© Copyright Hans J. Reich 2020 The analysis of complex NMR patterns is assisted by a general labelling method for spin systems introduced by Pople. Each set of chemically equivalent protons (or other nuclei) is designated by a letter of the alphabet. Nuclei are labeled
If groups of nuclei are magnetically equivalent, they are labeled _{3} groups are A, or _{3}X. A group of magnetically equivalent nuclei must have identical chemical shifts, and all members of the group must be coupled equally to nuclei outside the group. If nuclei are chemical shift equivalent but not magnetically equivalent, then they are labeled _{3}AA', BB'B'' or XX'. Thus in an A system the _{2}X_{2}A nucleus must have identical couplings to the two X nuclei. In an AA'XX' system, on the other hand, J_{AX} ≠ J_{AX'}. There are usually profound differences in the appearance of A compared to _{2}X_{2}AA'XX' patterns.
First order. Significant parameters:
First order. Significant parameters: Second order. Both
First order. Significant parameters: Second order. This is a very common pattern.
Second order. This pattern can only be accurately solved using computer simulation methods. Manual analysis as a distorted ABX or even AMX pattern will lead to approximate values of coupling constants, which in severe cases can be drastically wrong.
First order. Significant parameters:
Second order. Computer simulation required to solve.
First order. This is a very rare pattern. Second Order. Rare. Second order. Common pattern. Can be solved by hand, but there are several ambiguities. For example, one cannot distinguish
Second order. This is a common pattern. The entire multiplet is centrosymmetric (i.e., the
First or second order depending on ν Also commonly seen in 1,4-disubstituted dienes as well as in 1,2- and 1,3-disubstituted benzenes, 2- or 3-substituted pyridines and related aromatics.
First order. Very common pattern: ethyl groups: CH
Second order. Seen in ethyl groups CH Second order, but soluble by hand. Commonly seen in ethyl groups in chiral molecules where the CH
Second order. Part structures like -CH
Always second order. Commonly seen in monosubstituted phenyl groups. Monosubstituted cyclopropanes are also AA'BB'C systems.
A common type involves isolated allyl groups in achiral molecules.
Part structures like R Examples: 1, 2, 3, 4, 5. This spin system also appears in HC=CH-CH-CH_{2}.Examples: 1.
Seen in systems like -CH
Second order, but a common type, R
If R Common types are 1,1- or 1,2-disubstituted propyl groups. For the former, if R If R
First order. Common pattern: isopropyl groups: (CH
Not actually first order. However, a common type, n-propyl groups CH Next Section: Symmetry · Previous Section: Long-range Coupling · Home |