Encapsulating Proton Inside C60 Fullerene: A Density Functional Theory Study on the Electronic Properties of Cationic X+@C60 (X+ = H+, H3O+ and NH4+)

Confining protons into an enclosed carbon cage is expected to give rise to unique electronic properties for both the inner proton and the outer cage. In this work, we systematically investigated the geometric and electronic structures of cationic X⁺@C₆₀ (X⁺ = H⁺, H₃O⁺, and NH₄⁺), and their corresponding neutral species (X = H₂O, NH₃), by quantum chemical density functional theory calculations. We show that C₆₀ can trap H₂O, NH₃, H₃O⁺ and NH₄⁺ at the cage center and only slightly influence their geometries. The single proton clings to the inner wall of C₆₀, forming a C-H chemical bond. The encapsulated neutral species almost do not change the electronic structure of the C₆₀, while the internal cations have obvious effects. The charge transfer effect from the inner species to the C₆₀ cage was found for all X@C₆₀ (X = H₂O, NH₃) (about 0.0 e), X⁺@C₆₀ (X⁺ = H₃O⁺, NH₄⁺) (about 0.5 e) and H⁺@C₆₀ (about 1.0 e) systems. Encapsulating different forms of protons also regulates the fundamental physico-chemical properties of the hollow C₆₀, such as the HOMO-LUMO gaps, infrared spectra, and electrostatic potential, etc., which are discussed in detail. These findings provide a theoretical insight into protons' applications, especially in energy.