Phenanthrene-4-carboxylic acid and 1,2-dihydrophenanthrene-4-carboxylic acid

Phenanthrene-4-carboxylic acid, C15H10O2, crystallized in the centrosymmetric space group P2(1)/n, while 1,2-dihydrophenanthrene-4-carboxylic acid, C15H12O2, crystallized in the centrosymmetric space group Pbca. In each structure, there is a single type of hydrogen bond: it is of the cyclic dimer type about a center of symmetry. The Odonor...Oacceptor distances are 2.634 (2) and 2.651 (2) A, and the O-H...O angles are 176 (3) and 173 (2) degrees, respectively, for the two structures. In each structure, the carboxy H and O atoms are ordered. The phenanthrene core of the fully aromatic acid is roughly planar; the dihedral angle between the best-fit core plane and the carboxy group plane is 63.7 (1) degree. As expected, the hydrogenated ring of the second acid is much less nearly planar; the remaining naphthalenoid core is, however, roughly planar and the dihedral angle between this best-fit plane and the carboxy group plane is 60.4 (1) degree.