High Resolution Proton NMR (Edexcel International A Level Chemistry): Revision Note
High Resolution Proton 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
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
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) There are 3 different hydrogen environments - the methyl CH3 hydrogens, the CH hydrogen and the OH hydrogen. This means that there are 3 peaks
ii) By looking the 3 different environments on the data sheet, we can assign chemical shifts:
(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) There are 6 hydrogens in the (CH3)2, 1 hydorgen in the CH and 1 hydrogen in the OH. This gives an overall ratio of 6 : 1 : 1
iv) The methyl groups in (CH3)2CHOH have one neighbouring hydrogen in the CH, which means that its splitting pattern is a doublet (1 + 1 = 2)
The CH in (CH3)2CHOH has 6 neighbouring hydrogens in the 2 methyl groups, which means that it ssplitting pattern is a heptet ( 6 + 1 = 7) - this is more commonly just referred to as a multiplet
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