STRUCTURE OF 1,8-NAPHTHALENEDICARBOXYLIC ACID (NAPHTHALIC ACID), C12H8O4 - RING GEOMETRY AND HYDROGEN-BONDING EFFECTS

C12H8O4, M(r) = 216.19, orthorhombic, Pbca, a = 15.308 (2), b = 17.926 (2), c = 7.093 (2) angstrom, V = 1946.4 (5) angstrom3, Z = 8, D(x) = 1.48 g cm-3, lambda(Mo K-alphaBAR) = 0.71073 angstrom, mu = 1.05 cm-1, F(000) = 896, T = 296 K, R = 0.043 for 912 unique reflections having I > 3-sigma(I). In this structure, 1,8-naphthalenedicarboxylic acid departs significantly from planarity. While the average deviation of carbons from the best least-squares plane for the naphthalene core is 0.055 (4) angstrom, the carboxyl carbons adopt positions + 0.403 (4) and -0.350 (4) angstrom from that plane. The carboxyl groups are twisted in the same direction with respect to that plane and make an average dihedral angle of 42.6 (3)-degrees with it. The nearly equal lengths of the C-O distances within the carboxyl groups [mean value 1.267 (5) angstrom] indicate almost complete disorder of the H atoms of these groups. Hydrogen bonding occurs in cyclic dimer fashion between two molecules related by a center of inversion. The carboxyl hydrogens appear to be subject to a symmetric double-minimum potential and to be disordered via tunneling; there is no evidence that the carboxyl oxygens are disordered. The mean carboxyl H-O acceptor distance is 1.90 (7) angstrom.