Proton NMR (OCR A Level Chemistry A): Revision Note
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
Spin-Spin Splitting Patterns
Spin-Spin Splitting
A high resolution 1H NMR spectrum 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 or spin-spin coupling
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
Examiner Tips and Tricks
It is very rare that the spin-spin splitting of equivalent protons is covered in teaching because it is so rarely asked in exams
Equivalent protons do not cause spin-spin splitting
The simplest example of this is benzene
In benzene, all of the protons are equivalent
This means that they are seen as one singlet in the high resolution 1H NMR spectrum of benzene
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 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 splitting 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)
Tetramethylsilane as a Standard
Tetramethylsilane
The horizontal scale on an NMR spectrum represents chemical shift (δ)
Chemical shift is measured in parts per million (ppm) of the radio frequency needed for resonance compared to a reference chemical called tetramethylsilane, abbreviated to TMS
The displayed formula of tetramethylsilane
TMS is used universally as the reference compound for NMR as its methyl groups are particularly well shielded and so it produces a strong, single peak at the far right of an NMR spectrum
The signal from the hydrogen atoms in TMS is defined as having a chemical shift of 0 ppm
The NMR reference peak for TMS
Hydroxyl and amino protons
Organic compounds often contain protons from part of a functional group rather than being bonded to a carbon, examples include:
Alcohols, ROH
Carboxylic acids, RCOOH
Phenols, ArOH
Amines, RNH2
Amides, RCONH2
Amino acids, H2NCHRCOOH
These hydroxyl, OH, protons and amino, NH, protons can be involved in hydrogen bonding which causes their peaks to be broader than normal and they can appear at almost any chemical shift
The broadening of the peak means that they are not usually involved in spin-spin coupling and therefore appear as singlets
Deuterated solvents
Deuterated solvents such as D2O (heavy water) and CDCl3 can be used to identify OH and NH protons
This is achieved by:
Performing a standard proton NMR on the sample compound
A small amount of the deuterated solvent is added to the sample compound and mixed
A second proton NMR is then performed
Deuterium atoms from the solvent replace the OH and NH protons in the sample, e.g for ethanol
C2H5OH + CDCl3 ⇌ C2H5OD + CHCl3
This means that the second proton NMR will still have the peaks for the CH3 and CH2 protons of ethanol but will not have the OH peak as that proton has been exchanged
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