3.3.15 N.M.R

• Why is TMS used

  > TMS gives a single intense peak due to the 12 equivalent protons

  > TMS produces a peak which is very shielded compared to hydrogen atoms in organic molecules

  > TMS is non-toxic and it does not react with the sample

  > TMS has a low boiling point and it can be easily removed
• The sample for NMR must be dissolved in a solvent. The solvent must be proton-free to avoid solvent peaks in the spectrum examples Tetrachloromethane, CCl4. Deuterium has no spin so CDCl3 can be used (D=H2)
• Why is CCl3 and CDCl3 are used in H NMR spectroscopy and explain their properties

  CCl4 and CDCl3 have no protons and they won't produce a peak on the H NMR spectrum.

  > CCl4 is non polar as it is has symmetrical tetrahedral shape so dipoles cancel out. CCl4 is a good solvent to dissolve non polar compounds.

  > CDCl3 is polar as it has is non-symmetrical and has a dipole. So CDCl3 is used to dissolve polar compounds.
• Why is Si(CH3)4 used in H NMR spectroscopy and explain its properties

  > TMS / Si(CH3)4 has a very strong peak due to have 12 protons in a single environment. Therefore it is used as a reference to measure other NMR values. TMS is also inert so it is unlikely to react with other samples.
• Chemical shift depends on the molecular environment