Local Stability Analysis of Hydrogen Bonding and Other Non-Covalent Interactions

Abstract

Bader's quantum theory of atoms in molecules (QTAIM) is used to evaluate local atomic stabilities in clusters of molecules. The total energy of a molecular system is decomposed into atomic contributions determined quantum mechanically through evaluation of the electron density contained within atomic basins. Stability is defined by changes in the atomic energy. These stabilities are used to interpret energetic changes within molecules as they form non-covalent interactions, including hydrogen bonding, beryllium bonding and halogen bonding, as well as combined instances of each. The stabilities are then represented using a unique method of visualization, whereby atom size represents the magnitude of the energy change and atom colour represents sign of energy change (positive or negative). Local stabilities in small methanol (MeOH)n=2-4, formaldehyde (H2C=O)n=2-4, and water (H2O)n=2-6 clusters reveal a clear increase in the magnitude of atomic stability when cooperative interactions are present. This energy increase is not observed for non-cooperative or anti-cooperative interactions in formaldehyde and water clusters. For methanol clusters the cooperative stability is clearly localized at the hydroxyl group. Local atomic and molecular energies give new insight into the interaction of water wires with alkali metals, alkaline earth metals and halide ions and, finally, local atomic stabilities show the existence of strong cooperative effects for beryllium-hydrogen bond interactions and beryllium-halogen bond interactions, which are in some cases very intense.