Nuclear Magnetic Resonance Spectroscopy (Oxford AQA International A Level Chemistry)

NMR Principles

Combined techniques

• Organic chemists use a variety of techniques to determine an organic structure

  • In real laboratory practice, certain pieces of information might already be known

  • In this case, analytical techniques can be used to confirm this information

•  The typical sequence using the common analytical techniques would be:

  1. Elemental analysis - to determine the empirical formula

  2. Mass spectrometry - to determine the molecular mass, and consequently the molecular formula, as well as fragments of the molecule

  3. Infrared spectrometry - to primarily identify functional groups but also identify types of bond, e.g. C-C, C=C

  4. NMR spectrometry  

• Each technique provides valuable information that can be used to propose a structure, although some techniques provide more information than others

  • Elemental analysis is sometimes skipped as the combination of mass spectrometry and NMR can suggest the molecular formula without the need for elemental analysis

Nuclear magnetic resonance (NMR)

• Nuclear magnetic resonance 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

  • This explains why NMR spectroscopy gives information about the position of ¹³C or ¹H atoms in a molecule

Tetramethylsilane (TMS) as a standard

• In NMR spectroscopy, tetramethylsilane (TMS) is used as a reference compound

• The structural formula of TMS is Si(CH₃)₄

The structure of tetramethylsilane

• The organic compound is dissolved in TMS before being introduced to the magnetic field of the spectrometer

• TMS is an ideal chemical to use as a reference because it is:

  • Non toxic

  • Inert - does not react with the sample.

  • Volatile - easily separated from the sample molecule due to its low boiling point.

  • Produces one strong, sharp absorption peak on the spectrum.

¹H NMR and TMS

• All of the protons are in the same molecular environment

• Therefore, TMS gives a single sharp peak on the ¹H NMR spectrum

  • This peak has a very high intensity as it accounts for the absorption of energy from 12 ¹H nuclei

• This reference peak is assigned a value of zero on a ¹H NMR spectrum

• When a sample is analysed, its ¹H peaks are measured and recorded by their shift away from the sharp TMS peak

• This gives rise to the chemical shift values for different ¹H environments in a molecule

The ¹H NMR spectrum for tetramethylsilane

¹³C NMR and TMS

• All of the carbons are in the same molecular environment

• Therefore, TMS gives a single sharp peak on the ¹³C NMR spectrum

  • This peak has a high intensity as it accounts for the absorption of energy from 4 ¹³C nuclei

• When a sample is analysed, its ¹³C peaks are measured and recorded by their shift away from the sharp TMS peak

• This gives rise to the chemical shift values for different ¹³C environments in a molecule