Low Resolution Proton NMR
- Nuclear Magnetic Resonance (NMR) spectroscopy is used for analysing organic compounds
- All samples are measured against a reference compound – Tetramethylsilane (TMS)
Tetramethylsilane is the common reference compound for NMR spectroscopy
- TMS shows a single sharp peak on NMR spectra, at a value of zero
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TMS is also used because it is:
- Non toxic.
- Does not react with the sample.
- Easily separated from the sample molecule due to its low boiling point.
- Produces one strong, sharp absorption peak on the spectrum.
- Sample peaks are then plotted as a ‘shift’ away from this reference peak
- This gives rise to ‘chemical shift’ values for protons on the sample compound
- Chemical shifts are measured in parts per million (ppm)
Features of a 1H NMR spectrum
- NMR spectra shows the intensity of each peak against their chemical shift
- The area under each peak gives information about the number of protons in a particular environment
- The height of each peak shows the intensity / absorption from protons
- A single sharp peak is seen to the far right of the spectrum
- This is the reference peak from TMS
- Usually at chemical shift 0 ppm
A low resolution 1H NMR for ethanol showing the key features of a spectrum
Molecular environments
- 1H nuclei that have different neighboring atoms (said to have different chemical environments) absorb at slightly different field strengths
- The difference environments are said to cause a chemical shift of the absorption
- Ethanol has the structural formula CH3CH2OH
- There are 3 chemical environments: -CH3, -CH2 and -OH
- The hydrogen atoms in these environments will appear at 3 different chemical shifts
- Different types of protons are given their own range of chemical shifts
Worked example
How many different 1H chemical environments occur in 2-methylpropane?
Answer:
Two different 1H chemical environments occur in 2-methylpropane
- The three methyl groups are in the same 1H environment
- The lone hydrogen is in its own 1H environment
Chemical Shift Values for 1H Molecular Environments Table
- Protons in the same chemical environment are chemically equivalent
- 1,2-dichloroethane, Cl-CH2-CH2-Cl has one chemical environment as these four hydrogens are all exactly equivalent
- Each individual peak on a 1H NMR spectrum relates to protons in the same environment
- Therefore, 1,2-dichloroethane would produce one single peak on the NMR spectrum as the protons are in the same environment
Low resolution 1H NMR
- Peaks on a low resolution NMR spectrum refers to molecular environments of an organic compound
- Ethanol has the molecular formula CH3CH2OH
- This molecule as 3 separate environments: -CH3, -CH2, -OH
- So 3 peaks would be seen on its spectrum at 1.2 ppm (-CH3), 3.7 ppm (-CH2) and 5.4 ppm (-OH)
- The strengths of the absorptions are proportional to the number of equivalent 1H atoms causing the absorption and are measured by the area underneath each absorption peak
- Hence, the areas of absorptions of -CH3, -CH2, -OH are in the ratio of 3:2:1 respectively
A low resolution NMR spectrum of ethanol showing 3 peaks for the 3 molecular environments