Insights into the Chemical Bonding in Microhydrated Protons from Molecular Orbital and Population Analysis Methods

Abstract

Research works related to investigate molecular electronic structures and thermodynamic stabilities simply by looking into the position of concerned atomic nuclei in the theoretically optimized molecular geometries do not give quantitative picture of the chemical bonding in respect to electron density distributions around atoms and bonds between the atoms. Therefore, total molecular electron density surfaces, and molecular orbitals (MOs) iso-surfaces & contours must be analyzed deeply while interpreting chemical bonds and electronic interactions quantum mechanically. In this paper, how these electron density surfaces (EDSs) actually provide meaningful explanation to the orbital interactions and atomic bonding in microhydrated protons: Hydronium (H₃O⁺), Zundel (H₅O₂⁺), and Eigen (H₉O₄⁺), and their overviews are reported briefly. The MOs and Mulliken population analysis methods are used to extract their chemical bond related information from their own DFT derived molecular wavefunctions ( ), and then all the EDSs are examined thoroughly. The MOs analysis reveals that the total molecular electron density surface of each cation defines a closed 3D structural entity plus an exact distribution of electron cloud around the specific bonds, the iso-surface of each MO displays a delocalized type electron density properly along with approximating a region in which bonding electrons are likely to be found and confirming orbital type interactions in atomic bonding, and the contour lines of all the MOs measure the electron amplitudes quantitatively. Additionally, the Mulliken derived partial atomic charge value on each atom of them quantify and compare specific bonding interactions numerically.