Cryst. Growth Des., 2009, 9 (7), pp 3124–3128

Engineering Crystal Packing and Internal Dynamics in Molecular Gyroscopes by Refining their Components. Fast Exchange of a Phenylene Rotator by 2H NMR
Miguel A. Garcia-Garibay* and Carlos E. Godinez

Abstract
Using quadrupolar echo 2H NMR, we have determined that a relatively simple change on the periphery of the triptycene stators of molecular gyroscopes may have a profound effect on the packing arrangements, packing coefficients, and rotary dynamics of the central phenylene rotators. The previously reported crystal structure of 1,4-bis-[2-(9-triptycyl)-ethynyl]benzene (1) is characterized by the inclusion of meta-xylene and a very tightly interdigitated packing arrangement that effectively prevents the rotary motion of the phenylene rotator. Structural modifications to prevent this interdigitation led to the design and synthesis of 1,4-bis[2-(2,3,6,7,12,13-hexamethyl-10-propyl-9-triptycyl)ethynyl]benzene (2), which had been shown to crystallize in the desired manner but with the inclusion of bromobenzene. Using crystals of 2 with a 2H-labeled phenylene rotator, we determined by quadrupolar echo 2H NMR line shape analysis that rotation occurs by a 180° site exchange (2-fold flip) with frequencies in the MHz regime at low temperatures (150−183 K). From the temperature dependence of the rotational exchange frequency, we determined a barrier of 4.4 kcal/mol, which is only 1.4 kcal/mol higher than the internal barrier for ethane in the gas phase. Additional spectral narrowing observed at higher temperature was analyzed in terms of a model that considers larger amplitude excursions between the 180° jumps.