Low & High Resolution Proton NMR
Features of a 1H NMR spectrum
- An NMR spectrum shows the intensity of each peak against its chemical shift
- The area under each peak gives information about the number of protons in a particular environment
- The area under 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
- By definition the chemical shift is at 0 ppm
A low resolution 1H NMR for ethanol showing the key features of a spectrum
Molecular environments
- 1H nuclei that have different neighbouring atoms (said to have different chemical environments) absorb at slightly different field strengths
- The difference environments are said to cause a chemical shift away from TMS
- 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 |
R–CH | 0.5 - 2.0 |
N–CH |
2.0 - 3.0 |
O–CH Cl–CH Br–CH |
3.0 - 4.2 |
4.5 - 6.0 | |
6.2 - 8.0 | |
9.0 - 10.0 | |
10.0 - 12.0 | |
R–OH R–NH |
0.5 - 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 refer to molecular environments of an organic compound
- Ethanol has the molecular formula CH3CH2OH
- This molecule as 3 separate proton 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 and are proportional to 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
High resolution 1H NMR
- More structural details can be deduced using high resolution NMR
- The peaks observed on a high resolution NMR may sometimes have smaller peaks clustered together
- The splitting pattern of each peak is determined by the number of protons in neighbouring environments