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 in terphases show two different signals for the (CH2)(n) chain, with a ch emical shift difference of 2.6 ppm. They are characterized by selectiv e population inversion (SPI) measurements and contact time variation e xperiments. One signal belongs to more rigid chains with trans conform ations, the other one to more mobile chains with trans/gauche conforma tions. With decreasing temperature, more trans conformations are forme d. HPLC separations of vitamin A acetate isomers have been performed f rom 50 degrees C to 2 degrees C. The on-line coupling of HPLC with H-1 NMR spectroscopy yields information necessary for determination of th e structures of the separated isomers. The conformational change of th e C-30 interphases with the temperature can also be seen in the retent ion order of the vitamin A acetate isomers. At low temperatures, the m ost angled 9-cis isomer is the most strongly retained compound, wherea s at high temperatures, the linear all-trans isomer exhibits the stron gest interactions with the C-30 phase.