Charge Densities of Two Polymorphs of Hydrated 1,8-Bis(dimethylamino)naphthalene HydrochlorideSimilarities and Differences

For the two known polymorphic forms (T triclinic form in P1̅; and M monoclinic in P21/n) of the hydrated and protonated N,N,N,N-peri(dimethylamino)naphthalene chloride, DMANH+·2Cl−·H5O2+, experimental electron density distributions from the high resolution X-ray diffraction measurements have been obtained and analyzed. Theoretical electron densities from periodic ab initio computations have also been obtained. For one of the polymorphs, M, we have also applied high resolution neutron diffraction structural data (positions and ADPs for H-atoms) in multipole refinement of high resolution X-ray data. In the 3D crystal lattices of both polymorphs, the diaquahydrogen (Zundel) cations (H5O2+) with chloride anions form infinite ribbons arranged as condensed hexagons in their chair conformation stabilized by hydrogen bond interactions with the DMANH+ cations. For such ribbons one can observe an unusual charge density distribution in the diaquahydrogen (Zundel) cations: a concentration of positive charge on the hydrogen atoms included in strong [O···H···O]+ hydrogen bonds. Also, a significant polarization of the H5O2+ charge density in the direction of the chloride anions is observed in the experimental maps and is far less significant in theoretical charge density maps. The differences between the charge density distributions in both polymorphs are surprisingly small and observed as slightly different interaction paths between neighboring molecules. These differences are generated by tiny differences in intermolecular interactions present in both polymorphs. We studied these interactions using Hirshfeld surfaces. Analysis of the interaction energies between the ions shows that the strength of the chloride anion Cl(1)···diaquahydrogen (Zundel) cation interaction is twice as strong as its interaction with the DMANH+ cations.