Energetics of non-covalent interactions from electron and energy density distributions

Abstract

Non-covalent interactions dictate how molecules interact with their surroundings. Enhancing their knowledge is crucial to explain phenomena of utmost importance like self-assembly, chemical reactivity and crystallization. In this work, the possibility of investigating Non-Covalent Interactions (NCIs) by using the Reduced Density Gradient (RDG) in tandem with energy densities descriptors is explored. A sample of 30 molecular adducts, spanning dispersive, hydrogen bonds and XH⋯π interactions was considered. Potential relationships among molecule⋯molecule stabilization energies and energy densities were sought for. Adducts characterized by NCIs having similar physical origins exhibit an excellent linear correlation between stabilization energies and kinetic energy densities integrated over the volume enclosed by low-value RDG isosurfaces. Estimating stabilization energies this way is computationally unexpensive and applicable also to electron densities derived from experiment, where a reliable approximation to the kinetic energy density in the intermolecular regions is afforded through Abramov’s functional. Potential energy densities, when averaged over the basins enclosed by low-value RDG isosurfaces, assume different values according to the kind of interaction class (dispersive, HBs, XH···π, and so on). This observation provides a new recipe to disentangle how the various NCIs contribute to the total stabilization energy. Implications on the possibility of retrieving quantitative thermodynamic information from the topology of suitable scalar fields are discussed.