MESOPHASES OF ALKYLAMMONIUM SALTS .2. DISORDER AND MOTION IN THE LOW HOMOLOGS - [CH3(CH2)N-1]NX, N = 1, 2, 3 AND X = BR AND I ANALYZED BY SOLID-STATE C-13 NMR

Disorder and motion in crystals of tetra-n-alkylammonium halides of short chain length alkyl groups, [CH3(CH2)n-1]4NX, where n = 1, 2, 3 and X = Br and I, have been analyzed based on solid state NMR. The solid-mesophase phase transitions involve orientational motion and disorder of the cation as a whole, so that the mesophases must be identified as plastic crystals. Except for the methyl rotation, the conformational motion, i.e., the rotation about single-bonds, is limited to small-angle librations that do not reach other conformational isomers. The analyzed salts show, thus, a plastic-crystal mesophase without conformational disorder. The information on the disorder and motion, obtained initially by thermal analysis of the transition, agrees well with the more detailed NMR results.