The area under each peak in a 1H NMR spectrum is proportional to the number of hydrogen atoms in that environment. The spectrometer shows this as an integration trace: a line drawn across the spectrum that rises in a step at each peak. The height of each step is proportional to the area of the peak below it.
1. measure the height of each step
2. divide each height by the smallest one and find the simplest whole-number ratio
3. compare with the molecular formula: the numbers in the ratio should add up to the total number of H, or a factor of itThe integration trace gives the ratio of hydrogens, not the actual numbers. If the ratio adds up to fewer H than the molecular formula contains, multiply it up.Worked example: a compound C4H8O has three peaks with step heights of 1.2 cm, 1.8 cm and 1.8 cm.
Dividing by 1.2 gives 1 : 1.5 : 1.5, so the whole-number ratio is 2 : 3 : 3.
This adds up to 8, matching the 8 H in C4H8O. The compound has one CH2 group and two CH3 groups in different environments, which fits butanone, CH3COCH2CH3.Some spectra print the relative areas as numbers above each peak instead of drawing a trace; they are used in the same way.
1. measure the height of each step
2. divide each height by the smallest one and find the simplest whole-number ratio
3. compare with the molecular formula: the numbers in the ratio should add up to the total number of H, or a factor of itThe integration trace gives the ratio of hydrogens, not the actual numbers. If the ratio adds up to fewer H than the molecular formula contains, multiply it up.Worked example: a compound C4H8O has three peaks with step heights of 1.2 cm, 1.8 cm and 1.8 cm.
Dividing by 1.2 gives 1 : 1.5 : 1.5, so the whole-number ratio is 2 : 3 : 3.
This adds up to 8, matching the 8 H in C4H8O. The compound has one CH2 group and two CH3 groups in different environments, which fits butanone, CH3COCH2CH3.Some spectra print the relative areas as numbers above each peak instead of drawing a trace; they are used in the same way.