Amphidynamic Crystals of a Steroidal Bicyclo[2.2.2]octane Rotor: A High Symmetry Group That Rotates Faster than Smaller Methyl and Methoxy Groups

The
synthesis, crystallization, single crystal X-ray structure,
and solid state dynamics of molecular rotor 3 provided
with a high symmetry order and relatively cylindrical bicyclo[2.2.2]­octane
(BCO) rotator linked to mestranol fragments were investigated in this
work. By use of solid state 13C NMR, three rotating fragments
were identified within the molecule: the BCO, the C19 methoxy and
the C18 methyl groups. To determine the dynamics of the BCO group
in crystals of 3 by variable temperature 1H spin–lattice relaxation (VT 1H T1), we determined the 1H T1 contributions from the methoxy group C19 by carrying
out measurements with the methoxy-deuterated isotopologue rotor 3-d6. The contributions from the
quaternary methyl group C18 were estimated by considering the differences
between the VT 1H T1 of mestranol 8 and methoxy-deuterated mestranol 8-d3. From these studies it was determined that
the BCO rotator in 3 has an activation energy of only
1.15 kcal mol–1, with a barrier for site exchange
that is smaller than those of methyl (Ea = 1.35 kcal mol–1) and methoxy groups (Ea = 1.92 kcal mol–1), despite
their smaller moments of inertia and surface areas.