Proton NMR Spectroscopy (HL) (HL IB Chemistry)

• Nuclear Magnetic Resonance (NMR) 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
• In ¹H NMR, the magnetic field strengths of protons in organic compounds are measured and recorded on a spectrum
• Samples are irradiated with radio frequency energy while subjected to a strong magnetic field
• The nuclei can align themselves with or against the magnetic field
• Protons on different parts of a molecule (in different molecular environments) absorb and emit (resonate) different radio frequencies
• All samples are measured against a reference compound – Tetramethylsilane (TMS)
  • TMS shows a single sharp peak on an NMR spectrum, 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 an NMR spectrum

• An NMR spectrum shows the intensity of each peak against their chemical shift
• The area under each peak is proportional to the number of protons in a particular environment
• The height of each peak shows the intensity/absorption from protons

Low resolution proton NMR

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

Chemical environments

• Hydrogen atoms of an organic compound are said to reside in different chemical environments
• Eg. Methanol has the molecular formula CH₃OH
• There are 2 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

¹H NMR Chemical Shifts

• Protons in the same environment are chemically equivalent
• Each peak on an NMR spectrum relates to protons in the same environment
• Peaks on a low resolution NMR spectrum refer to environments of an organic compound
  • E.g. 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)

Low resolution proton NMR of ethanol

Three signals are observed in the low resolution proton NMR of ethanol which correspond to three molecular environments

• The area under each peak is determined by computer and an integration trace overlaid on the spectrum
• The integration trace has stepped lines whose steps are in the same proportion as the peak areas
• This makes it easier to determine the relative abundance of the different proton environments