Peak Splitting in Proton NMR (HL) (DP IB Chemistry): Revision Note
Peak Splitting in Proton NMR
In the first part of NMR spectroscopy, we have seen that the nuclei of H atoms:
Behave as tiny magnets
Interact with an applied magnetic field
Only atoms with odd mass numbers show signals on NMR spectra and have the property of nuclear spin
Table showing nuclei which posses spin
Nuclei | Protons | Neutrons | Spin |
---|---|---|---|
1H | 1 | 0 | ✓ |
2H | 1 | 1 | x |
12C | 6 | 6 | x |
13C | 6 | 7 | ✓ |
19F | 9 | 10 | ✓ |
31P | 15 | 16 | ✓ |
Atoms with odd mass numbers either align themselves:
With the external magnetic field (lower energy state)
Against the external field (higher energy state)
Samples are irradiated with radio frequency energy while subjected to a strong magnetic field
Energy from the radio frequency end of the electromagnetic spectrum excites the nuclei
This causes them to resonate or ‘flip’ between the lower and higher energy states
Protons in different molecular environments absorb and emit (resonate) different radio frequencies
The magnetic field strength of protons in organic compounds is measured and recorded on a spectrum
The resonance energy is unique to specific H atoms in molecules that are located in the same chemical environment
Information from the spectrum tells us the number of different H environments
A reminder about low resolution 1H NMR:
Low resolution 1H NMR of ethanol

Chemical shift
The horizontal scale on an NMR spectrum represents chemical shift (δ)
Chemical shift:
Is measured in parts per million (ppm) of the magnetic field strength needed for resonance
Measured against a standard or reference chemical called tetramethylsilane, abbreviated to TMS

TMS is used universally as the reference compound for NMR as its methyl groups are particularly well shielded
This means that it produces a strong, single peak at the far right of an NMR spectrum
The signal from the carbon atoms in TMS is defined as having a chemical shift of 0 ppm
So, TMS gives a strong single reference peak on an NMR spectrum at 0 ppm:

The chemical shift values of peaks on an 1H NMR spectrum give information about the likely types of proton environment in a compound
Relevant data for interpreting ¹H NMR spectra is provided in the IB Chemistry data booklet (Section 21)
Worked Example
Use the 1H NMR data table located in the IB Chemistry data booklet (section 21) to assign the 3 proton peaks for propanal.

Answer:
From the molecule, there are 3 different proton environments:
CH3-, highlighted in blue
-CH2-, highlighted in green
-CHO, highlighted in red
There are 3 peaks on the 1H NMR spectrum:
9.7 ppm
2.6 ppm
1.0 ppm
Using the 1H NMR data table, each peak can be assigned to a proton environment:
9.7 ppm = R-CHO
2.6 ppm = R-CO-CH2-
1.0 ppm = -CH3

Peak splitting
High resolution NMR provides more structural detail than low resolution NMR
Each signal in a high resolution spectrum can appear as a cluster of smaller peaks
This is know as multiplicity
Splitting occurs because neighbouring protons influence the magnetic field experienced by a proton
It depends on how the spins align with or oppose the external field
A neighbouring proton can either align with or oppose the magnetic field
When aligned:
The neighbouring spin slightly increases the effective magnetic field
This causes a slightly higher chemical shift
When opposed
The neighbouring spin slightly reduces the effective magnetic field
This causes a slightly lower chemical shift
One neighbouring proton
If a proton has one neighbouring proton, there are two possible alignments:
Protons aligned
Protons opposed

This causes the single low resolution 1H NMR peak to split into two equal peaks
The peaks are equal because of their 1:1 intensity ratio
This is called a doublet
This pattern is only obtained when there is one neighbouring proton
Two neighbouring protons
If a proton has two neighbouring protons, there are four possible combinations of alignment:
First neighbour | Second neighbour | Field strength | Frequency |
---|---|---|---|
+ | + | stronger | 1 |
+ | – | unchanged | 2 |
– | + | unchanged | |
– | – | weaker | 1 |
Two of the possible combinations have the same outcome on field strength
They are equivalent peaks
So, three separate peaks are obtained
But one peak is twice as strong as the others
The peaks have an 1:2:1 intensity ratio
This is called a triplet
This pattern is obtained when a proton is next to a -CH2- group
Three neighbouring protons
If a proton has three neighbouring protons, there are eight possible alignments:
First neighbour | Second neighbour | Third neighbour | Field strength | Frequency |
---|---|---|---|---|
+ | + | + | strong increas | 1 |
+ | + | – | slight increase | 3 |
+ | – | + | slight increase | |
– | + | + | slight increase | |
+ | – | – | slight decrease | 3 |
– | + | – | slight decrease | |
– | – | + | slight decrease | |
– | – | – | strong decrease | 1 |
Three of the eight combinations cause the same increase in field strength (+, +, -)
Another three cause the same shift decrease in field strength (+, -, -)
These groups form two medium-intensity peaks
Overall, four separate peaks are obtained
But two peak are three times as strong
The peaks have an 1:3:3:1 intensity ratio
This is called a quartet
This pattern is obtained when a proton is next to a -CH3- group
The n+1 rule and splitting patterns
The number of neighbouring protons determines how a signal is split
This follows the n+1 rule
A proton with n equivalent neighbouring protons produces n + 1 peaks
Each pattern has a characteristic name, intensity ratio and shape:
Number of adjacent protons (n) | Splitting pattern using the n+1 rule the peak will split into .... | Relative intensities in splitting pattern | Shape |
---|---|---|---|
0 | 1, singlet | 1 | ![]() |
1 | 2, doublet | 1 : 1 | ![]() |
2 | 3, triplet | 1 : 2 : 1 | ![]() |
3 | 4, quartet | 1 : 3 : 3 : 1 | ![]() |
Final summary
An NMR spectrum provides several types of information:
The number of signal groups shows the number of different proton environments
The chemical shift indicates the general environment of the protons
The area under each peak (integration) shows the relative number of protons in each environment
The multiplicity (splitting pattern) reveals how many protons are on adjacent atoms
This information is often enough to deduce the structure of an organic molecule
Other techniques are often used alongside NMR to confirm the structure, such as:
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