¹H NMR (AQA A Level Chemistry)
Revision Note
¹H NMR
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
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 on neighbouring environments
The number of peaks a signal splits into = n + 1
(Where n = the number of protons on the adjacent carbon atom)
High resolution 1H NMR spectrum of ethanol showing the splitting patterns of each of the 3 peaks. Using the n+1, it is possible to interpret the splitting pattern
Each splitting pattern also gives information on relative intensities
A doublet has an intensity ratio of 1:1 – each peak is the same intensity as the other
In a triplet, the intensity ratio is 1:2:1 – the middle of the peak is twice the intensity of the 2 on either side
In a quartet, the intensity ratio is 1:3:3:1 – the middle peaks are three times the intensity of the 2 outer peaks
Integrated Spectra
Integrated spectra
In 1H NMR, the relative areas under each peak give the ratio of the number of protons responsible for each peak
The NMR spectrometer measures the area under each peak, as an integration spectra
This provides invaluable information for identifying an unknown compound
The 1H NMR of methyl chloroethanoate, ClCH2COOCH3, will show an integration spectra in the peak area ratio of 2:3
2 for the protons in the CH2
3 for the protons in CH3
Spin-Spin Splitting
Spin-Spin Splitting
A 1H NMR peak can show you the structure of the molecule but also the peaks can be split into sub-peaks or splitting patterns
These are caused by a proton's spin interacting with the spin states of nearby protons that are in different environments
This can provide information about the number of protons bonded to adjacent carbon atoms
The splitting of a main peak into sub-peaks is called spin-spin splitting
The n+1 rule
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, it shows you the number of hydrogen atoms on the immediately adjacent carbon atom
These are the splitting patterns that you need to be able to recognise from a 1H spectra:
1H NMR peak splitting patterns table
Splitting patterns must occur in pairs, because each protons splits the signal of the other
There are some common splitting pairs you will see in a spectrum however you don't need to learn these but can be worked out using the n+1 rule
You will quickly come to recognise the triplet / quartet combination for a CH3CH2 because it is so common
Common pair of splitting patterns
A quartet and a triplet in the same spectrum usually indicate an ethyl group, CH3CH2-
The signal from the CH3 protons is split as a triplet by having two neighbours
The signal from the CH2 protons is split as a quartet by having three neighbours
Here are some more common pairs of splitting patterns
Common pairs of splitting patterns
1H NMR spectrum of propane
The CH2 signal in propane (blue) is observed as a heptet because it has six neighbouring equivalent H atoms (n+1 rule), three either side in two equivalent CH3 groups
The CH3 groups (red) produce identical triplets by coupling with the CH2 group
Worked Example
For the compound (CH3)2CHOH predict the following:
i) the number of peaks
ii) the type of proton and chemical shift (using the Data sheet)
iii) the relative peak areas
iv) the split pattern
Answers:
i) 3 peaks
ii) (CH3)2CHOH at 0.7 - 1.2 ppm, (CH3)2CHOH at 3.1 - 3.9 ppm, (CH3)2CHOH at 0.5 - 5.5 ppm
iii) Ratio 6 : 1 : 1
iv) (CH3)2CHOH split into a doublet (1+1=2), (CH3)2CHOH split into a heptet (6+1=7)
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