2-Carboxyl­atopyridinium–4-nitro­phenol (1/1)

In the title 1:1 adduct, C(6)H(5)NO(3)·C(6)H(5)NO(2), both mol­ecules are almost planar (r.m.s. deviations for the non-H atoms = 0.027 and 0.023 Å for 4-nitro­phenol and 2-carboxyl­atopyridinium, respectively). The pyridine mol­ecule crystallizes as a zwitterion (nominal proton transfer from the carb­oxy­lic acid group to the N atom in the ring). In the crystal, inversion dimers of the zwitterions linked by pairs of N—H⋯O hydrogen bonds generate R (2) (2)(10) loops; two 4-nitro­phenol mol­ecules link to the dimer by O—H⋯O hydrogen bonds, generating a four-molecule aggregate. These are linked by C—H⋯O inter­actions, forming a three-dimensional network.