Hydrogen bond studies. 85. A very short, asymmetrical, intramolecular hydrogen bond: A neutron diffraction study of pyridine‐2,3‐dicarboxylic acid (C7H5NO4)

Abstract

A neutron diffraction study of pyridine‐2,3‐dicarboxylic acid (quinolinic acid) has been carried out. The intensities for 3780 reflexions were measured at the Brookhaven High Flux Beam Reactor. The structure was refined to an R(F 2) of 0.054 using starting parameters from a previous x‐ray study. The short intramolecular hydrogen bond [O ··· O: 2.398(3) Å] has no symmetry restriction operating on the hydrogen atom position and is asymmetric. The two O ··· H distances are 1.163(5) and 1.238(5) Å, respectively, and the O–H–O angle is 174.4(4)°. The asymmetry of the bond can be explained by the intramolecular environment. The strain in the molecule caused by the short hydrogen bond results in long carbon‐carbon bonds to the carboxyl groups [1.516(2) and 1.541(2) Å]. The C–O bond lengths [1.219(3), 1.263(2), 1.222(3), and 1.270(2) Å] vary depending on the hydrogen‐bond involvement of the oxygen atoms. The mean C–H distance is 1.087 Å and the N–H distance is 1.036(4) Å. The nitrogen atom is involved in an intermolecular N–H ··· O hydrogen bond [N ··· O: 2.725(2), H ··· O: 1.845(4) Å and angle N–H ··· O: 140.5(2)°]. There are a number of short intermolecular O ··· H–C contacts. The refined neutron scattering length for the nitrogen atom is b̄ N=0.925(4)×10−12 cm.