Chemical shift

In NMR, the chemical shift represents the resonance frequency for a proton, relative to a reference compound. Chemical shift is denoted by the symbol (δ) and is measured in ppm (parts per million).

In a proton NMR spectrum, the chemical shift gives information on the chemical environment of the targeted proton. The chemical shift value is highly impacted by the structure of the analysed compound, especially by electronegative elements or effects. One reason for this is that electronegative elements pull electron density away from the proton, which increases the chemical shift value. This effect is known as 'de-shielding', since it leaves the proton more exposed to the externally applied magnetic field.

Another way the chemical shift can be impacted is through protons of OH and NH groups, NMR spectra of compounds such as alcohols and amines have a broad peak representing the OH and NH protons. This is because these protons are especially labile and have interactions with the solvent, where the protons are exchanged at a faster rate than the NMR is probed. Appearing as a broad peak on the spectrum, whereas the protons that do not undergo fast exchange appear sharp and defined.

Figure 1. Example of proton NMR spectrum, with chemical shift (δ) in ppm on the x-axis.

Below is an overview of the chemical shift values of protons in common functional groups. Note that only the protons in a compound will give rise to a signal in proton NMR analysis.

Figure 2. Overview of Chemical shift ranges in ppm for common functional groups.