21.1 Spectroscopic identification of organic compounds

High-Resolution 1H NMR spectroscopy

  • Usually, 1H NMR contain multiple peaks
  • In a high resolution H NMR spectrum, single peaks present in low resolution can split into further clusters of peaks
  • These multiple peaks result from spin-spin coupling

Main features of 1H NMR spectra

  1. Number of different absorptions (peaks)
    • Corresponds to the number of different chemical environments occupied by protons
  2. Area under each peak
    • Proportional to the number of hydrogen atoms in that chemical environment
  3. Chemical shift
    • Since spinning electrons create their own magnetic field. This results in a lower frequency at which they resonate. This chemical shift is measured in ppm relative to the standard tetramethylsilane (TMS)
  4. Splitting pattern
    • If the number of adjacent protons is equivalent to n then the peak will be split into (n+1) peaks

TMS as the standard reference

  • All the protons are in the same environment so it gives strong single peak
  • It is not toxic and unreactive
  • It is volatile; can be easily removed from sample
  • Tetramethylsilane

Single crystal X-ray crystallography

  • Single crystal X-ray crystallography can be used to identify the bond lengths and bond angles of crystalline compounds

 

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