ORDER PARAMETERS OF THE MESOGENIC CORE IN 2 NEMATIC LIQUID-CRYSTALS CONTAINING A 4-UNIT LINK

A series of compounds containing a four-unit link, 4'-n-alkylphenyl-4- n-hexyloxycinnamates, has been synthesized, and their mesomorphic beha viour has been determined. The orientational ordering of two compounds in this series has been studied in detail by 2D carbon-13 nuclear mag netic resonance with variable angle sample spinning. The carbon-proton dipolar coupling constants were obtained and the order parameters wer e calculated for different molecular segments (two phenyl rings, the c entral link containing an ethylenic motif, and the methylene carbons o f the alkoxy chain). The temperature dependence of the local order par ameters of each fragment were determined using these data and C-13 che mical shifts which were measured as a function of temperature. In thes e non-rigid molecules, the ratio between the order parameters of the t wo phenyl rings was found to be temperature dependent. The ring bearin g the double bond is more ordered at low temperatures compared to the ring bearing the short alkyl chain, but this behaviour is reversed nea r the clearing point. The position of the major axis of the rigid core was calculated from the order parameter of one ring and the order par ameter of the ethylenic link. It was found that this major axis forms a constant angle with respect to the para axis of the phenyl ring conn ected to the ethylenic group. On the other hand, the angle of this maj or axis with respect to the para axis of the phenyl ring connected to the carboxylic group is temperature dependent, varying from 11-degrees at T/T(NI)=0.92 to 7-degrees at T/T(NI)=0.98.