Quantum−Mechanical Investigation of Chemical Energetics and Electronic Stabilities of Microhydrated Protons [H+(H2O)n]

Abstract

Though proton hydration is a spontaneous phenomenon that occurs as a result of reorientation of the polar H₂O molecules in the immediate vicinity of the proton, its ultimate role to give rise to an unusually high diffusion rate of the protons in batteries, fuel cells, and Nafion series proton exchange membranes has captivated the interest of researchers for a long time. Interestingly, such anomalously high proton conductivity is already found to be caused due to the formation of several hydrated states of the protons out of which most possible stable states are hydronium (H₃O⁺), zundel (H₅O₂⁺), and eigen (H₉O₄⁺) cations. This study is mainly aimed to probe deep into the molecular−level of these three hydrated protons by applying DFT−based quantum mechanical model, and to unveil their electronic stabilities, ground state electronic structures/ energies, binding energies, and relative thermodynamic stabilities theoretically. The DFT derived ground state electronic structures of them are observed as flattened like a wind−blown umbrella, linear and flat, and triangular planar shape respectively. All these three 3D geometries are examined quantitatively through electron density mapping (mapped with the total density) techniques, and found no structural deviations. Besides this, the relative thermodynamic stability of them is closely monitored and found the order as H₉O₄⁺ > H₅O₂⁺ > H₃O⁺. The significance and originality of this study lies in enlightening most of the structural and energetic properties of the three most stable hydrated states of the protons, and in producing their structural data sets that are very essential while modeling fuel cell and redox flow battery simulators.