Electron density, electrostatic potential, and spatial organization of ammonium hydrooxalate oxalic acid dihydrate heteromolecular crystal from data of diffraction experiment at 15 K using synchrotron radiation and theoretical calculations

A high-precision diffraction study at 15 K using synchrotron radiation and theoretical calculation of a heteromolecular crystal ammonium hydrooxalate oxalic acid dihydrate NH4 +•C2HO 4 -•C2H2O4• 2H2O (1) were carried out. The calculation was performed with the Kohn-Sham method taking into account periodic boundary conditions. The joint experimental and theoretical study allowed one to locate positions of hydrogen atoms and to reliably establish peculiar features of the electron density and electrostatic potential distributions in 1. Interatomic and molecular interactions were characterized based on the electron density properties within the framework of a quantum topological theory. The bond order indices were calculated from the experimental electron density without using the orbital notions. A new approach based on visualization of the ellipsoids whose semiaxes depend on the principal values of the electron density curvature at the bond critical points was used. It was found that charge transfer between ammonium cation and hydrooxalate anion in 1 dominates other electrostatic interactions in the crystal. Based on analysis of peculiar features of the electron density and electrostatic potential distributions in the crystal of 1, it was found that spatial organization of the crystal in hand is also governed by one more, weaker, electrostatic factor that originated from the presence of well-localized regions behind protons on the extensions of the lines of covalent bonds at the periphery of the molecules. In those regions, the electrostatic potential is higher than in other directions due to anisotropy of the electron density distribution. This feature mainly ensures directed complementary electrostatic interaction between corresponding fragments with negatively charged regions of neighboring molecules, such as the lone electron pairs and π-electrons. © 2013 Springer Science+Business Media, Inc.