NMR studies of correlations between molecular motions and liquid-crystalline phase transitions in two hydrogen-bonded carboxylic acid–pyridyl complexes. Part 2.—The alkyl regions

Abstract

A ¹³C solid-state NMR study of two hydrogen-bonded carboxylic acid–pyridyl complexes is presented. The complexes are formed between 4-pentyl benzoic acid (PH complex) in one case and 4-pentyl cyclohexanoic acid (CH complex) in the other and 1,2-bis-(4-pyridyl)ethane. Two different two-dimensional NMR techniques are employed to analyse the motion of the alkyl regions of these two complexes. One technique separates the ¹³C chemical shift anisotropy powder lineshapes of each ¹³C site according to the isotropic ¹³C chemical shift of that site. Subsequent quantitative analysis of the powder lineshapes indicates that the C₅ alkyl chain of the PH complex is static on the NMR timescale at 298 K, whilst the C₅ chain in the CH complex undergoes diffusive rotational motions at the same temperature. A second experiment separates the ¹H dipolar-broadened lines according to the ¹³C chemical shift of the ¹³C spins to which each ¹H nucleus is bound. The results of this experiment suggest that the C₅ chain in PH remains fairly static at 373 K, but that in CH undergoes motions of greater amplitude at the higher temperature. These different mobilities can be understood in terms of 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 mobilities of the two complexes in their solid phases.