CIE A Level Chemistry (9701) exams from 2022

Revision Notes

8.1.5 Proton (1H) NMR Spectroscopy

Interpreting & Explaining Proton (1H) NMR Spectra

  • 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

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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