Molecular motion in solid all-trans retinoic acid (vitamin A acid) by proton NMR.

An investigation of the molecular dynamics of all-trans retinoic acid (vitamin A) has been carried out in the solid state by proton NMR. Measurements were made of T1 at 14 and 25 MHz and of second moment at 25 MHz in the temperature range 55 to 420 K for both monoclinic and triclinic forms. In the monoclinic state relaxation was attributed to methyl group reorientation below 250 K and to conformational motion of the cyclohexene ring above this temperature. Activation energies and time constants were derived for the motions. In the triclinic state the same motions were active, but the cyclohexene ring motion was less restricted and some of the methyl group motions were more restricted indicating that hindrances to the motions were at least partly determined by the intermolecular packing arrangements.