LIQUID-CRYSTALS SHOWING A MOLECULAR REARRANGEMENT - A DYNAMIC NMR-STUDY OF A 2-BENZOYLOXYTROPONE MESOGEN

The [1, 9] sigmatropic rearrangement has been studied by Dynamic NMR s pectroscopy for a 2-benzoyloxy tropone mesogen. The proton spectra of the compound in CDCl3 and C6D6 as isotropic solvents have been recorde d in the ranges 238-328 K and 277-343 K, respectively, and the rate co nstants for the dynamic process have been determined by line shape ana lysis. One-dimensional H-2 NMR spectra of the compound in its own meso phases and in the nematic phase E63, recorded between 300-330 K and 29 8-353 K respectively, show that the exchange process is in the slow li mit for this kind of spectra and allow the study of orientational orde r. The values of the rate constants in anisotropic environments were y ielded by the analysis of two-dimensional exchange H-2 NMR spectra in the range 310-330 K for the pure compound and 328-348 K for the compou nd in E63. The trends of the kinetic constants against temperature, an alyzed by the Eyring equation, give the activation parameters of the r earrangement. The activation enthalpies are lower in the isotropic tha n in the anisotropic phases, and the activation entropies result, resp ectively, slightly negative and about zero. The frequency of the excha nge in the mesophases of the pure compound is significantly lower than the typical frequency of molecular reorientation in nematic and S-C p hases.