NMR-STUDIES OF CORRELATIONS BETWEEN MOLECULAR MOTIONS AND LIQUID-CRYSTALLINE PHASE-TRANSITIONS IN 2 HYDROGEN-BONDED CARBOXYLIC ACID-PYRIDYLCOMPLEXES .1. THE AROMATIC REGIONS

A C-13 solid-state NMR study of two hydrogen-bonded carboxylic acid-py ridyl complexes is presented. The complexes are formed between 4-penty lbenzoic acid in one case and 4-pentylcyclohexanoic acid in the other and 1,2-bis-(4-pyridyl)ethane. The former complex exhibits a transitio n to a liquid-crystalline smectic A phase at 421 K and clears at 452 K , whilst the latter melts directly to an isotropic phase at 404 K. Qua ntitative analysis of the spinning sidebands in magic-angle spinning e xperiments allows the nature and rates of the motion of the aromatic r egions of both complexes to be identified. The degrees of freedom avai lable to both complexes are discussed with reference to their differen t physical properties.