5: Infrared spectroscopy

Infrared (IR) spectroscopy is used to probe covalent bonds in an organic molecule, and give us information on different functional groups present. Though IR spectroscopy doesn’t give us the same depth of information that an NMR spectrum does, it is a quick way to learn whether certain functional groups are present in a molecule, and so is used as a complementary technique to NMR spectroscopy. 

Theory behind infrared (IR) spectroscopy

Infrared radiation has the right energy to cause vibrational transitions in molecules. There are different types of vibrational modes, of two general types: stretching and bending. 

Stretching:

Bending:

To understand why different bonds vibrate at different frequencies, it can be helpful to imagine a covalent bond connecting two atoms as being a system of two balls (the atoms) and a spring (the bond). Infrared radiation is absorbed by the bond, or spring, causing it to vibrate. If one of the balls is made heavier, the frequency at which the spring vibrates decreases. Conversely, if the spring is made more rigid, the frequency that it vibrates increases. 

The same is true for covalent bonds connecting atoms.

Consider a carbon atom connected by a single covalent bond to one of three atoms: hydrogen, oxygen, chlorine.

The C-H bond vibrates with the highest frequency, then C-O, then C-Cl. This is the case where one of our atoms is made heavier, and the vibrational frequency decreases.

A C≡C triple bond vibrates at a much higher frequency than a C=C double bond, which vibrates at a higher frequency than a C-C single bond. In our analogy, increasing bond strength is making the spring more rigid.

How do we translate this into reading IR spectra?

Reading an IR spectrum

An IR spectrum measures % Transmittance on the y-axis, against frequency in wavenumbers (cm-1) on the x-axis. Unlike other spectra we look at in this depth study, the baseline for an IR spectrum is at the top of the y-axis, and the peaks point towards the x-axis.

There are 2 main regions to look at on an IR spectrum:

1. Between 1800 and 1630 cm-1, the C=O stretch will appear as a strong, sharp peak.

2. The O-H stretch is strong and broad, between 3500 and 3000 cm-1. An N-H stretch also appears in this region, usually sharper, though less intense.

Let's compare the IR spectra of ethanol (left) and acetic acid (right):

Both ethanol and acetic acid have very broad, intense peaks between 3500 and 3000 cm-1. This is due to the O-H stretch, found in both molecules. Acetic acid also has a carbonyl (C=O) functional group. This appears as a very sharp, intense peak around 1700 cm-1. This is absent from the IR spectrum of ethanol because ethanol does not contain this functional group.

Look at the spectra of phenol and aniline below, to compare an OH stretch to an NH stretch. Notice that both peaks appear in the same region, but the NH stretch is typically less broad, and less intense. 

An IR spectrum can be thought of as a molecular barcode – and as such is highly specific to the molecule and can give us a wealth of information. For our purposes, being able to identify a few key regions will be enough to interpret the spectrum. 

You may find the following table helpful when characterising your spectra:

The region below 1400 cm-1 is known as the “fingerprint region” – we rarely need to interpret this region of the spectrum.

Remember that IR is always most useful when combined with other spectroscopic and spectrometric techniques.