Temperature-dependent behavior of C-30 interphases. A solid-state NMR and LC-NMR study

Temperature-dependent C-13 CP/MAS NMR investigations of C-30 bonded interphases show two different signals for the (CH2)(n) chain, with a chemical shift difference of 2.6 ppm. They are characterized by selective population inversion (SPI) measurements and contact time variation experiments. One signal belongs to more rigid chains with trans conformations, the other one to more mobile chains with trans/gauche conformations. With decreasing temperature, more trans conformations are formed. HPLC separations of vitamin A acetate isomers have been performed from 50 degrees C to 2 degrees C. The on-line coupling of HPLC with H-1 NMR spectroscopy yields information necessary for determination of the structures of the separated isomers. The conformational change of the C-30 interphases with the temperature can also be seen in the retention order of the vitamin A acetate isomers. At low temperatures, the most angled 9-cis isomer is the most strongly retained compound, whereas at high temperatures, the linear all-trans isomer exhibits the strongest interactions with the C-30 phase.