NMR

• Different types of NMR

  • 13C NMR
  • 1H (proton) NMR
• 13C NMR


  There is only around 1% C13 in organic molecules but modern NMR machines are sensitive enough to give a full spectra for C13
• C13 NMR spectrum


  There is one signal (peak) for each set of equivalent C atoms
• Equivalent hydrogen atoms


  In an 1H NMR spectrum, there is one signal for each set of equivalent H atoms. The intensity (integration value) of each signal is proportional to the number of equivalent H atoms it represents.
• Ethanol


  • 3 groups of different hydrogen atoms: CH3 (ratio 3), CH2 (ratio 1), CH (ratio 2)
• Solvents


  Samples are dissolved in inert solvents with no 1H atoms, e.g. CCl4, CDCl3. These solvents have no H atoms so will not give any peaks in the 1H NMR spectrum.
• Calibration and shift


  A small amount of TMS (tetramethylsilane) is added to the sample to calibrate the spectrum. The spectra are recorded on a scale known as the chemical shift (δ), which is how much the field has shifted away from the field for TMS.
• Chemical shift (δ)


  A relative scale of how far the frequency of the proton signal has shifted away from that for TMS, measured in parts per million (ppm).
• 1H NMR shift


  The δ depends on what other atoms/groups are near the H – more electronegative groups gives a greater shift.
• Hydrogen bonded to N or O


  Usually do not couple with other protons and appear as singlets on the 1H NMR spectra.
• Spin-Spin coupling in 1H NMR

  Splitting of peak = number of inequivalent H's on neighbouring C atoms + 1
• Elemental analysis


  Used to determine the empirical formula
• Molecular formula


  Determined from the molecular ion peak in the mass spectrum
• IR spectra


  Used to identify main bonds/functional groups
• NMR spectra


  Used to give details of the carbon chain
• The final structure is C-CH3-C-CH3-CH3-CH2-C-O-O-CH2-CH3