MESOPHASES OF ALKYLAMMONIUM SALTS .4. DISORDER AND MOTION IN THE HIGHHOMOLOGS - 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 o f 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 s tate C-13 NMR over a wide range of temperatures covering all phase tra nsitions seen by calorimetry. It is proven that this group of molecule s cannot form plastic crystals. Their mesophases show only conformatio nal disordering of C-C single bonds below isotropization. In most case s, the observed phase transitions in calorimetry can be correlated wit h abrupt changes of the chemical shift of certain carbon atoms in the alkyl chains. The bonds at the chain ends tend to become gradually con formationally mobile and disorder at temperatures much below the first observable transition. The increased mobility is revealed by a gradua l 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 wit h a possible change in the nitrogen-to-anion distance, as shown by sig nificant chemical shift changes of the inner-most carbon atoms. Orient ational and positional disordering is only possible on isotropization (final step of melting).