.
  2. Use a frequency table to identify the functional groups for each frequency
  3. Use the fingerprint region (below 1500 cm-1) to confirm or elaborate on structural elements.
  4. Do not try to assign every single peak in the spectrum
  5. Cross check wherever possible
  6. Take note of negative and positive evidence
  7. In some cases, some band intensities may vary significantly for the same group.
  8. If sample is in solution, some frequency bands are solvent-sensitive. Be cautious when using small wavenumber changes
Infra-red spectroscopy is a great identification of a sample if it is used in conjunction with other analytical methods such as nuclear magnetic resonance, mass spectroscopy and elemental analysis.