Introduction to Structure Determination  
NMR: Integration  
Introduction   Infrared   Ultraviolet   Mass Spectrometry   NMR   Crystallography   Combined  
 
In the 1H NMR spectrum, the amount of radiation absorbed for each chemical shift is proportional to the number of nuclei in that environment. This is extremely helpful when combined with an analysis of the number of equivalent nuclei and their chemical shifts.

The NMR spectrometer calculates the area of each peak in the spectrum ("integration"). (The bands in NMR spectra are usually so narrow that the height of the peak is usually a good enough guide.) The spectrum displayed by the spectrometer shows the integral and the ratio of the peak areas.

The 1H NMR spectrum of ethanol is shown below with the integral line shown in orange.

There are 3 1H environments in ethanol, CH3CH2OH, due to the 3 H in the CH3 group, 2 H in the CH2 group and 1 H on the OH group. There are therefore 3 signals in the spectrum with relative areas of 3 : 2 : 1.

Notice that the integration is over each set of lines as each signal is split by coupling.

The integration usually gives the relative areas of the signals.
 
1H NMR spectrum of ethanol

Integrals are not normally displayed on 13C NMR spectra and the peak heights are not a reliable indicator of the number of nuclei in each environment. This is a consequence of the way in which 13C NMR is recorded. As 13C is only 1% abundant, the NMR signal is extremely weak and is barely noticeable above the noise. As a consequence, the spectrometer re-scans the spectrum many times and adds them together so that the true signals grow but any random noise averages to zero. The time between scans is quite short and means that not all nuclei have had a chance to 'relax' (i.e. realign their spin with the magnetic field) before the next scan is recorded. Such nuclei are unable to absorb for this scan.


 
 © Prof Adam Bridgeman, School of Chemistry, The University of Sydney, 2024