¹³C and ¹H NMR spectra tell us a lot about the hydrocarbon skeleton of a molecule, and mass spectroscopy weighs the molecule as a whole. But none of these techniques reveal much about functional groups. Some functional groups, for example C O or C=C, can be seen in the ¹³C NMR spectrum because they contain carbon atoms, but many, such as ethers or nitro groups, cannot be seen at all by NMR—they show their presence only by the way they affect the chemical shifts of nearby H or C atoms.

Infrared (IR) spectroscopy, however, provides a direct way of observing these functional groups because it detects the stretching and bending of bonds rather than any property of the atoms themselves. It is particularly good at detecting the stretching of unsymmetrical bonds of the kind found in functional groups such as OH, C=O, NH₂, and NO2, and for this reason IR spectroscopy complements NMR beautifully as a method for structural analysis. NMR requires electromagnetic waves in the radio-wave region of the spectrum to make nuclei flip from one state to another. The amount of energy needed for stretching and bending individual bonds, while still very small, is rather greater, and therefore corresponds to much shorter wavelengths. These wavelengths lie in the infrared, just to the long wave length side of visible light (wavelengths between 10 and 100 mm). When the carbon skeleton of a molecule vibrates, all the bonds stretch and relax in combination and by and large these absorptions are unhelpful. However, some bonds stretch essentially independently of the rest of the molecule, and we can use these to identify functional groups. This occurs if the bond is either:

• much stronger or weaker than others nearby, or

• between atoms that are much heavier or lighter than their neighbours Indeed, the relationship between the frequency of the bond vibration, the mass of the atoms, and the strength of the bond is essentially the same as Hooke’s law for a simple har-monic oscillator. Hooke’s law shows that the frequency of the vibration ν is proportional to the square root of a force constant f—more or less the bond strength—and inversely propor- tional to the square root of a reduced mass μ, that is, the product of the masses of the two atoms forming the bond divided by their sum:

The precise maths is less important to us as chemists than the simple result.

● Stronger bonds vibrate faster and so do lighter atoms.

Infrared spectra are simple absorption spectra. The sample is dissolved in a solvent (or sometimes deposited on the surface of an inert NaCl plate) and exposed to infrared radiation. The wavelength scanned across the spectrum and the amount of infrared energy able to pass through the sample are plotted against the wavelength of the radiation. Just to make the num bers work out nicely, IR spectra don’t usually indicate the wavelength but instead a value known as the ‘wavenumber’, in cm⁻¹, which is simply the number of wavelengths in one centimetre. For a typical bond this will fall between 4000 (short wavelengths, i.e. high frequency) and 500 (long wavelengths, i.e. low frequency). Strong bonds, and light atoms, vibrate fast, so you expect to see these bonds at the high wavenumber end of the spectrum, always plotted at the left-hand end. To illustrate what we mean, here are some typical values for the IR frequencies of a selection of bonds grouped in two ways. Firstly, a series of bonds to increasingly heavy atoms (D, deu-terium, has twice the mass of H, and Cl has about twice the mass of O) and secondly a series of bonds of increasing strength.

Here’s what a typical IR spectrum actually looks like: notice that the wavenumber scale runs from high to low but also that absorption maxima are shown upside down (IR spectra plot ‘transmission’)—you might say that IR spectra are upside down and back to front. If you look carefully you will also see that the scale changes in the middle to give more space to the more detailed right-hand half of the spectrum.

This is the spectrum of cyanoacetamide, the compound shown on the right. The overall shape of the spectrum is characteristic of this compound, but as chemists we need to be able to interpret the spectrum, and we can do this by dividing it up into regions, just as we did with the NMR spectra.