Ch at xx
For acyclic systems, if the X and/or Y groups are small, then the populations of the anti and gauche conformations will be close to statistical (1:2).
Some examples (both A and B shown): ) is nearly zero, which makes L ≈ N.
Thus the two lines of the M quartet will appear at or close to the chemical shift of the N doublet, leaving only a doublet and one ab quartet, whose doublets will be on both sides of the N M lines (simulation A).
These features can be seen in the dioxolane spectrum, which has been simulated using the parameters shown (red trace). Since this is actually an AA'BB' pattern, the N lines each split into two closely spaced ones. Unsymmetrical 1,1-disubstituted cyclopropanes often have AA'XX' patterns that have a quartet-like appearance.
Because the A/X shift difference is diastereotopic in nature, most of these patterns tend toward AA'BB' except at very high field strength. Because J, the M quartet is essentially a doublet, and the L quartet is very strongly leaning (see the spectra and simulation below).
It is possible to specify multiple score / member pairs.
As 300 MHz a more "normal" AA'BB' pattern is seen ) are large enough that the expected first-order AB quartet is not seen, but a much more complicated pattern.In this case, as in the p-disubstituted benzene case above the "AB" character dominates the appearance of the multiplet, largely because the coupling interactions between either of the AB protons and the A'B' protons are much weaker than the A to B and the A' to B' couplings (i.e.
Below is an example of an AA'A''A'''BB'B''B''' system in a pyrrolidine ring. In such cases the degeneracy between spin states is no longer present, and first order systems result. A monosubstituted benzene is an AA'BB'C or AA'MM'X system.In fact, there is free rotation around almost all such bonds in acyclic molecules at accessible temperatures.The appearance of more complicated patterns is the result of a preference for the anti conformation over the gauche (or vice versa), and has nothing to do with the rate of rotation. This type of AA'BB' pattern is much less common than type (a).A simulated spectrum is shown below If we recalculate the spectrum after setting the meta couplings J = 0 then it becomes essentially first order (it would be completely first order if the chemical shifts between A, M and X were made larger).For this reason, some spin systems which are formally of the AA'XX' type, but in which there is no significant spin-spin coupling between the equivalent protons show first order spectra.The two spectral parameters which control appearance of the spectrum are chemical shift difference ν-Y fragment.