132
Number of signals and symmetry in flexible systems
Rule:| In the case of impossibility to fit different conformations by any symmetry operation, the protons involved show different chemical shifts. |
In general you will observe this behaviour if you have CH2 groups and at least one asymmetric carbon atom together in one molecule.
Example: Within the compound
 |
 |
with one asymmetric carbon atom (C*) it is impossible to fit the conformations:
with any symmetry operation. Each moment of the free rotation there interact different chemical environments with both methylene protons H(1) und H(2). Both methylene protons show their individual signal, as demonstrated within the experimental spectrum.
And now back to more fundamental problems