Overview of Chemical Shifts in H-NMR

The chemical shift of hydrogens is caused by the electron distribution in the molecule -- the movement of the electrons produce small magnetic fields that affect the net field experienced by each hydrogen nucleus (proton). In general, electrons surrounding an atom move in such a way so as to create a field at the atom that tends to counteract the applied magnetic field. The electrons are said to "shield" the proton from the applied magnetic field and this means that less energy is necessary to excite the proton from one spin state to another and therefore its chemical shift occurs at lower frequency than it would otherwise. For example, the acidic hydrogens in carboxylic acids come at higher chemical shifts than most other types of hydrogens since there is a lower electron density around them, thus they are relatively "deshielded".

This simple picture is complicated by the fact that movement of electrons in bonds (particlarly pi bonds and delocalized systems) result in induced magnetic fields that may either shield or deshield adjacent protons. For example, the chemical shift of hydrogens attached to an aromatic ring are higher than one would expect based on considering the electron density around the hydrogen. (Interestingly, if a hydrogen were to be placed in the center of an aromatic ring its chemical shift would be much lower than normal.) Luckily, in most simple organic molecules these so called "anisotropic" effects are fairly predictable. However, you should realize that in more complex molecules, especially those with a rigid 3-dimensional structure, hydrogens may be placed in unusual environments that result in unusual chemical shifts.


Typical H-NMR Shift Ranges
Chemical Shift (d)
Type of Proton
Examples
(Chemical shift in ppm.)
Comments
0.8-1.5 ppm
Alkane C-H
The greater the substitution on the carbon bearing the hydrogen, the further downfield (higher frequency) the resonance occurs. (This is a general trend, add approximately 0.2-0.4 ppm for each additional alkyl group.)
1.6-2.7 ppm
Allylic, benzylic, adjacent to sp2 carbon
(Terminal alkyne hydrogens are usually in the 2.3-3.0 ppm range.)
2.4-4.5 ppm
Adjacent to an electronegative atom
The more electronegative the atom the greater the chemical shift.
4.5-6.5 ppm
Alkene =CH
6.5-8.5 ppm
Aromatic
In alkyl substituted aromatic rings, the aromatic hydrogens normally have similar chemical shifts and may appear as either a broad singlet or complex multiplet.
9.5-10.5 ppm
Aldehyde
Usually show small (1-3 Hz) coupling with adjacent protons.
10.5-13.0 ppm
Carboxylic Acid OH
Carboxylic acid hydrogens occur as broad or very broad singlets due to chemical exchange. Their exact chemical shift depends on concentration, temperature, and solvent.
1-5 ppm
Alcohol OH
Amine NH
Alcohol OH and Amine NH hydrogens often occur as singlets (due to chemical exchange), even when there are other hydrogens 3 bonds away. In some cases, when very pure, they can show typical 3-bond couplings. *The exact chemical shift depends on the concentration, temperature, and solvent used. Addition of D2O will normally cause all hydrogens on non-carbon atoms to exchange with deuteriums, thus making these resonances "disappear". (Amide NH and Phenolic OH come at higher chemical shifts: 5-8 ppm.)
Multiple functional groups For hydrogens on sp3 carbons, chemical shift effects are approximately additive. For hydrogens on sp2 carbons, one must also consider resonance effects and how these change the electron distribution.

This page is maintained by John Hanson. Please e-mail comments to hanson@ups.edu.