4.9.3 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)

 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

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

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

Chemical Shift Values for ¹H Molecular Environments 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

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

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