Structure of 1,8-naphthalenedicarboxylic acid (naphthalic acid), C12H8O4: ring geometry and hydrogen-bonding effects

C12H8O4, Mr = 216.19, orthorhombic, Pbca, a = 15.308 (2), b = 17.926 (2), c = 7.093 (2) A, V = 1946.4 (5) A3, Z = 8, Dx = 1.48 g cm-3, lambda (Mo K alpha) = 0.71073 A, mu = 1.05 cm-1, F(000) = 896, T = 296 K, R = 0.043 for 912 unique reflections having I greater than 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) A, the carboxyl carbons adopt positions +0.403 (4) and -0.350 (4) A 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) A] 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) A.