Proton (¹H) NMR spectroscopy — common mistakes

When deducing a structure from a ¹H NMR spectrum, always use a systematic approach. First, count the number of signals to find the number of proton environments. Second, use the integration ratio to determine the relative number of protons in each environment. Third, analyse the splitting patterns using the n+1 rule to establish which groups are adjacent. Finally, use the chemical shifts to identify the type of environment. Write down your reasoning for each piece of evidence before proposing a final structure.

Due to rapid acid-catalysed exchange, the -OH proton is not bonded to a single oxygen atom for long enough to maintain a consistent spin state relative to its neighbours. This rapid exchange 'averages out' the coupling interaction, so the -OH peak typically appears as a broad singlet and does not split adjacent proton signals (nor is it split by them).

The solvent used for ¹H NMR must not contain any protons (¹H atoms) itself, as it is present in a much larger quantity than the sample and would produce an enormous signal, overwhelming the spectrum. Therefore, deuterated solvents are used, where the hydrogen atoms have been replaced by their isotope, deuterium (²H or D). Common examples include chloroform-d (CDCl₃), deuterium oxide (D₂O), and acetone-d₆ ((CD₃)₂CO).

Usually, no. If the two protons on a CH₂ group are chemically equivalent (which they most often are), they will not split each other. However, in some complex, rigid molecules (like certain cyclic compounds), protons on the same carbon can become non-equivalent (diastereotopic) and can indeed split each other. This is generally considered beyond the scope of the A-Level syllabus.