A Charge Density Analysis of Cationic and Anionic Hydrogen Bonds in a “Proton Sponge” Complex

The charge density distribution in crystals of the 1,2-dichloro hydrogen maleate salt formed by 1,8-bis(dimethylamino)naphthalene (DMAN) has been obtained by high-resolution X-ray and neutron diffraction at 100 K. The asymmetric [Me2N−H···NMe2]+ hydrogen bond in the monoprotonated cation is characterized by nonlinear interaction lines, constrained by crystallographic symmetry to lie in the plane of the DMAN molecule; the critical point situated between the proton and the acceptor nitrogen atom lies in a region of positive Laplacian of the charge density. The lone pair of this nitrogen atom is polarized in the direction of the hydrogen bond. The N−C(aromatic) bond paths are slightly curved. Magnitudes of the charge density at critical points in covalent and hydrogen bonds within both ions are compared with values obtained from an ab initio MO single point calculation for a cation−anion pair. Properties of the charge density in a number of weak C−H···O interionic interactions are found to be systematically related to the donor−acceptor separation. A series of ab initio calculations on a model formate ion−benzene complex shows that the density at C−H···O hydrogen bond critical points is systematically lower than the density of the superposed moieties, but by an amount that is on the threshold of current experimental measurements analyzed with multipole-based models.