Proton NMR spectroscopy
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Summary
Proton NMR spectroscopy
In a nutshell
Proton NMR utilises the odd number of nucleons in the hydrogen nuclei to measure their absorption at different frequencies. Splitting is a feature of proton NMR which can tell you the number of hydrogen atoms on an adjacent carbon.
NMR
Proton nuclear magnetic resonance (NMR) is how hydrogen nuclei, protons, react in a magnetic field. Each peak on a spectrum represents a hydrogen environment. The number above the peak gives the ratio of the area under the peaks. This shows the relative number of hydrogens present in the hydrogen environment. They are given as ratios. This is because they show the relative number of hydrogens, not the total amount.
Splitting patterns
Splitting patterns are a feature of high-resolution proton NMR. The splitting pattern of each peak is determined by the number of protons on adjacent carbon atoms.
; where is the number of protons on adjacent carbon atoms.
Number of proton on adjacent carbon | Splitting pattern | splitting pattern ratio | Shape |
| | , singlet | |  |
| | , doublet | |  |
| | , triplet | |  |
| | , quartet | |  |
Splitting pattern | Example | Explanation |
Singlet |  | For the proton , there are no protons on the adjacent carbon atom, so a singlet it produced. |
Doublet |  | For the proton , there is one proton () on the adjacent carbon atom, so a doublet is produced. |
Triplet |  | For the proton , there are two protons () on the adjacent carbon atom, so a triplet is produced. |
Quartet |  | For the proton , there are three protons () on the adjacent carbon atom, so a quartet is produced. |
Integration traces
Integration traces show the area under the peaks more clearly. This shows the number of hydrogens in a particular environment using an integration line.
 | |
There are two hydrogens in environment a and three hydrogens in environment b, so the integration trace shows their areas in the ratio . | There are six hydrogens in environment a and one hydrogen in environemnt b, so the integration trace shows their areas in the ratio . |
Hydrogen-free solvents
In proton NMR, if a sample has to be dissolved, it can not be done in a solvent which contains atoms, as they would appear on the spectra. This will make it difficult to distinguish specific peaks.
Deuterated solvents are often used. Hydrogen atoms in water have been replaced by deuterium or . This doesn't show up on the NMR spectra as it has an even number of nucleons (one proton and one neutron), unlike water which only has one proton. can also be used.
Chemical shifts
To correspond to each peak in an NMR spectrum with a hydrogen environment, a table with the chemical shifts and hydrogen environments is provided. Hydrogen environments can overlap which can mean that matching peaks to chemical shifts may not always be easy.
NMR CHEMICAL shifts relative to (SOLVENt) | |||||
| Hydrogen environment | Example | |||
| | Alcohol | | |||
| | Alkane | | |||
| | Alkyl next to | | |||
| | Alkyl next to aromatic ring | | |||
| | Hydrogen on alkyl amine | | |||
| | Aryl amine | | |||
| | Alkyl next to a halogen | | |||
| | Alkene | | |||
| | Phenol, phenylamine | | |||
| | Amide | | |||
| | Aromatic ring | | |||
| | Aldehyde | | |||
| | Carboxylic acid | | |||
FAQs - Frequently Asked Questions
What is peak splitting?
Peak splitting is the splitting of proton peaks which shows the number of hydrogens on adjacent carbon atoms.
What is the difference between proton NMR and carbon NMR?
Proton NMR looks at the hydrogen environments and the amount of protons whilst carbon NMR looks at the carbon environments and the amount of carbons.
What is proton NMR used for?
Proton NMR is a spectroscopic technique which can help identify or confirm the structure of organic compounds or those that contain protons.