Relative Kinetic Stability Study of Hydronium, Zundel, and Eigen Cations through Quantum Mechanical Molecular Orbitals Approach

Abstract

Despite the existence of Hydronium (H₃O⁺), Zundel (H₅O₂⁺), and Eigen (H₉O₄⁺) states of proton as premier cationic species in several pH−related aqueous systems including electrochemical and biological fluid matrices, their critical roles in a wide range of industrial, chemical, and physical processes, and their typical thermodynamic and energetic instabilities, a comprehensive study focused on their relative kinetic stabilities through quantum mechanical molecular orbitals (QMO) approach is seldom reported. This insight is mainly aimed to compute their HOMO−LUMO interactions and band gaps ( ) separately using hybrid density functional B3LYP method, and to determine most significant global quantum−chemical reactivity descriptors (QCDs) along with assessing the use of QCDs properties for predicting their relative chemical stabilities based on electronic localizations, electronic polarizabilities, and electrophilicities. The general results show that the H₉O₄⁺ form relatively: (1) has the smallest HOMO−LUMO energy gap ( , (2) has a maximum tendency for losing electrons ( ) (less firmly−held electron cloud), (3) is the chemically softest  or reactive species having significant electronic polarizabilities, and (4) has the least degree of electron loving propensity (electrophilicity) ( ) than those for the H₅O₂⁺ ( ) and H₃O⁺ (  states. The originality of this study lies in elucidating relative kinetic stabilities of the three most stable hydrated states of the protons quantitatively through DFT based QMO approach.