How To Interpret Infrared Spectra

Infrared spectroscopy is a very commonly used analytical technique used to determine organic compounds in Chemistry. It is often used alongside other techniques in order to fully establish the identity of an unknown compound (or in some cases, identify compounds present in a mixture). IR spectroscopy helps to identify which functional groups are present in a sample.

The concept involves the use of infrared radiation passing through an unknown sample. Different chemical bonds in a compound absorb infrared radiation to different degrees. Each different absorbance is measured and recorded. From this, a spectrum (basically a diagram showing the output of the analysis) is produced. A typical spectrum will consist of curves and "peaks" - commonly called bands - that represent different functional groups.

So since each chemical bond absorbs radiation differently, they appear in different regions of a spectrum and tend to have defining physical characteristics. For example, you should describe the appearance of a band as strong or weak, sharp or broad.

When defining the region of a band, use wavenumber ranges to describe its position in the spectrum. For example, a strong, sharp band in the range of 1620 - 1680 cm^-1. (cm^-1 is referred to as `wavenumbers`).

As a general rule of thumb, bands in the range 1500cm^-1 and below are not included in the analysis. This is the fingerprint region and the peaks here are not very distinguishable anyway.

Now that you know the language used in describing bands, these are the most characteristic bands to look out for in organic compounds:

O-H band/peak

This is a strong, broad peak that is commonly found in the range of 2500 - 3550 cm^-1. There are 2 sub-categories of the OH band - OH bonds found in alcohol compounds are found in the higher range of 3230 - 3550 cm^-1 and OH bonds found in carboxylic acid (-COOH) compounds are found in the lower range of 2500 - 3000 cm^-1

C=O band/peak

The carbonyl bond absorbs radiation in the range of 1680 - 1750cm-1 and produces a very strong, sharp peak. Look out for this peak if you suspect the compound is an aldehyde, ketone or carboxylic acid.

C=C band/peak

Unsaturated compounds, like alkenes or compounds with a benzene ring, will produce a strong, sharp peak in the region of 1620 - 1680 cm-1. Not to be confused with the carbonyl peak.

Note: These numbers may vary slightly from the numbers provided in your textbook or data sheet, but be sure to refer to the ranges provided to you in answering any exam questions.