The Pople nomenclature system classifies spin systems based on the difference between their chemical shifts. Coupled spins are denoted by capital letters with subscripts indicating the number of equivalent nuclei. When the coupled nuclei have well-separated chemical shifts, they are assigned letters that are far apart in the alphabet, such as A and X. When the difference in chemical shifts is small, coupled nuclei are named using adjacent letters of the alphabet (AB, MN, or XY).
A proton coupled to two other protons can be an AMX, ABX, or ABC system, based on their chemical shifts. Similarly, an ethyl group is an A2X3 system when attached to an achiral electron-withdrawing group and an ABX3 system if the group is chiral. However, the ethyl groups in triethyl silane would be an A2B3 system.
The Pople system also classifies magnetically nonequivalent nuclei. Two nuclei with equivalent chemical shifts (belonging to the same spin set) can be magnetically nonequivalent if they do not couple equally to each nucleus in every other spin set. Such nuclei are denoted as A and A′. Thus, in p-chloronitrobenzene, a complex spectrum is obtained from the four aromatic protons, which form an AA′XX′ system.
