Interpreting Infrared Spectra

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.

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.

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:

Identifying functional groups

Functional groups can be determined by the absorbed frequencies. Several examples of these characteristic absorptions are shown below.

1. Carbonyls (C=O) – Acetone

C=O is characterised by a sharp peak at 1715 cm^-1. Also found in carboxylic acids and esters.

2. Alcohols (C-O and O-H): Ethanol

O-H is characterised by a strong broad peak at 3200 – 3550 cm^-1. For C-O bond there is a sharp peak at 1050 cm^-1.

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

O-H bond in carboxylic acid is characterised by a broad peak at 2500 – 3300 cm^-1. Notice that there are also sharp peaks at 1710 cm^-1 C=O and 1300 cm^-1 C-O.

4. Esters (C=O and C-O): Ethyl Acetate

C-O is characterised by a sharp peak at 1210 cm^-1 and 1750 cm^-1 for C=O bond. (Note the absence of a broad peak between 2500 and 3500 cm^-1, thus no O-H bond present).

5. Hydrocarbons (C-C, C=C, C≡C): Heptane, Heptene and Heptyne

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.

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

The =C–H stretch in aromatics is observed at 3330-3335 cm^-1. Note this is higher frequency than is the –C–H stretch in alkanes. The C=C sharp peak can be found at 1550-1700 cm^-1.

7. Amide (C=O, N-H): Propanamide

Amide is characterised by a strong C=O band at 1650 cm^-1 (note this is below the carbonyl absorption for aldehyde and ketone). Absorptions for the NH (symmetric and asymmetric) band is in the range of 3200-3400 cm^-1. The broadness of the bands is likely due to hydrogen bonding.

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

N-H is characterised by a medium peak at 660-900 cm^-1. C-N stretch is found at 1029-1200 cm^-1. Note the N-H bend which shows a symmetric and asymmetric band in the range of 3500 cm^-1.

9. Nitrile (C≡N): Propionitrile

C≡N is characterised by the sharp peak at 2250 cm^-1. (Note this is in the same region as the alkyne C≡C)

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:

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)

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.

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