LIQUID-CRYSTALS SHOWING A MOLECULAR REARRANGEMENT - A DYNAMIC NMR-STUDY OF A 2-ACYLOXYTROPONE MESOGEN AND 2 RELATED-COMPOUNDS

The [1,9] sigmatropic rearrangement has been studied by Dynamic NMR sp ectroscopy for two 2-acyloxyuopone mesogens and for 2-acetyloxytropone . The proton spectra of the compounds in CDCl3 have been recorded in t he range 213-328 K and the rate constants for the dynamic process dete rmined by line-shape analysis. Evidences of the same rearrangement wer e found in the H-2 NMR spectra of the three molecules in the nematic p hase E63 between 293 and 350 K. Values of the rate constants were yiel ded also by 2D-exchange spectra in the slow rate limit (300-320 K). Th e trends of the kinetic constants against temperature, analyzed by the Eyring equation, give the activation parameters of the rearrangement. The activation energies in the nematic solvent are slightly higher th an the corresponding ones in CDCl3. By extrapolating these results, we can infer a range of values for the frequency of the process in the l iquid-crystalline phases of compounds I and II.