NMR and Chromatography

• TLC Stationary Phase - plastic sheet with a thin layer of silica coated on the surface
• Distance travelled by the solvent depends on:

  • solubility of the solvent
  • affinity for the plate
  • both of these depend on polarity
• Rf = distance travelled by spot / distance travelled by solvent
• TLC - Most plates are coated in silica which is relatively polar. Less polar molecules will travel further up the plate as they have a lower affinity for silica.
• TLC - the more polar the solvent, the further that polar molecules will travel.
• Column Chromatography

  • consists of stationary phase in a tube with loaded sample at the top and a tank to run the mobile phase through.
  • samples are collected at set time intervals from the bottom of the column.
  • less polar molecules arrive first followed by more polar ones.
• HPLC - high pressure liquid chromatography

  • tightly packed column with a liquid solvent under pressure
• NMR

  • The nucleus of some atoms have nuclear spin, although many atoms do not
  • Atoms with an odd numbered nucleus have a spin
  • A nucleus with spin generates a small magnetic field
• 2 main pieces of structural information from 13C NMR:

  • number of different carbon environments - from the number of signals
  • the type of environment - from the chemical shift
• Number of carbon environments - this is not necessarily the number of carbons

  • each carbon is unique = each carbon is an environment
  • no difference between 2+ carbons = count as the same environment
• Chemical Shift

  The spectra are recorded on a scale known as the chemical shift, which is how far the signal is away from the signal for TMS in parts per million.

  It depends on how shielded the atom is.
• TMS

  To calibrate the spectrum, a small quantity of tetramethylsilane is added to samples as this produces a signal providing an internal standard to which other peaks are compared.
• TMS is used because:

  • signal is further right than most due to low electronegativity of Si.
  • only gives one signal as all 4 carbon atoms are chemically equivalent.
  • non -toxic
  • inert
  • low boiling point so can be removed
• Hydrogen NMR - 4 pieces of structural information:

  • number of different hydrogen environments - from the number of signals
  • type of environment - from chemical shift
  • number of hydrogens in an environment - from the integration
  • number of hydrogens in adjacent environments - from the splitting pattern
• Number of H environments

  • Hydrogens attached to the same carbon are always identical
  • If carbons are identical, any hydrogens attached to them are identical
• Relative intensity of signals

  • In H NMR, the area of the signal is proportional to the number of H atoms it represents
• Splitting patterns - Spin-spin splitting (coupling or multiplicity)

  • The number of lines a signal is split into gives information about neighbouring H atoms
  • number of lines = 1 + number of H atoms on adjacent carbon atoms
  • (n+1 rule)
• Double coupling

  • On mid-chain carbons, the protons can couple to both of the carbon groups next to it. This gives a peak which may be a pentet, hextet or just a mess.
• H NMR - if sample is solid

  • dissolve in solvent: CDCl3
  • duterium nucleus does not have spin due to even number of protons