Low Resolution Proton NMR (Edexcel International A Level Chemistry)

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Low Resolution Proton NMR

  • Nuclear Magnetic Resonance (NMR) spectroscopy is used for analysing organic compounds
  • All samples are measured against a reference compound – Tetramethylsilane (TMS)

Structural formula of TMS, downloadable AS & A Level Chemistry revision notes Tetramethylsilane is the common reference compound for NMR spectroscopy

  • TMS shows a single sharp peak on NMR spectra, at a value of zero
  • TMS is also used because it is:

    • Non toxic.
    • Does not react with the sample.
    • Easily separated from the sample molecule due to its low boiling point.
    • Produces one strong, sharp absorption peak on the spectrum.
  • 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 1H 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

  • 1H nuclei that have different neighboring atoms (said to have different chemical environments) absorb at slightly different field strengths
  • The difference environments are said to cause a chemical shift of the absorption
    • Ethanol has the structural formula CH3CH2OH
    • There are 3 chemical environments: -CH3, -CH2 and -OH

  • The hydrogen atoms in these environments will appear at 3 different chemical shifts
  • Different types of protons are given their own range of chemical shifts

Worked example

How many different 1H chemical environments occur in 2-methylpropane?

Answer:

Two different 1H chemical environments occur in 2-methylpropane

    • The three methyl groups are in the same 1H environment
    • The lone hydrogen is in its own 1H environment

Worked example environments answer, downloadable AS & A Level Chemistry revision notes

Chemical Shift Values for 1H Molecular Environments Table

Chemical shift values for 1H molecular environments table, downloadable AS & A Level Chemistry revision notes

edexcel-proton-nmr-table

  • Protons in the same chemical environment are chemically equivalent
    • 1,2-dichloroethane, Cl-CH2-CH2-Cl has one chemical environment as these four hydrogens are all exactly equivalent

  • Each individual peak on a 1H NMR spectrum relates to protons in the same environment
    • Therefore, 1,2-dichloroethane would produce one single peak on the NMR spectrum as the protons are in the same environment

1,2-dichloroethane, downloadable AS & A Level Chemistry revision notes

Low resolution 1H NMR

  • Peaks on a low resolution NMR spectrum refers to molecular environments of an organic compound
    • 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)
    • The strengths of the absorptions are proportional to the number of equivalent 1H atoms causing the absorption and are measured by the area underneath each absorption peak
    • Hence, the areas of absorptions of -CH3, -CH2, -OH are in the ratio of 3:2:1 respectively

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

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.