Rodríguez-Molina, B.; Pérez-Estrada, S.; Garcia-Garibay, M. A. ?Amphidynamic Crystals of a Steroidal Bicyclo[2.2.2]octane Rotator: A High Symmetry Group That Rotates Faster than Smaller Methyl and Methoxy Groups? J. Am. Chem. Soc. 2013, 135, 10388-10395, DOI: 10.1021/ja4024463.
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.