How to teach spectroscopy at post-16

Spectroscopy is the study of the interaction of light and matter. The discovery of how to determine the structure and properties of matter using light over the past two centuries has revealed the sub-microscopic realm. In our everyday lives, for example, spectroscopy allows police officers to accurately test blood alcohol content at the roadside. They analyse breath samples using infrared (IR) spectroscopy in a handheld device. At its most complex, spectroscopy provides an atomic level map of the structure of proteins and how drug molecules interact with them. For example, the Diamond Light Source synchrotron in Didcot, which cost around £260 million to build, was central to solving the structures of proteins in the SARS-CoV-2 virus and in the search for novel drugs to treat the disease it causes, Covid-19.

What students need to know

The electromagnetic (EM) spectrum ranges from low-energy radio waves and microwaves to high-energy X-rays and gamma rays. When a photon (the particle of EM radiation) interacts with matter, the matter absorbs the energy and a change occurs. In nuclear magnetic resonance (NMR) spectroscopy, the spin of atomic nuclei flip from being aligned with the external magnetic field to being aligned against it. In ultraviolet-visible (UV-vis) spectroscopy, electrons in atoms or molecules are promoted from lower energy levels to higher energy levels. In IR spectroscopy, the vibrations of molecules increase from lower energy levels to higher energy levels. In all cases, the energy level of the matter is quantised, meaning it can only exist at fixed levels. Therefore, only photons with energy equal to the difference between energy levels can be absorbed and reemitted. Spectra show a series of peaks related to the energy of the photons absorbed (or released once the matter emits energy). Databases link peak values with structural elements, giving us insight into the overall structure of the matter being analysed.

Common misconceptions

Some students see individual structural elements as being isolated in molecules. This misconception develops early on when students study IR spectroscopy, and single wavenumbers are used to describe a particular bond. The molecular environment surrounding an individual bond changes the energy levels of that bond. For example, a carbonyl in an aldehyde absorbs IR at a lower wavenumber than one in an acyl chloride. In post-16 structural analysis, students have to process broad ranges of values supplied in datasheets. It is important for students to integrate all the data available when determining a structure to ensure they don’t make impossible suggestions. For example, if the empirical formula of the substance shows the presence of only carbon, hydrogen and oxygen, then acyl chloride is not a possibility.

The quantity of information in spectra rapidly increases with the complexity of the molecules, and students can become overwhelmed

Ideas for your classroom

Be selective in which substances you introduce students to early on. The quantity of information in spectra rapidly increases with the complexity of the molecules, and students can become overwhelmed. This is why I particularly like the set of NMR spectra from Knockhardy. They build up the complexity of molecules slowly over the 16 spectra, from a simple haloalkane to a diketone.

Checking for understanding

Students can get good at spectra interpretation without necessarily understanding how the spectra come about. You can assess how much they understand about the underlying concepts by having detailed discussions with them. With larger classes, one-to-one discussions can be harder, so consider setting writing tasks. For example, ask students to write some concise explanations. You could ask them to explain why the hexaaquacopper(II) complex is blue or why single bonds generally have higher IR wavenumbers than double bonds. Or you could ask about the correlation between the electronegativity of adjacent atoms and the chemical shift of hydrogen nuclei in NMR spectra.