Precessional Motion in Crystalline Solid Solutions of Ionic Rotors

The order–disorder phase transition associated with the uprise of reorientational motion in (DABCOH2)2+, in the supramolecular salts of general formula [1·(DABCOH2)]X2 (where 1=12-crown-4, DABCO=1,4- iazabicyclo[2.2.2]octane, and X=Cl- or Br-), has been investigated by variable temperature X-ray diffraction on single crystals and powder samples, as well as by DSC and solid-state NMR spectroscopy (SSNMR). The two compounds undergo a reversible phase change at 292 and 290 K, respectively. The two crystalline materials form solid solutions [1·(DABCOH2)]Cl2xBr2(1-x) in the whole composition range (0 < x<1), with a decrease in the temperature of transition to a minimum of ca 280 K, corresponding to x=0.5. Activation energy values for the dynamic processes, evaluated by variable-temperature 13C magic-angle spinning (MAS) SSNMR and line-shape analysis are ca. 50 kJmol-1 in all cases. Combined diffraction and spectroscopic evidence has allowed the detection of a novel dynamic process for the (DABCOH2)2+ dications, based on a room temperature precessional motion that is frozen out below the disorder–order transition; to the best of the authors’ knowledge this phenomenon has never been observed before.