DEUTERIUM AND C-13 NMR OF THE SOLID AND DISCOTIC PHASES OF 3 BENZENEHEXA-N-ALKANOATES

Deuterium and carbon-13 NMR measurements are reported of the solid and discotic phases of three homologues of the benzenehexa-n-alkanoate series, BHAn, with n (the number of carbons in the alkanoate chains) equal to 6, 7, and 8. All compounds show a high degree of solid polymorphism, and the two higher homologues exhibit a discotic mesophase. The transition between the solid phases is associated with an increase of disorder in the side chains reflecting the increase in the number of populated conformational states. Contrary to a previous interpretation the interconversion rate between the conformers is fast on the NMR time scale and the evolution of the spectrum with temperature mainly reflects changes in the distribution of the populations. Line shape and relaxation measurements are used to estimate relative populations and interconversion rates between different chain conformations. In all solid phases there is no reorientation of the benzene core on the NMR time scale except for the high-temperature phase (1) of benzenehexa-n-hexanoate (BHA6). The dynamic NMR features of this phase are very similar to those of the discotic mesophases of benzenehexa-n-heptanoate and benzenehexa-n-octanoate (BHA7 and BHA8) and are consistent with fast overall molecular reorientation. On this basis, phase I of BHA6 is classified as a discotic mesophase.