How NMR works
- Nuclear Magnetic Resonance (NMR) spectroscopy is used for analysing organic compounds
- Only atoms with odd mass numbers show signals on NMR spectra and have the property of nuclear spin
- In 1H NMR, the magnetic field strengths of protons in organic compounds are measured and recorded on a spectrum
- Samples are irradiated with radio frequency energy while subjected to a strong magnetic field
- The nuclei can align themselves with or against the magnetic field
- Protons on different parts of a molecule (in different molecular environments) absorb and emit (resonate) different radio frequencies
- All samples are measured against a reference compound – Tetramethylsilane (TMS)
- TMS shows a single sharp peak on an NMR spectrum, at a value of zero
- Sample peaks are then plotted as a ‘shift’ away from this reference peak
- This gives rise to ‘chemical shift’ values for protons on the sample compound
- Chemical shifts are measured in parts per million (ppm)
Features of a NMR spectrum
- An NMR spectrum shows the intensity of each peak against their chemical shift
- The area under each peak is proportional to the number of protons in a particular environment
- The height of each peak shows the intensity/absorption from protons
A low resolution 1H NMR for ethanol showing the key features of a spectrum