MESOPHASES OF ALKYLAMMONIUM SALTS .4. DISORDER AND MOTION IN THE HIGH HOMOLOGS - A SOLID-STATE C-13 NMR-STUDY ON (CNH2N+1)4NBR, N = 8, 10, 12, AND 18 AND [CH3(CH2)11]4NI

Conformational disorder and motion in crystals of some high homologs of symmetric tetra-n-alkylammonium halides, (CnH2n+1)4NBr, where n = 8, 10, 12, and 18, and [CH3(CH2)11]4NI, have been analyzed using solid state C-13 NMR over a wide range of temperatures covering all phase transitions seen by calorimetry. It is proven that this group of molecules cannot form plastic crystals. Their mesophases show only conformational disordering of C-C single bonds below isotropization. In most cases, the observed phase transitions in calorimetry can be correlated with abrupt changes of the chemical shift of certain carbon atoms in the alkyl chains. The bonds at the chain ends tend to become gradually conformationally mobile and disorder at temperatures much below the first observable transition. The increased mobility is revealed by a gradual up-field shifting of the resonance peaks with increasing temperature. This feature explains the entropy deficit in total entropy of fusion observed by calorimetry. Crystal structure changes can be coupled with a possible change in the nitrogen-to-anion distance, as shown by significant chemical shift changes of the inner-most carbon atoms. Orientational and positional disordering is only possible on isotropization (final step of melting).