Orientation Studies on Fluorobenzenes in Nematic Liquid Crystals by NMR

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Abstract
The orientation of C6H6, 1,2-, 1,3-, and 1,4-C6H4F2, 1,3,5-C6H3F2, 1,2,4,5- and 1,2,3,5-C6H2F4 and C6F6 in nematic liquid crystalline solvents has been determined by 1H and 19F magnetic resonance measurements. Most of these substances have been studied in two solvents, in 4,4′-di-n-hexyloxyazoxybenzene and in a mixture (3:2) of 4-heptanoyloxy- and 4-hexanoyloxy-4′-ethoxyazobenzene. Orientation parameters (S-values) of the solute molecules are given and their free energies are approximately determined as functions of the orientation. The fluorobenzenes show some correlation in their free energies and in their S-values that are theoretically expected by regarding only dispersion forces and assuming localized contributions from the substituted bonds. The results confirm that the influence of permanent electrical dipole moments is relatively small. Although the orientation in the two solvents is similar, a close comparison shows that there are differences, which are assumed to be due to specific interactions.