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Wize University Chemistry Textbook > Structure Elucidation (IR Spectroscopy, NMR, Mass Spectrometry)

Introduction to NMR

Mass SpectrometryPrevious Section
1H NMRNext Section
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Nuclear Magnetic Resonance (NMR) Spectroscopy



NMR Spectroscopy works by introducing a sample into a very strong magnet, tuned to a particular ‘spin-active’ atom isotope, and scanning with variable radio frequency waves. Different chemical environments (i.e. functional groups or multiple bonds) will result in different ‘chemical shifts’ which NMR will reveal.

The two atoms of particular interest to organic chemists are hydrogen (1H, 100% abundant) and carbon (13C, 1% abundant).

The scanning radio frequency results in ‘peaks’ that will be in a particular region (measured in ppm) of a NMR spectrum:


Downfield = Deshielded; Upfield = Shielded




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Factors that Influence Chemical Shift


1. Proximity to Electronegative Atoms


Electronegative atoms will produce downfield shifts in 1H spectra. The closer the atom is to the electronegative atom, the more pronounced the shift will be:

Integration of peaks in 1H NMR spectra is related to number of hydrogens and is often given as the lowest whole number ratio - more on this later!


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Carbon (13C) NMR signals will also shift downfield due to the presence of electronegative atoms. More electronegative atoms will result in a more downfield signal.

2. Hybridization of Atoms


The hybridization of atoms also has an influence on chemical shift. The most downfield shifts are observed with sp2 carbon atoms, and the hydrogens bound to them.





3. Aromaticity


Aromaticity creates an electron cloud that is reflected in the chemical shifts of the atoms. Take for example the cyclic and acylic molecules shown below:



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Which proton will have a more downfield NMR shift in each pair?





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Which carbon will have the lowest ppm value in each pair?