Short N center dot center dot center dot O hydrogen bonds in the 1 : 1 adduct of 4,4 `-bipyridyl and oxalic acid

Oxalic acid, C2O4H2, and 4,4'-bipyridyl, C10H8N2, crystallize in a 1:1 ratio. The asymmetric unit consists of one oxalic acid (OXA) molecule and one 4,4'bipyridyl (BPY) molecule in general positions, together with one half-OXA molecule and one half-BPY molecule; the latter two molecules are centrosymmetric. The molecules are linked in two parallel independent chains by strong O - H center dot center dot center dot N hydrogen bonds. In one chain there is one independent O - H center dot center dot center dot N hydrogen bond [N center dot center dot center dot O = 2.557 (3) angstrom] and the molecules lie on centres of symmetry and are therefore constrained to have planar central portions. The second chain contains two independent O - H center dot center dot center dot N hydrogen bonds [O center dot center dot center dot N = 2.549 (3) and 2.581 (3) angstrom] and both molecules are twisted about their central bonds