AQA A Level Chemistry 3.3.15 Nuclear Magnetic Resonance Spectroscopy Exam Questions

About AQA A Level Chemistry 3.3.15 Nuclear Magnetic Resonance Spectroscopy

NMR spectroscopy is the most powerful structure-determination technique on the AQA A Level course. You need to interpret carbon-13 NMR spectra by counting carbon environments and using chemical shift values, and interpret proton NMR spectra using chemical shift, integration ratios and splitting patterns from the n+1 rule. You also need to explain why TMS is used as the standard and why deuterated or proton-free solvents are used. NMR is often combined with infrared spectra, mass spectra and chemical tests from 3.3.6 to identify an unknown compound. Examiners set deduction questions where you must justify each piece of evidence and draw the final structure.

What you need to know

• Carbon-13 NMR Each peak represents a different carbon environment. Count environments by looking for symmetry. Chemical shifts from the data booklet show the type of carbon, e.g. C=O in the range 160 to 220 ppm.
• Proton NMR environments Each peak represents a different hydrogen environment. Chemical shift values indicate neighbouring groups, for example hydrogens on a carbon next to C=O or O shift further downfield.
• Integration The relative area under each peak, shown as an integration trace or ratio, gives the relative number of hydrogens in each environment.
• Splitting A peak is split into n+1 lines by n hydrogens on adjacent carbon atoms: singlet (0), doublet (1), triplet (2), quartet (3). An ethyl group gives a triplet and a quartet.
• TMS Tetramethylsilane, Si(CH3)4, is the standard at 0 ppm. It gives a single strong peak because all 12 hydrogens are equivalent, and it is inert, non-toxic and volatile, so it is easily removed.
• Solvents Samples are dissolved in deuterated solvents such as CDCl3 or in proton-free solvents such as CCl4, so the solvent gives no peaks in a proton NMR spectrum.

Common mistakes in 3.3.15 Nuclear Magnetic Resonance Spectroscopy

• Counting hydrogens on the same carbon when using the n+1 rule. Only hydrogens on adjacent carbons cause splitting.
• Ignoring symmetry and so counting too many environments, for example in propanone or propan-2-ol.
• Treating the integration ratio as the actual number of hydrogens. It gives the simplest ratio, which may need scaling up.
• Splitting peaks for O-H or N-H protons. These usually appear as singlets and do not split neighbouring peaks.
• Explaining deuterated solvents as 'not interfering' without saying they contain no 1H atoms, so they give no signal.

Try three 3.3.15 Nuclear Magnetic Resonance Spectroscopy exam-style questions