Proton NMR spectroscopy

• a proton NMR spectrum provides 4 important pieces of info about a molecule - provides similar information to a carbon-13 NMR spectrum but for protons:

  • the number of different proton environments - from the number of peaks
  • the types of proton environment - from the chemical shift

  2 extra pieces of info:

  • the relative numbers of each type of proton - from integration tracers or ration numbers of the relative peak areas
  • the number of non-equivalent protons adjacent to given proton - from the spin-spin splitting pattern
• as with carbon-13 NMR spectra, there are broad categories for types of proton as the chemical shift value increases

  • factors such as solvent, concentration and substituents may move a peak outside these ranges
• Equivalent and non-equivalent protons:

  • if 2 or more protons are equivalent, 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 - absorb at different chemical shifts
• Relative numbers of each type of proton:

  • the NMR spectrometer measures the area under each peak as an integration trace (the area under a curve)
  • the integration trace is shown either an an extra line on the spectrum or as a printed number of the relative peak areas
  • provides invaluable info for identifying an unknown compound
• Spin-spin coupling:

  • 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
  • provides info about the no. of protons bonded to adjacent carbon atoms
• the n+1 rule:

  • the splitting of a main peak into sub-peaks is called spin-spin coupling or spin-spin splitting
  • the number of sub-peaks is one greater than the number of adjacent protons causing the splitting
• • for a proton with n protons attached to an adjacent carbon atom, the number of sub-peaks in a splitting pattern = n + 1
• when analysing spin-spin splitting, you are really seeing the number of hydrogen atoms on the immediately adjacent carbon atom
• n = 0

  n+ 1 = 1

  splitting pattern = singlet

  relative peak areas within splitting = 1

  no H on adjacent atoms
• n = 1

  n+1 = 2

  splitting pattern = doublet

  relative peak areas within splitting = 1:1

  adjacent CH
• n = 2

  n+1 = 3

  splitting pattern = triplet

  relative peak areas within splitting = 1:2:1

  adjacent CH2
• n=3

  n+1=4

  splitting pattern = quartet

  relative peak areas within splitting = 1:3:3:1

  adjacent CH3
• these patterns are very common in NMR spectra - this degree of splitting continues with more adjacent protons
• another common pattern is for CH(CH3)2 where a CH proton has 6 protons on the adjacent carbon atoms

  • gives the heptet (7) splitting pattern
• More complex splitting:

  in CH(Ch3)2 the 2 CH3 groups are in the same environment

  sometimes adjacent protons may have different environments

  • in molecules such as CH3CH2CH2COOH , the central -CH2- would be split differently by the CH3 and CH2 protons
  • the resulting splitting would then show as a multiplet
  • more advanced work allows multiplets to ab analysed
• aromatic protons:

  • aromatic protons are expected to absorb in the range of 6.2-8.0 ppm
  • splitting does occur but this can be difficult to interpret
  • only expected to interpret aromatic protons as groups of protons often forming one or more multiplets
• • the relative peak areas within spin-spin coupling follow a pattern called Pascal's triangle
  • as the number of sub-peaks increases, the new extra peak has a relative area equal to the sum of the peak areas immediately above it
• spin-spin coupling in pairs:

  • in an NMR spectrum if you see one splitting pattern there must always be another
  • splitting patterns occur in pairs bc each proton splits the signal of the other
  • can make it easy to spot a structural feature when analysing a molecule
• • there are several very common splitting pairs that you may see in a spectrum
• Hydroxyl and amino protons:

  • organic compounds may contain protons that are not bonded to carbon atoms e.g. organic compounds often contain -OH and -NH protons
• the functional groups involved include:

  • alcohols, ROH, phenols, ArOH and carboxylic acids, RCOOH
  • amides, RNH2, amides, RCONH2 and amino acids, RHC(NH2)COOH
• • in solution, NH and OH protons may be involved in hydrogen bonding and the NMR peaks are often broad and of variable chemical shift
  • OH and NH peaks can occur at almost any chemical shift
  • carboxyl COOH protons are more predictable, absorbing at 10-12ppm
  • the broadening of the peaks also means that OH and Nh protons are not usually involved in spin-spin coupling
  • all this makes assigning OH and NH protons difficult
• Proton exchange:

  1. chemists have devised a technique called proton exchange for identifying -OH and -NH protons
  2. proton NMR spectrum is run as normal
  3. small volume of deuterium oxide, D2O, is added, the mixture is shaken and a second spectrum is run