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 spectroscopy for a 2-benzoyloxy tropone mesogen. The proton spectra of the compound in CDCl3 and C6D6 as isotropic solvents have been recorded in the ranges 238-328 K and 277-343 K, respectively, and the rate constants for the dynamic process have been determined by line shape analysis. One-dimensional H-2 NMR spectra of the compound in its own mesophases and in the nematic phase E63, recorded between 300-330 K and 298-353 K respectively, show that the exchange process is in the slow limit for this kind of spectra and allow the study of orientational order. The values of the rate constants in anisotropic environments were yielded 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 compound in E63. The trends of the kinetic constants against temperature, analyzed by the Eyring equation, give the activation parameters of the rearrangement. The activation enthalpies are lower in the isotropic than in the anisotropic phases, and the activation entropies result, respectively, slightly negative and about zero. The frequency of the exchange in the mesophases of the pure compound is significantly lower than the typical frequency of molecular reorientation in nematic and S-C phases.