Smectogenic C~* 2, 6-bis-arylidene derivativesof 3(R)-ethylcyclohexanone exhibiting inversion of spontaneous polarization sign

摘要

Temperature measurements of ~1H NMR spectra in (CD_3)_2SO solutions have been carried out for 2,6-bis-[4-(4-meth-oxybenzoyloxy)-benzylidene]-3(R)-methylcyclohexanone as a model compound for smectogenic C~* homologues demonstrating an inversion of the spontaneous polarization sign to clarify an opportunity of configurational and conformational changes with the temperature rise as origins of this effect. In the temperature ranges 30 - 110°C, spectral evidences in favor of the possible temperature-induced E→Z isomerization were not revealed. At the same time, changes found in vicinal spin-spin couplings in combination with data of molecular simulation by the semi-empirical PM3 method allowed to determine an appreciable displacement of the chair →← twist-boat conformational equilibrium for the cyclohexanone ring with temperature in direction of the increase by 15 % of the fraction of twist-forms. However, it was demonstrated with molecular simulation this process cannot result in a substantial alteration in the value and direction of the dipole moment. This concerns also intra-molecular rotation in the exocyclic fragments of isolated molecules resulting in conformers with quite different dipole moments, but with the very close energies and populations in the whole temperature interval studied. The single factor responsible for the inversion of the spontaneous polarization sign can be surmised to be changes in conformational energetics.