Identification of hydrogen bonds using quantum electrodynamics

Abstract

A method for the identification of hydrogen bonds was investigated from the viewpoint of the stress tensor density proposed by Tachibana and following other works in this field. Hydrogen bonds are known to exhibit common features with ionic and covalent bonds. In quantum electrodynamics, the covalent bond has been demonstrated to display a spindle structure of the stress tensor density. Importantly, this spindle structure is also seen in the hydrogen bond, although the covalency is considerably weaker than in a typical covalent bond. Distinguishing it from the ionic bond is most imperative for the identification of the hydrogen bond. In the present study, the directionality of the hydrogen bond is investigated as the ionic bond is nearly isotropic, while the hydrogen bond exhibits the directionality. It was demonstrated that the hydrogen bond can be distinguished from the ionic bond using the angle dependence of the largest eigenvalue of the stress tensor density.quantum electrodynamics, hydrogen bond, stress tensor density | INTRODUCTIONChemical bonds have been actively investigated from the viewpoint of quantum theory since Heitler and London, which involved the evaluation of chemical bonds based on quantum mechanics. [1][2][3] Specifically, topological analyses of wave functions, such as the quantum theory of atoms in molecules (QTAIM) [4,5] and the topology of the electron localization function (ELF), [6] have attracted considerable attention. Based on these evaluations, a chemical bond can be classified as covalent or ionic using topological parameters, such as rρ, where ρ denotes the electron density. The aforementioned real-space analyses are relatively novel trends used for the investigation of chemical bonds, [7][8][9] particularly the hydrogen bond. [10] Our group and coworkers have recently developed a real-space analysis method using stress tensor density, which was originally proposed by Tachibana and is a new density quantity based on quantum electrodynamics (QED). [11] The approach offers a new platform for gaining a better understanding of chemical bonds. Notably, the classification of chemical bonds has been previously carried out utilizing the stress tensor density. [12][13][14][15] Previous studies included the identification of the covalent, metallic, and ionic bonds. [16][17][18][19] The bond order based on QED was particularly well established for covalent bonds and shows an excellent correlation with the internuclear distances of numerous chemical bonds. [16,20] The bond orders of other bonds are currently being investigated and will undoubtedly be reported in the near future.In the present work, we extend the scope of real-space analysis utilizing the stress tensor density to identify the hydrogen bond. The identification of ionic bonds is also confirmed using models involving periodic boundary conditions as previous studies on the ionic bond used cluster models. [19] As hydrogen bonds exhibit similar analogous distribution patterns of the stress tensor density as those of ionic bonds, as ...