The chemical nature of very strong hydrogen bonds in some categories of compounds

In the current research, chemical nature of very strong hydrogen bonds in their three fundamental cases, resonance assisted hydrogen bond [RAHB], negative charge assisted hydrogen bond [(−)CAHB], and positive charge assisted hydrogen bond [(+)CAHB] is studied. The results are obtained at B3LYP/6-311++G** and MP2/6-311++G** level of theories. Attention is focused on topological parameters such as electron density, its Laplacian, kinetic energy density, potential energy density and energy density at the bond critical points (BCP) of O⋯H and O-H bonds from Bader's atoms in molecules (AIM) theory. Charge transfer energies based on natural bond orbital (NBO) analysis are also considered. Our results show that these hydrogen bonds are partially electrostatic and partially covalent in nature, in which the covalent contribution increases as the stabilization energy of hydrogen bond increases. In addition, it is shown that, as the O-H-O angle in intramolecular hydrogen bonds approaches to 180°, the charge transfer energy from oxygen lone pairs to antibonding NBO of O-H increases. In the investigated systems, double minimum no barrier (DM/NB) potential energy surface (PES) is obtained for hydrogen transfer between the two oxygens. AIM analysis based on DFT calculation for the transition states (TSs) show that the hydrogen atom is connected to the oxygens with two almost identical covalent bonds with some contribution of electrostatic interaction, while MP2 calculation predict two covalent O-H bonds in some cases.