Nuclear Magnetic Resonance (NMR)

  • Nuclear Magnetic Resonance (NMR) is based on the magnetic properties of certain nuclei such as hydrogen-1 (1H) and carbon-13 (13C).

  • In this technique, the sample to be investigated is placed in a strong magnetic field. The nuclei in the sample absorb and re-emit electromagnetic radiation, producing a spectrum that can be analysed.

  • 1H NMR spectra are the most common uses as most organic compounds contain hydrogen.

  • Each peak in an NMR spectrum represents a different chemical environment for the nuclei.

  • The size of the peaks (integration) gives the ratio of the different types of proton (for 1H NMR) or carbon (for 13C NMR) in the molecule.

  • The chemical shift, measured in ppm, indicates how far the signal is from the reference signal (usually TMS for 1H and 13C NMR - tetramethylsilane).

  • The chemical shift can help us to identify the type of environment the hydrogen or carbon is in. For example, a deshielded (downfield) proton in a 1H NMR often indicates a proton attached to an electronegative atom.

  • The splitting of peaks, also known as spin-spin coupling, can be used to determine the number of adjacent protons or the number of hydrogen atoms on the adjacent carbon atoms. This is known as the “n + 1 rule”.

  • The multiplicity (singlet, doublet, triplet, etc) can help to provide information about the proximity of other nuclei in the molecule.

  • Knowing how to interpret NMR spectra is key to understanding the structure of organic compounds.

  • It’s also important to understand how factors like chemical shift, integration, and multiplicity are influenced by the structure of the molecule.

  • Molecular symmetry can also affect the NMR spectra. Symmetrical molecules tend to have fewer signals because many of the protons are in the same environment.

  • It’s crucial to be able to interpret both 1H (proton) and 13C (carbon) NMR spectra as both provide different types of information about the structure of a molecule.

  • For more complex molecules, 2D NMR techniques like COSY (correlation spectroscopy) and HSQC (heteronuclear single quantum correlation) can be applied to help elucidate the structure.