Charge Density Distribution in the “Proton Sponge” Compound 1,8-Bis(dimethylamino)naphthalene

Abstract

This is the first low-temperature, high-resolution X-ray and neutron diffraction study of charge distribution in an uncomplexed proton sponge compound. In the parent compound of this class, 1,8-bis(dimethylamino)naphthalene (DMAN), a small degree of asymmetry of the naphthalene moiety is induced by weak, asymmetric intermolecular interactions. The only manifestation of this asymmetry is at the level of the Laplacian of the charge distribution. Comparing the DMAN molecule with its cation shows the consequences of protonation, such as shortening of the C−C, and lengthening of the C(ar)−H and C−N bond lengths. These changes are even more apparent in CP properties, with ρb on average smaller in the cation than in the neutral molecule. Relationships between CP and structural parameters viz. ρb vs Laplacian, Laplacian vs bond length, and ρb vs bond length depend on bond type, and are different for DMAN and its cation. Comparison with ab initio calculated values reveals some systematic discrepancies. Only two cr...