NMR Spectroscopy

Made using OCR Chemistry Textbook. Pages 84-93. Gent and Richie.

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  • Created by: M
  • Created on: 19-12-12 20:24
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  • NMR Spectroscopy
    • Background
      • Nuclear spin
        • In nucleus, nucleons (protons and neutrons) with opposite spins pair up
          • If uneven no. of nucleons, small residual nuclear spin= magnetic field. Nuclei with magnetic spin can line up with or against a magnetic field
            • Those that oppose MF have a higher energy than those aligned, stronger MF= larger gap btn these energies
      • Resonance
        • Cycle of excitation and relaxation
          • Nucleus in low-energy spin state can be promoted to upper- energy. = excitation. Emits energy as radiation= relaxation.
            • In NMR energy supplied by low-energy radio-frequency radiation
      • Nuclear Shielding
        • An atoms e- shield the nucleus from some of the applied magnetic field.
        • Extent of shielding depends on e- density of near atoms so atoms in diff enviroments have different resonance frequencies= diff signals on NMR spectrum
      • Chemical Shift
        • is a scale that compares the frequency of an NMR absorption with the frequency of  a reference peak of TMS
          • Tetramethylsilane, 12 equivalent protons so single signal @ 0 ppm.
            • When NMR spectra is recorded small amount of TMS is added to calibrate.
            • TMS can easily be removed from sample later.
    • Solvents for NMR
      • Usually carried out in solution but can't use solvent that will produce a signal.
        • So, deuterated solvents are used ie. CDCl3
        • Sample recovered by evaporating off solvent
      • Deuterium = even nucleons (1P, 1N)  = no signal
    • Carbon-13 NMR
      • Carbon-13 = sensitive to nuc. shielding= chemical shift 0-230ppm
      • Interpreting
        • No. of peaks= No. of different C. environments
        • Work out type of environment from chemical shift
          • Presence of  electronegative group= significant c.s.
    • Proton NMR
      • Narrow range of c.s.= overlaps. More abundant than C-13 so smaller sample needed.
      • Interpreting
        • No. of peaks= no. of H environments
        • Chemical shifts= types of  environm.
        • Relative peaks areas= proportions of H per environ.
          • = Integration trace
        • Spin-spin coupling= information about adjacent protons
          • Arises from interactions of spin states of protons on adjacent c's
            • Adjacent Hs may align their spins states in different ways
            • N+1 rule.
          • Only happens btn non-equivalent Hs
  • Only happens btn non-equivalent Hs


Qamar :)

What you could do is use the notes to make your own.

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