NMR studies of correlations between molecular motions and liquid-crystalline phase transitions in two hydrogen-bonded carboxylic acid-pyridyl complexes .1. The aromatic regions

A C-13 solid-state NMR study of two hydrogen-bonded carboxylic acid-pyridyl complexes is presented. The complexes are formed between 4-pentylbenzoic acid in one case and 4-pentylcyclohexanoic acid in the other and 1,2-bis-(4-pyridyl)ethane. The former complex exhibits a transition 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. Quantitative analysis of the spinning sidebands in magic-angle spinning experiments allows the nature and rates of the motion of the aromatic regions of both complexes to be identified. The degrees of freedom available to both complexes are discussed with reference to their different physical properties.