Low Resolution Proton NMR (Edexcel International A Level (IAL) Chemistry): Revision Note
Exam code: YCH11
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
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 different 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
Environment of proton | Chemical shift range, δ / ppm |
|---|---|
ROH | 1.9-3.9 |
RCH3 | 0.7-1.2 |
RNH2 | 1.0-4.5 |
R2CH2 | 1.2-1.4 |
R3CH | 1.4-1.6 |
RCOCH- | 2.1-2.6 |
ROCH- | 3.1-3.9 |
RCH2Cl or Br | 3.1-4.2 |
RCOOCH- | 3.7-4.1 |
RC=CH- | 4.5-6.0 |
RCHO | 9.0-10.0 |
RCOOH | 10.0-12.0 |
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 structural 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
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