NMR-STUDIES OF CORRELATIONS BETWEEN MOLECULAR MOTIONS AND LIQUID-CRYSTALLINE PHASE-TRANSITIONS IN 2 HYDROGEN-BONDED CARBOXYLIC ACID-PYRIDYLCOMPLEXES .2. THE ALKYL 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 l benzoic acid (PH complex) in one case and 4-pentyl cyclohexanoic aci d (CH complex) in the other and 1,2-bis-(4-pyridyl)ethane. Two differe nt two-dimensional NMR techniques are employed to analyse the motion o f the alkyl regions of these two complexes. One technique separates th e C-13 chemical shift anisotropy powder lineshapes of each C-13 site a ccording to the isotropic C-13 chemical shift of that site. Subsequent quantitative analysis of the powder lineshapes indicates that the C-5 alkyl chain of the PH complex is static on the NMR timescale at 298 K , whilst the C-5 chain in the CH complex undergoes diffusive rotationa l motions at the same temperature. A second experiment separates the H -1 dipolar-broadened lines according to the C-13 chemical shift of the C-13 spins to which each H-1 nucleus is bound. The results of this ex periment suggest that the C-5 chain in PH remains fairly static at 373 K, but that in CH undergoes motions of greater amplitude at the highe r temperature. These different mobilities can be understood in terms o f differences in the molecular packing in the solids. The formation of a liquid-crystalline phase for PH and the lack of such a phase for CH can be understood, at least in part, on the basis of the different mo bilities of the two complexes in their solid phases.