Molecular propellers are a kind of atropisomerism involving substances.
They are called molecular propellers because of their analogy with the propellers of airplanes or boats.
Molecules of this type consist of two or more subunits.
They may be single atoms or a combination of atoms.
The term molecular propeller is not confined to cases where the subunits are identical.
An example of 3 bladed propellers is tri – ortho-substituted triaryl borane.
If the three aryl ring is not co-planar, the molecule is chiral.
The interconversion of stereoisomers has been considered in terms of Flip Mechanism, defined as a passage of one or more rings through the plane perpendicular to that of the central atom.