H NMR

• A proton NMR spectrum provides four important pieces of information about a molecule

  1. the number of different proton environments
  2. the types of proton environments present
  3. the relative numbers of each type of proton
  4. the number of non-equivalent protons adjacent to a given proton
• the number of different proton environments = the number of peaks
• the types of proton environments present can be figured out from the chemical shift
• the relative numbers of each type of proton - from integration traces or ratio numbers of the relative peak areas
• the number of non-equivalent protons adjacent to a given proton - from the splitting pattern
• The chemical shift is referenced against TMS at 0 ppm
• The chemical shift range of about 12 ppm is much narrower than the C-13 spectra range of about 220 ppm
• Factors such as solvent, concentration, and substituents may move a peak outside of chemical shift ranges
• If two or more protons are equivalent i.e. have the same chemical environment they will absorb at the same chemical shift, increasing the size of the peak
• Protons of different types have different chemical environments and are non-equivalent - they absorb at different chemical shifts
• A good way of visualising equivalent and non-equivalent protons is to look for any plane of symmetry in the molecule
• For proton NMR - the ratio of the relative areas under each peak gives the ratio of the number of protons responsible for each peak
• The NMR spectrometer measures the area under each peak as an integration trace (mathematically, integration means the area under a curve)
• The integration trace is shown either as an extra line on the spectrum or as a printed number of the relative peak areas
• The integration provides invaluable information for identifying an unknown compound
• A proton NMR peak can also be split into sub-peaks or splitting patterns. These are caused by the proton's spin interacting with the spin states of nearby protons that are in different environments.
• Splitting provides information about the number of protons bonded to adjacent carbon atoms
• The n + 1 rule is when the number of sub-peaks is one greater than the number of adjacent protons causing the splitting
• When analysing splitting, you are really seeing the number of hydrogen atoms on the immediately adjacent carbon atom
• Splitting pattern names

  • 1 peak is a singlet
  • 2 peaks is a doublet
  • 3 peaks is a triplet
  • 4 peaks is a quartet
• If the peak is a singlet the neighbouring carbon has 0 hydrogen atoms attached to it
• If the peak is a doublet the neighbouring carbon has 1 hydrogen atom attached to it, adjacent CH
• If the peak is a triplet the neighbouring carbon has 2 hydrogen atoms attached to it, adjacent CH2
• If the peak is a quartet the neighbouring carbon has 3 hydrogen atoms attached to it, adjacent CH3
• Another common pattern is for CH(CH3)2 where a CH proton has six protons on the adjacent carbon atoms. This gives the heptet (7) splitting pattern.
• When environment adjacent protons have different environments the central group would be split differently, resulting the splitting to be shown as a multiplet.
• The splitting pattern for aromatic protons is interpreted as groups of protons often forming one or more multiplets.
• In an NMR spectrum, if you see one splitting pattern there must always be another - splitting patterns occur in pairs because each proton splits the signal of the other
• A very common splitting pattern to spot is CH3CH2 which has a triplet/quartet combination
• In H-NMR, assigning OH and NH protons to peaks is difficult because they are not usually involved in splitting, which causes the broadening of its peaks.
• Chemists have devised a technique called proton exchange for identifying -OH and -NH protons
• Proton exchange

  1. A proton NMR spectrum is run as normal
  2. A small volume of deuterium oxide, D2O, is added, the mixture is shaken and a second spectrum is run
• During proton exchange, Deuterium exchanges and replaces the OH and NH protons in the sample with deuterium atoms
• For example, proton exchange with methanol:

  CH3OH + D20 = CH3OD + HOD
• E.g. The second spectrum is essentially being run on CH3OD. As deuterium does not absorb in this chemical shift range, the OH peak disappears.
• Interpreting proton NMR spectra

  1. Analyse the types of proton present and how many of each type
  2. Analyse the splitting patterns to find information about adjacent protons
  3. Using the data sheet with analyse the chemical shifts for the types of proton
  4. Combine the information to suggest a structure
• Predicting a H NMR spectrum

  1. Draw the structure and identify the number of chemical environments
  2. Use data sheet to predict the chemical shifts
  3. Predict the relative peak heights from the number of each type of proton
  4. Predict the splitting patterns from the number of H atoms on adjacent C atoms