AQA Chem4: N.M.R

Revision notes on NMR from AQA Chem4

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  • Created by: anna
  • Created on: 09-06-13 15:25

Carbon 13 NMR

  • NMR stands for Nuclear Magnetic Resonance Spectroscopy
  • there are two isotopes of carbon, 12 and 13, only 1% are 13 however modern instruments are sensitive enough to obtain a carbon 13 spectrum
  • carbon atoms in different environments are affected differently  the magnetic field
  • C atoms are surrounded by electrons in adjacent covalent bonds and the carbon nulei are shielded by these electrons
  • the degree of shielding depends on the electron density surrounding the nucleus
  • NMR produces a graph of energy absorbed vertically against chemical shift horizontally
  • chemical shift is measured in parts per million (ppm) from a defined zero related to a compound called tetramethylsilane, TMS
  • the number of peaks on the graph is the number of different carbon environments in the molecule 
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H low resolution NMR

H (proton) NMR gives us information on:

  • the number of hydrogen atoms in the compound
  • the position of the H atoms in the compound
  • the number of different environments of the H atoms

 

  • the sample is dissolved in a proton free solvent e.g. CCl4 or CDCl3 - this is so they do not produce any absorptions which interfere with the spectra
  • the NMR spectra are recorded with the peaks giving information about the H atoms in the molecule
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The Reference Standard

 

 

  • chemical shift is measured relative to an internal standard
  • this is tetramethylsilane (CH3)4Si, TMS
  • it has 12 equivalent H atoms and so produces a single intense peak
  • its peak is upfield from nearly all other H atom peaks
  • it is non-toxic and inert and easily vapourised
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Chemical Shift and Peaks

Chemical Shift:

  • chemical shift is a measure of the movements caused by shielding and deshielding, atoms with high shielding are moved upfield
  • electronegative atoms cause deshielding so the peak moves downfield

 

Information about the peaks:

  • the number of peaks shows the number of different H atom environments
  • the area under the peaks shows the number of equivalent H atoms, it is quoted as a ratio or given as an intergration trace
  • the position of the peaks shows information about the position of H atoms in the molecule using the H NMR shift data
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H high resolution NMR

High resolution H NMR involves peak splitting:

  • splitting of peaks occurs when neighbouring non-equivalent hydrogen atoms are present
  • non-equivalent adjacent H atoms interact (couple) with each other
  • this results in the signal being split into several peaks
  • splitting is only observed between nuclei with different chemical shifts
  • only H atoms on adjacent C atoms should be considered
  • O-H groups are non-splitting
  • H atoms without non-equivalent neighbouring H atoms are not split so single peaks are produced
  • the number of peaks is equivalent to the number of non-equivalent adjacent protons (n+1 rule)
  • a single peak is a singlet, a double peak is a doublet, a triple peak is a triplet and a quadruple peak is a quartet
  • example: a CH3-CH2 would create a triplet peak and a quartet peak
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