In the crystal structure of the title compound, C(15)H(15)NO, the two benzene rings are twisted from each other by a dihedral angle of 47.97 (4)°. The crystal structure is stabilized by weak inter-molecular C-H⋯O and C-H⋯π inter-actions, and π-π inter-actions with a centroid-centroid distance of 3.8493 (5) Å are observed.
A second monoclinic polymorph of methyl 4-hydroxy-benzoate, C(8)H(8)O(3), is reported. The unit-cell dimensions are different from those of the previously reported monoclinic form [Vujovic & Nassimbeni (2006 ▶). Cryst. Growth Des.6, 1595-1597]. The asymmetric unit contains three crystallographically independent mol-ecules, as observed in the previous form. The crystal structure is stabilized by inter-molecular O-H⋯O and C-H⋯O hydrogen bonds and C-H⋯π inter-actions, which link the mol-ecules into a three-dimensional network.
In the title compound, C(5)H(7)N(2) (+)·C(7)H(5)O(3) (-), the carboxyl-ate mean plane of the 4-hydroxy-benzoate anion is twisted by 8.78 (5)° from the attached ring. The cations and anions are linked via O-H⋯O, N-H⋯O and C-H⋯O hydrogen bonds, forming a three-dimensional network. In addition, π-π inter-actions involving the benzene and pyridinium rings, with centroid-centroid distances of 3.5500 (6) and 3.6594 (6) Å, are observed.
In the title 1:1 adduct, C(7)H(5)NO(4)·C(7)H(7)NO(2), the nitro group of the 4-nitro benzoic acid is twisted from the attached ring by 4.40 (8)°. In the crystal, the mol-ecules are linked into ribbon-like structures along [150] and [10] via O-H⋯O, N-H⋯O, N-H⋯N and C-H⋯O inter-molecular hydrogen bonds.
The asymmetric unit of the title compound, C(5)H(7)N(2) (+)·C(7)H(4)NO(4) (-)·C(7)H(5)NO(4), consists of an amino-pyridinium cation, a 4-nitro-benzoate anion and a neutral 4-nitro-benzoic acid mol-ecule. The pyridine ring forms dihedral angles of 64.70 (5)° and 70.37 (5)°, respectively, with the benzene rings of 4-nitro-benzoic acid and 4-nitro-benzoate. In the crystal structure, the cations, anions and the neutral 4-nitro-benzoic acid mol-ecules are linked by O-H⋯O and N-H⋯O hydrogen bonds, forming a two-dimensional network parallel to (001). Adjacent networks are cross-linked via C-H⋯O hydrogen bonds and π-π stacking inter-actions [centroid-centroid distances 3.6339 (6) and 3.6566 (6) Å].