Infrared Spectroscopy & Molecule Identification

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson we will learn how to read the output from infrared (IR) spectroscopy and how to use it to help identify molecules. We will also learn how IR absorption relates to global warming.

Infrared Spectroscopy

We can use infrared (IR) spectroscopy as a tool to help identify molecules and determine their molecular structure. But how do we understand the output? It just looks like a bunch of lines and curves. What do these lines have to do with alkenes, carboxylic acids, and other chemical groups? When we understand how to read an IR output, then we can identify patterns that can be helpful in identifying the structure.

IR works by vibrating the molecules and then measuring how much they bend or stretch. The faster or more they bend and stretch, the higher and stronger a peak will be observed. So the output is the wavenumber on the x-axis and the transmittance % on the y-axis. IR is mostly useful in determining what functional groups are present. It is difficult to tell from an IR spectrum how the molecule is oriented.

The output from IR spectroscopy can look very confusing with a bunch of curves and lines, but by understanding what it means we can often identify the molecule; here bromomethane is shown
Bromomethane spectrum

The most easily used portion of an IR spectrum is the portion above 1400/cm (also written 1400 cm-1) because the peaks and curves are more obvious to identify. Below 1400/cm is called the fingerprint region. This region is helpful in comparing an unknown compound with a known IR spectrum to see if they are the same, but since the bands can overlap so much it is hard to tell what is really going on. If a compound does not match the fingerprint region perfectly, then it could be an impure mixture.

Functional Groups

The first thing we need to understand in reading the IR output is functional groups. Functional groups are specific molecular groups that are often found on molecules and can be easily identified. These include alkanes, alkenes, alcohols, amines, carboxylic acids, phosphor groups, and halogens. Each functional group has a specific wavelength that is shown on the output.

For example alkanes (or more specifically the C-H sp3 bond) will have a peak around 2850-3000/cm which will look like a combination of 2-3 bands. But an alkene (or the C-H sp2 bond) will have a peak around 3020-3100/cm and an alkyne (the C-H sp bond) will have a peak above 3300/cm. The C-H from the aldehyde will have a double band around 2690-2840/cm and the C=O from the aldehyde will show a band at 1720-1740/cm.

This table shows some of the most common functional groups and the associated IR range.

IR Functional Groups Chart

Example Spectrum

Let's first look at this IR spectrum:

Ethanol IR spectrum

We see that there is a very broad curve at 3358/cm. This broad curve on the high end of the spectrum is very indicative of an alcohol. If this alcohol is part of another functional group (such as a carboxylic acid) then it will be a little lower on the spectrum and there would be other bands to identify what functional group it is from.

Next we see several peaks at 2974/cm, 2927/cm, and 2887/cm. Since these are narrow bands in the 2850-3000/cm range we know that these are from alkanes.

So we know that this molecule has an alcohol and alkanes. So it could be methanol, ethanol, propanol, 2-propanol, etc. So how do we determine how many alkanes there are and where the alcohol is situated? This comes from looking at the bands in the fingerprint region. These bands do tell you the exact configuration, but they start overlapping too much to really be able to tell where one peak begins and another one ends. Instead we can simply compare it to several known alcohol spectrums. We will see when we compare this spectrum to an ethanol spectrum that the fingerprint area will match up. If it almost matches, but not quite, then we can still say it is ethanol, but with impurities.

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