Experimental and theoretical charge density study of polymorphic isonicotinamide−oxalic acid molecular complexes with strong O···H···N hydrogen bonds

Two polymorphs of the 2:1 molecular complex of isonicotinamide and oxalic acid have been characterized by combined X-ray charge density and single-crystal neutron diffraction studies at 100 K. Both polymorphs show strong O−H···N intermolecular hydrogen bonding between the acid and the pyridine base. As is typical of short, strong hydrogen bonds (SSHBs), the covalent O−H bonds are considerably elongated to 1.161(3) and 1.235(5) Å, and the H···N interactions are correspondingly short at 1.398(3) and 1.313(6) Å in Forms I and II, respectively. The neutron diffraction data indicate no pronounced H dynamics in the SSHBs, and in the case of Form II the SSHB can be described as quasicentered. In addition to the experimental charge densities, theoretical charge densities have been determined from ab initio calculations within the full periodic environment of the crystalline state. The SSHBs are found to be covalent in nature according to the topological analysis of the experimental and theoretical charge densities and application of the source function. Aside from the SSHBs, moderate N−H···O and weak C−H···O interactions are also present in the molecular complexes, for which hydrogen bond energies are estimated from energy densities and independent ab initio calculations. Finally, an attempt is made to evaluate the intermolecular interactions governing the manifestation of polymorphism in this compound