Orbital-Free Quantum Crystallographic View on Noncovalent Bonding: Insights into Hydrogen Bonds, π∙∙∙π and Reverse Electron Lone Pairs∙∙∙π Interactions

A detailed analysis of a complete set of the local potentials that appear in the Euler equation for electron density is carried out for noncovalent interactions in the uracil derivative using experimental X-ray charge density. The interplay between the quantum theory of atoms in molecules and crystals and the local potentials and corresponding inner-crystal electronic forces of electrostatic and kinetic origin is explored. Novel physically grounded bonding descriptors derived within the orbital-free quantum crystallography provided the detailed examination of pi-stacking and intricate C=O...pi interactions and nonclassical hydrogen bonds. The donor-acceptor character of these interactions is revealed by analysis of Pauli and von Weizsäcker potentials together with more well-known functions. Partitioning of crystal space into atomic-like potential basins led us to the definite description of the charge transfer. In this way, our analysis throws light on aspects of these closed-shell interactions hitherto hidden from the description.