Rotamers- assigned by a simple NMR experiment

Sussex Drug Discovery Centre

Rotamers are conformational isomers where interconversion by rotation around a single bond is restricted and an energy barrier has to be overcome in order to convert one conformer to another.  When this rotation strain barrier is high enough to allow for the isolation of the conformers then the isomers become atropoisomers. Rotamers are however not separable and their existence normally complicates the 1H NMR interpretation. Variable temperature (VT) NMR is the generally preferred method for  studying the equilibration of the rotamers and at low temperatures the spectrum is assigned to the frozen equilibrium and multiple peaks are observed while at higher temperatures the spectrum simplifies as the equivalent peaks are averaged out. Other methods for NMR simplification include the introduction of a complexing agent and solvent switching. All these techniques are inconvenient to the organic synthetic chemist when working on small scale. To overcome this problem, Steve Ley and…

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