AQA A Level Chemistry

Revision Notes

7.10.1 Principles of NMR

Principles of NMR

  • Nuclear Magnetic Resonance (NMR) spectroscopy is used for analysing organic compounds
  • Atoms with odd mass numbers usually show signals on NMR
  • In 1H NMR, the magnetic field strengths of protons in organic compounds are measured and recorded on a spectrum
  • Protons on different parts of a molecule (in different molecular environments) emit different frequencies when an external magnetic field is applied
  • All samples are measured against a reference compound – Tetramethylsilane (TMS)
    • TMS shows a single sharp peak on NMR spectra, 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

  • NMR spectra shows the intensity of each peak against their chemical shift
  • The area under each peak gives information about the number of protons in a particular environment
  • The height of each peak shows the intensity/absorption from protons
  • A single sharp peak is seen to the far right of the spectrum
    • This is the reference peak from TMS
    • Usually at chemical shift 0 ppm

Analytical Techniques - Features of a 1H NMR Spectrum, downloadable AS & A Level Chemistry revision notes

A low resolution 1H NMR for ethanol showing the key features of a spectrum

Molecular environments

  • Hydrogen atoms of an organic compound are said to reside in different molecular environments
    • Eg. Methanol has the molecular formula CH3OH
    • There are 2 molecular environments: -CH3 and -OH
  • The hydrogen atoms in these environments will appear at 2 different chemical shifts
  • Different types of protons are given their own range of chemical shifts

Chemical shift values for 1H molecular environments table

Analytical Techniques - Chemical shift values for 1H molecular environments table (1), downloadable AS & A Level Chemistry revision notes Analytical Techniques - Chemical shift values for 1H molecular environments table (2), downloadable AS & A Level Chemistry revision notes

  • Protons in the same molecular environment are chemically equivalent
  • Each peak on a NMR spectrum relates to protons in the same environment

Low resolution 1H NMR

  • Peaks on a low resolution NMR spectrum refers to molecular environments of an organic compound
    • Eg. Ethanol has the molecular formula CH3CH2OH
    • This molecule as 3 separate environments: -CH3, -CH2, -OH
    • So 3 peaks would be seen on its spectrum at 1.2 ppm (-CH3), 3.7 ppm (-CH2) and 5.4 ppm (-OH)

Analytical Techniques - Low Resolution NMR of Ethanol, downloadable AS & A Level Chemistry revision notes

A low resolution NMR spectrum of ethanol showing 3 peaks for the 3 molecular environments

High resolution 1H NMR

  • More structural details can be deduced using high resolution NMR
  • The peaks observed on a high resolution NMR may sometimes have smaller peaks clustered together
  • The splitting pattern of each peak is determined by the number of protons on neighbouring environments

The number of peaks a signal splits into = n + 1
(Where n = the number of protons on the adjacent carbon atom)

Analytical Techniques - High Resolution NMR of Ethanol, downloadable AS & A Level Chemistry revision notes

High resolution 1H NMR spectrum of Ethanol showing the splitting patterns of each of the 3 peaks. Using the n+1, it is possible to interpret the splitting pattern

  • Each splitting pattern also gives information on relative intensities
    • E.g. a doublet has an intensity ratio of 1:1 – each peak is the same intensity as the other
    • In a triplet, the intensity ratio is 1:2:1 – the middle of the peak is twice the intensity of the 2 on either side

1H NMR peak splitting patterns table

Analytical Techniques - 1H NMR peak splitting patterns table, downloadable AS & A Level Chemistry revision notes

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