From Molecular Propellers to Molecular Gears The torsional motions of two or more internal
rotors in a molecule may be so strongly coupled that correlated rotation becomes
energetically preferred over the independent rotation of the individual groups. 1 In ap-
propriately structured molecules this phenomenon may result in the formation of
stereoisomers, as first dem-onstrated for molecular propellers. 2 For example, it is possible
underordinary laboratory conditions to separate two conformational diastereomers of (2 …

Empirical force-field calculations show that bis (9-triptycyl) methane has a Cs ground state
and that the securely meshed triptycyl groups undergo nearly unhindered cogwheel rotation
through a C 2 transition state, with an activation energy of≈ 1.0 kcal mol-1. These features
lead to note-worthy stereochemical attributes in appropriately substituted derivatives and
analogs. Bis (9-triptycyl) carbinol has been prepared as an example of a compound that is
chemically achiral, even though all conformations are chiral under the constraint of gear …