How NMR Works & ¹³C NMR (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about the principles of NMR and carbon-13 NMR.

The two go together because every NMR spectrum is measured against the same reference compound, TMS, and a carbon-13 spectrum is that measurement applied to the carbon atoms in a molecule.

Nuclear magnetic resonance spectroscopy is used for analysing organic compounds, and it gives information about the positions of carbon-13 and hydrogen-1 atoms in a molecule. Every sample is measured against a reference compound, tetramethylsilane, or TMS. TMS gives a single sharp peak at zero, and every other peak is plotted as a shift away from it. Those chemical shifts are what a carbon-13 spectrum is made of: each carbon environment in the molecule sits at its own shift.

We'll start with the principles, which set up the reference and the chemical shift scale. Then what a carbon-13 spectrum actually looks like, and finally how you count carbon environments to work out how many peaks to expect.

NMR spectroscopy is used for analysing organic compounds, and it tells you about the positions of carbon-13 and hydrogen-1 atoms in a molecule. All samples are measured against tetramethylsilane. TMS shows a single sharp peak on the spectrum at a value of zero, and sample peaks are then plotted as a shift away from that reference peak. That is what gives the chemical shift values, and chemical shifts are measured in parts per million. Everything on a carbon-13 spectrum is a distance from this one reference point.

Atoms with odd mass numbers usually show signals on NMR. In carbon-13 NMR, the magnetic field strengths of the carbon-13 atoms in the compound are measured and recorded on a spectrum. Most of the carbon in an organic molecule is carbon-12, but a small quantity of the molecules contain carbon-13, and it is those atoms that give the signals. A carbon-13 spectrum displays sharp single signals, with none of the complicated splitting patterns you get in a proton NMR spectrum. The height of each signal is not proportional to the number of carbon atoms in that environment. As with proton NMR, tetramethylsilane is the standard reference point at zero parts per million, so each signal's position is its chemical shift from TMS.

Carbon atoms in different chemical environments give resonances at different chemical shifts. The carbon-13 environments in a molecule are identified in a similar way to the proton environments in hydrogen-1 or proton NMR. Take propanone: it has two different carbon environments, so two signals will be present on its carbon-13 spectrum. Counting the environments tells you how many peaks to expect, and the shift of each one places it on the scale that TMS anchors.

You are often asked why tetramethylsilane is suitable for NMR analysis. TMS is non toxic, it does not react with the sample, it is easily separated from the sample molecule because of its low boiling point, and it produces one strong sharp absorption peak on the spectrum. Counting the number of carbon-13 resonances should also be the first step in analysing a spectrum. For example, the three isomers of dihydroxybenzene can be told apart quickly by considering the symmetry of the molecules, and therefore the number of resonances expected in their spectra.

TMS is the reference compound for NMR, and it sets the zero point. Every other peak is plotted as a chemical shift away from it, measured in parts per million. A carbon-13 spectrum gives sharp single signals, with no splitting patterns, and their heights are not proportional to the number of carbon atoms. Each carbon environment in the molecule gives one peak, at its own shift from TMS.