CIE A Level Chemistry复习笔记8.1.5 Proton (1H) NMR Spectroscopy

• 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

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

• 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)

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)

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