Crystal structures of optically active polyamides derived from di-O-methyl-L-tartaric acid and 1,n-alkanediamines: A study combining energy calculations, diffraction analysis, and modeling simulations

The crystal structures of a series of optically active polyamides derived from L-tartaric acid and 1,n-alkanediamines (n = 2, 4, 6, and 8) have been investigated. Experimental data provided by X-ray diffraction of powders and fibers as well as by electron diffraction of single crystals were used to determine the lattice parameters for each polymer. A triclinic unit cell with space group P1 was found to be shared by the whole series. Semiempirical quantum mechanical calculations revealed that the preferred conformation for these polytartaramides entailed the tartaric acid moiety in a gauche arrangement with the amide groups rotated out of the plane containing the all-trans polymethylene segment. Crystal models compatible with the crystallographic data were built and refined against X-ray diffraction intensities with the linked-atom least-squares (LALS) methodology. The favored structure appeared to consist of hydrogen-bonded pleated sheets packed with a stagger similar to that found in the alpha-form of nylon 66. Modeling performed with CERIUS, a program enabling on-line observation of the simulated X-ray and electron scattering as the molecular model is adjusted, led to substantially similar results.