2-Carboxyl-atopyridinium-4-nitro-phenol (1/1). In the title 1:1 adduct, C6H5NO3·C6H5NO2, both mol-ecules are almost planar (r.m.s. deviations for the non-H atoms = 0.027 and 0.023 Å for 4-nitro-phenol and 2-carboxyl-atopyridinium, respectively).  The pyridine mol-ecule crystallizes as a zwitterion (nominal proton transfer from the carb-oxy-lic 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-nitro-phenol mol-ecules link to the dimer by O-H⋯O hydrogen bonds, generating a four-molecule aggregate.  These are linked by C-H⋯O inter-actions, forming a three-dimensional network.
C