E016: Crystal Structures Of 2,6-Disubstituted Naphthalenes

E016

Crystal Structures of 2,6-Disubstituted Naphthalenes. James A. Kaduk, Amoco Corporation, P.O. Box 3011 MC F-9, Naperville IL 60566

The crystal structures of 2,6-naphthalenedicarboxylic acid (NDA) and dimethyl-2,6-naphthalenedicarboxylate (NDC) have been solved ab initio using a combination of X-ray powder diffraction and computational chemistry techniques. These two crystal structures, and that of 2,6-dimethylnaphthalene (DMN), have been refined by the Rietveld technique, using laboratory and synchrotron data. The NDA structure was solved by manipulating the centrosymmetric molecule in real space (cell determined from a twinned crystal) to form the expected hydrogen-bonded chains and to minimize molecular overlap. The NDC structure was solved by manipulating the molecule in real space, simultaneously monitoring the calculated powder pattern and intermolecular interactions. The initial orientation was defined by analogy to the structure of DMN. One conformation of the centrosymmetric NDC molecule packs much more efficiently than the other. The crystal structures provide a basis for understanding the crystal morphologies and microstructures.

DMN crystallizes in the orthorhombic space group Pbca, with a = 7.4544(4), b = 6.0826(6), c = 20.0946(12) Å, V = 911.1(1) Å³, and Z = 4. The structure consists of a herringbone stacking parallel to a, resulting in loosely-bound layers perpendicular to c. NDA crystallizes in the triclinic space group P, with a = 3.7061(8), b = 7.4688(14), c = 8.5352(22) Å, _ = 86.62(2), _ = 85.49(2), _ = 87.99(2)_, V = 235.00 Å³, and Z = 1. The structure consists of loosely-packed hydrogen-bonded chains along [11]. NDC crystallizes in the monoclinic space group P2₁ /c, with a = 13.41931(14), b = 6.14869(5), c = 7.15257(5) Å, _ = 100.400(1)_, V = 580.47(1) Å³ at 27_C, and Z = 2. The structure consists of layers of NDC molecules perpendicular to a. The ester group is twisted 16_ out of the mean ring plane. This twist "costs" < 3 Kcal/mole, and results in a very efficient packing. The conformations of the carboxyl groups in NDA and NDC differ.