¹H NMR (AQA A Level Chemistry): Revision Note
Exam code: 7405
¹H NMR
Features of a 1H NMR spectrum
An NMR spectrum shows signal intensity plotted against chemical shift (δ), measured in parts per million (ppm)
The area under each peak (the integration) indicates the relative number of protons in each chemical environment
Although peaks vary in height, it is the area of each peak, not its height, that is used to determine the proton ratio
A single sharp peak appears at the far right of the spectrum at 0 ppm
This is the reference signal from tetramethylsilane (TMS), which is used as an internal standard
Chemical environment
¹H nuclei in different chemical environments absorb radiofrequency radiation at slightly different magnetic field strengths
These differences in environment cause a change in the resonance position, known as the chemical shift
For example, ethanol (CH₃CH₂OH) has three different proton environments: CH3, CH2, and OH
The hydrogen atoms in each of these environments produce separate signals at different chemical shifts
Different types of protons appear within characteristic chemical shift ranges, which can be used to help identify functional groups
Worked Example
How many different 1H environments occur in 2-methylpropane?
Answer:
Two different 1H 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 environments table
Environment of the proton | Chemical shift range (δ) / ppm |
|---|---|
ROH | 0.5-5.5 |
RCH3 | 0.7-1.2 |
RNH2 | 1.0-4.5 |
R2CH2 | 1.2-1.4 |
R3CH | 1.4-1.6 |
RCOCH- | 2.1-2.6 |
ROCH- | 3.1-3.9 |
RCH2Cl or Br | 3.1-4.2 |
RCOOCH- | 3.7-4.1 |
RC=CH- | 4.5-6.0 |
RCHO | 9.0-10.0 |
RCOOH | 10.0-12.0 |
Protons in the same environment are chemically equivalent
1,2-dichloroethane, Cl-CH2-CH2-Cl has one environment as these four hydrogens are all exactly equivalent
Each 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
In a low-resolution ¹H NMR spectrum, each peak corresponds to a different proton (hydrogen) environment in an organic molecule
Ethanol has the molecular formula CH3CH2OH and contains three different proton environments: CH3, CH2 and OH
Therefore, three peaks are observed in its spectrum, typically at approximately 1.2 ppm (CH3), 3.7 ppm (CH2) and around 1–5 ppm for the OH proton (the exact position of the OH peak can vary)
The intensity of each signal is measured by the area under the peak (the integration)
The area is proportional to the number of equivalent ¹H atoms responsible for that signal.
For ethanol, the integration ratio of the CH3: CH2: OH peaks is 3: 2: 1, respectively.
High-resolution 1H NMR
High-resolution ¹H NMR provides additional structural information by showing splitting patterns
In a high-resolution spectrum, a signal may be split into several smaller peaks, known as a multiplet
The splitting pattern is caused by spin–spin coupling between a proton and nonequivalent protons on adjacent carbon atoms
The number of peaks in a split signal follows the n + 1 rule, where n is the number of equivalent protons on neighbouring carbon atoms:
The number of peaks a signal splits into = n + 1
Each splitting pattern also provides information about the relative intensities of the peaks within a multiplet:
A doublet has an intensity ratio of 1:1, meaning both peaks have equal intensity
A triplet has an intensity ratio of 1:2:1, with the central peak having twice the intensity of each outer peak
A quartet has an intensity ratio of 1:3:3:1, with the two central peaks each having three times the intensity of the outer peaks
These intensity ratios follow the pattern of Pascal’s triangle
Integrated Spectra
Integrated trace
In ¹H NMR spectroscopy, the relative areas under each peak show the ratio of the number of protons responsible for each signal
The spectrometer produces an integration trace, which measures the area under each absorption peak
This provides important information for identifying unknown compounds, as it reveals the relative number of protons in different chemical environments
For example, the ¹H NMR spectrum of methyl chloroethanoate, ClCH2COOCH3, shows two proton environments
The CH2 group contributes two protons, and the CH3 group contributes three protons
Therefore, the integration ratio of the peaks is 2 : 3
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)
Spin-Spin Splitting
Spin-spin splitting
Spin–spin splitting in ¹H NMR provides structural information in addition to chemical shift and integration data
A signal in a high-resolution ¹H NMR spectrum may be split into smaller peaks, known as a multiplet
This splitting occurs due to interactions between the spin states of a proton and those of nearby nonequivalent protons in adjacent chemical environments
This effect allows us to determine how many hydrogen atoms are attached to neighbouring carbon atoms
The n+1 rule
The n + 1 rule states that if a proton has n equivalent protons on an adjacent carbon atom, its signal will be split into n + 1 peaks
Therefore, by analysing the splitting pattern, you can determine the number of hydrogen atoms on the immediately adjacent carbon atom
You should be able to recognise common splitting patterns such as singlets, doublets, triplets and quartets in a ¹H NMR spectrum
1H NMR peak splitting patterns table
Number of adjacent protons (n) | Splitting pattern using the n+1 rule, the peak will split into .... | Relative intensities in the 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 |  |
Splitting patterns occur in corresponding pairs because neighbouring protons split each other’s signals
For example, if one set of protons appears as a triplet, the adjacent set will usually appear as a quartet, provided there are no complicating factors
These patterns do not need to be memorised, as they can be determined using the n + 1 rule.
A common example is the CH3CH2 group:
The CH3 signal appears as a triplet (split by two neighbouring protons), and the CH2 signal appears as a quartet (split by three neighbouring protons)
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:
1H NMR spectrum of propane
In propane (CH3CH2CH3), the two CH3 groups are chemically equivalent
The central CH2 group has six neighbouring equivalent protons (three from each adjacent CH3 group). According to the n + 1 rule, where n = 6, the CH₂ signal appears as a heptet (7 peaks)
Each CH₃ group is adjacent to the CH2 group, which contains two equivalent protons
Therefore, each CH3 signal is split into a triplet (n + 1 = 2 + 1 = 3)
Because the two CH3 groups are equivalent, they produce identical triplet signals
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)
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