Module 6.3.2 - Spectroscopy

• NMR spectroscopy

  A method of determining molecular structure that uses the relative position of carbons and hydrogens determined by the relative shielding and spins of electrons observed when a molecule is exposed to a magnetic field

  
  

  usually relevant for 1H and 13C
• Nuclear spin


  Like electrons, nuclei spin and because of this spinning of a charged particle (positively charged), it generates a magnetic field
• What does C13 NMR tell us about a molecule

  -the number of different carbon environments(no. of peaks)-the types of carbon environment present (chemical shift)
• Carbon environment in NMR

  The environment of a carbon atom can be determined by looking at the sequence of bonds the carbon atom has to other atoms. If two carbon atoms have the same bond sequence they will have the same environment.
• Working out the carbon environments of a molecule (example 1)

  This molecule has3 carbon environments

  1st carbon:CH3 CH2 CHO

  2nd carbon:CH3CH2CHO

  3rd carbon:CH3 CH2CHO

  As you can see the highlighted, all have different neighbours on each side

  
• Working out the carbon environments of a molecule (example 2)

  This molecule has 2 carbon environments

  1st carbon: CH3 CO CH3

  2nd carbon: CH3CO CH3

  3rd carbon: CH3 COCH3

  As you can see the highlighted, the 1st and 3rd carbon have the same neighbours (same sequence of groups on each side) therefore being part of the same carbon environment

  
• Carbon-13 NMR spectrum of


  propanone 2 carbon environments

  Carbon-1: δ - 205 ppm C=O

  Carbon-2: δ - 32 ppm CH3C=O

  2 peaks (= carbon environments)

  
• Working out the carbon environments of a molecule (example 3)


  This molecule has 5 carbon environments and therefore 5 peaks

  Look for symmetry within the molecule Same carbon environment on the methyl groups, carbon attached to methyl group etc.

  
• proton NMR spectroscopy


  a type of NMR spectroscopy which analyses 1H nuclei. The number of peaks on the spectrum shows the number of proton environments and the chemical shifts show the type of environments.
• Integration trace


  The integration trace (equivalent to the area under each peak) shows the relative number of protons in each environment. The splitting pattern can be used to work out the number of adjacent protons
• Proton environment

  protons/H bound to a carbon which are bonded to the same sequence of groups are part of the same environment

  if different groups/is unique, then they can be identified as different environments
• The number of environments is equal to the...…


  number of peaks on the NMR spectrum
• Working out the proton environments of a molecule (example 1)

  4 environments therefore 4 peaks

  Protons on the same carbon are generally part of the same proton environment so they count as 1 Look for symmetry within the molecule (in this case there are none)

  
• Working out the proton environments of a molecule (example 2)

  2 environments therefore 2 peaks

  In this molecule, there is symmetry so there will be an environment/peak for the 2 equivalent COOH protons and an environment/peak for the 4 equivalent protons in the 2 CH2 groups.

  
• The size of the peaks is equal to the...…


  number of atoms in that environment
• Explain why sometimes the chemical shift is sometimes outside of the range provided on NMR spectrum


  this could be due to extra shifting groups on the carbon
• Working out the ratio of the size of the peaks in proton NMR (butanoic acid)

  3:2:2:1

  
  

  3 hydrogens, 2 hydrogens, 2 hydrogens, 1 hydrogen

  
• Spin-spin coupling


  In an NMR spectrum, the interaction between spin states of non-equivalent nuclei that results in the splitting of a signal.
• n+1 rule


  if a signal is split by N neighboring equivalent protons, it will be split into N+1 peaks

  Peaks on a H NMR spectrum always split into the number of hydrogens on the neighbouring carbon(s),plus one.

  
• Spin-spin splitting patterns

  singlet- no H on adjacent carbon atoms

  
  

  doublet- 1 H on adjacent carbon atom CH

  
  

  triplet- 2 H on adjacent carbon atom CH2

  
  

  quartet- 3 H on adjacent carbon atom CH3

  
• Splitting of peaks

  number of split peaks = number of H on adjacent C groups + 1

  
• Spin-spin coupling pairs (common ones)

  4 peaks-3 peaksCH2-CH3

  A quartet (4 splitting pattern) indicates the adjacent carbon atom contains 3 hydrogens CH3 (4-1=3)

  a triplet (3 splitting pattern) indicates the adjacent carbon atom contains 2 hydrogens CH2 (3-1=2)

  
• The exception in the splitting of peaks

  peaks for H on OH/NH do not split and they are usually broad forming a singlet splitting pattern
• How are -OH and -NH protons identified

  -A proton NMR spectrum is first run as normal

  -Add D2O (deuterium oxide)and shake the mixture

  - Run the spectrum again and since Deuterium substitutes the H1 proton, this peak will disappear highlighting the -OH and -NH protons

  D= 2H
• Interpreting proton NMR spectra

  1) Analyse thetypes of proton present and how many of each type (identify peaks, ratios etc.)

  2) Analyse the splitting patterns using the "n+1" rule

  3) Analyse the chemical shifts for the types of proton

  4) Combine information to suggest a structure