The compound we use as a reference sample is usually tetramethyl silane, TMS. This is silane (SiH₄) with each of the hydrogen atoms replaced by methyl groups to give Si (CH₃)₄. The four carbon atoms attached to silicon are all equivalent and, because silicon is more electropositive than carbon, they are fairly electron-rich (or shielded), which means they resonate at a frequency a little less than that of most organic compounds. This is useful because it means our reference sample is not bang in the middle of our spectrum! The chemical shift, δ, in parts per million (ppm) of a given nucleus in our sample is defined in terms of the resonance frequency as:

No matter what the operating frequency (i.e. strength of the magnet) of the NMR machine, the signals in a given sample (e.g. ethanol) will always occur at the same chemical shifts. In ethanol the (red) carbon attached to the OH resonates at 57.8 ppm whilst the (green) carbon of the methyl group resonates at 18.2 ppm. Notice that by definition TMS itself resonates at 0 ppm. The carbon nuclei in most organic compounds resonate at greater chemical shifts, normally between 0 and 200 ppm. Now, let’s return to the sample spectrum you saw on p. 54 and which is reproduced below, and you can see the features we have discussed. This is a 100 MHz spectrum; the horizontal axis is actually frequency but is usually quoted in ppm of the field of the magnet, so each unit is one ppm of 100 MHz, that is, 100 Hz. We can tell immediately from the three peaks at 176.8, 66.0, and 19.9 ppm that there are three different types of carbon atom in the molecule.

¹³C NMR spectrum of lactic acid