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 O-donor... O-acceptor distances are 2.634(2) and 2.651(2)Angstrom, 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)degrees. 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)degrees.