Structures of 2,6-disubstituted naphthalenes.

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. 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) Å(3) 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 P1;, with a = 3.7061 (8), b = 7.4688 (14), c = 8.5352 (22) Å, alpha = 86.62 (2), beta = 85.49 (2), gamma = 87.99 (2) degrees, V = 235.00 (6) Å(3) and Z = 1. The structure consists of loosely packed hydrogen-bonded chains along [11;1]. NDC crystallizes in the monoclinic space group P2(1)/c, with a = 13.41931 (14), b = 6.14869 (5), c = 7.15257 (5) Å, beta = 100.400 (1) degrees, V = 580.47 (1) Å(3) at 300 K and Z = 2. The structure consists of layers of NDC molecules perpendicular to a. The ester group is twisted 20 degrees out of the mean ring plane in NDC. The conformations of the carboxyl groups in NDA and NDC differ. MP2 calculations suggest that the observed twist in NDC corresponds to an increase in conformational energy of 9 kJ mol(-1).