At -150oC however, the equilibrium between the two conformations
is extremely slow, so now signals are seen for each of the two conformations,
giving eight lines in total as shown in the diagram. The two conformations
A and B are diastereomers of one another and so have different physical
properties including solubility. Thus one of the two conformations (B)
is less soluble in the NMR solvent than the other conformation, and crystalizes
out. This leaves just the other conformation (A) in the solution, so only
the four lines corresponding to this conformation are observed. If the
crystals of conformation B are redissolved, then a spectrum corresponding
to just the four lines of conformation B will be observed. If the solution
of either conformation A or conformation B is allowed to warm to room temperature,
then the rate at which the two conformations interchange will increase
as the temperature is raised, and by the time the solution reaches room
temperature the equilibrium between A and B will have been reestablished
and the original four line, average spectrum will be observed.
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