Evolution of lone pair of excess electrons inside molecular cages with the deformation of the cage in e2@C60F60 systems

Density functional theory study of hydrogen and oxygen reactions on NiO(100) and Ce doped NiO(100)

CONTEXT

This study aims to reveal the reaction mechanisms of H2 and O2 on the NiO(100) and Ce-doped NiO(100) surfaces using the density functional theory (DFT) combined with the on-site Coulomb correction (DFT + U) method. It was found that H2 and O2 react favorably on the reduced surfaces of both materials. However, after the oxygen vacancy is filled, the activation energy for the reaction between H₂ and lattice oxygen increases. Ce doping reduces this activation energy to 1.64 eV (compared to 3.16 eV for pure NiO(100)). The enhanced activity of lattice oxygen due to Ce doping is attributed to the charge transfer in the Ce-O bond, which leads to the electronic localization around O atoms and weakens the activation energy barrier. Moreover, the presence of Ce facilitates the formation of a sub-stable OH intermediate on the reduced surface, ensuring the sustainability of the reaction. This study provides a theoretical basis for the design of high-performance nickel-based hydrogen deoxidizers and contributes to promoting the research and development process of nickel-based catalysts in related fields.

METHODS

The calculations were performed using the Vienna ab initio simulation package (VASP) module of the MedeA® software. The exchange-correlation energy calculations are performed using the Perdew, Burke and Ernzerhof (PBE) functional within the generalized gradient approximation (GGA). The transition states were calculated using the MedeA® Transition State Search Module, based on the climbing-image nudged elastic band (CI-NEB) method.

Nature of partial sigma bond

This study investigates the formation of partial sigma (σ) covalent bonds in experimentally synthesizable biradicals formed from hydrogenated and fluorinated C8, C20, and C60 cage structures, by assessing their stability, geometry, and bonding character in singlet and triplet states using restricted B3LYP-D3/6-31+G(d,p) theory, natural bond orbital (NBO) analysis, and complete active space self-consistent field (CASSCF) method. The results show that these partial σCC bonds have Wiberg bond orders of 0.38 to 0.48 and bond lengths ranging from 2.62 Å to 5.93 Å. Cage size influences the characteristics of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), with electrons favoring more antibonding orbitals in smaller cages where electrons reside more on the exterior of the cage and favoring bonding orbitals in larger ones where electrons are more in the interior. Fluorination enhances electron density on bonding orbitals. The analysis further clarified that the differentiation between antibonding and bonding features of HOMOs and LUMOs extends beyond merely electron transfer from s- to p-atomic orbitals, also noting possible interactions of the same symmetry repel. The study also introduces hyperconjugation from α-position CH bonds as a factor in stabilizing partial σ-bond formation. The results also caution against the use of broken symmetry methodology in unrestricted SCF wavefunctions for biradicals, such as those in this study as it may cause large spin contamination and thus errors in the calculated electronic properties results.

Metalloborospherenes with a Stabilized Classical Fullerene-like Borospherene B40 Act as Nonlinear Optical Switches, Electron Reservoirs, Molecular Capacitors, and Molecular Reactors

Using a new method of η5-Li and η6-Mg atoms capping the faces of the classical fullerene-like borospherene Td B40, we theoretically predict an exohedral metalloborospherene Td Mg10Li12&B40 molecule. Remarkably, a newfangled endoexo cage isomerism is proposed. Further, embedding Mg atoms in the Td B40 cage forms endohedral derivatives. Due to the intramolecular pull-push electron transfer relay, these obtained molecules possess unequal multilayered and alternant spherical charge distribution. The outer is an excess electron layer, bringing a molecular nonlinear switch character and an electron reservoir behavior with strong electron-donating and -accepting abilities. The middle (Mg2+)10(Li+)12 and the outer layers together constitute an electric double layer, presenting the behavior of a molecular capacitor where the electronic charge-discharge process occurs in the outer excess electron layer. The inner part is an empty cage B4026- with a strong negative electric field. The valence electrons of the embedded Mg atoms are transformed into new excess electrons and added in the outer excess electron layer, also exhibiting the charging behavior of the molecular capacitor. Considering the chemical reaction in the inner cage, the embedded Mg atom is ionized, forming an Mg2+ cation and 2e under the strong negative electric field; meanwhile, 2e is powerfully pushed into the outer excess electron layer. This chemical process shows a generalized Coulomb explosion, and thus the exohedral metalloborospherene molecules with cage B4026- may act as molecular reactors. The new species mark the genesis of classical fullerene-like borospherene chemistry and stimulate their applications in molecular nonlinear optical and nanoelectronics.

About Perfluoropolyhedranes, Their Electron-Accepting Ability and Questionable Supramolecular Hosting Capacity

Polyhedral molecules are appealing for their eye-catching architecture and distinctive chemistry. Perfluorination of such, often greatly strained, compounds is a momentous challenge. It drastically changes the electron distribution, structure and properties. Notably, small high-symmetry perfluoropolyhedranes feature a centrally located, star-shaped low-energy unoccupied molecular orbital that can host an extra electron within the polyhedral frame, thus producing a radical anion, without loss of symmetry. This predicted electron-hosting capacity was definitively established for perfluorocubane, the first perfluorinated Platonic polyhedrane to be isolated pure. Hosting atoms, molecules, or ions in such "cage" structures is, however, all but forthright, if not illusionary, offering no easy access to supramolecular constructs. While adamantane and cubane have fostered numerous applications in materials science, medicine, and biology, specific uses for their perfluorinated counterparts remain to be established. Some aspects of highly fluorinated carbon allotropes, such as fullerenes and graphite, are briefly mentioned for context.

Role of Excess Electrons in Nonlinear Optical Response

The excess electron is a kind of special anion with dispersivity, loosely bounding and with other fascinating features, which plays a pivotal role (promote to about 10(6) times in (H2O)3{e}) in the large first hyperpolarizabilities (β0) of dipole-bound electron clusters. This discovery opens a new perspective on the design of novel nonlinear optical (NLO) molecular materials for electro-optic device application. Significantly, doping alkali metal atoms in suitable complexants was proposed as an effective approach to obtain electride and alkalide molecules with excess electron and large NLO responses. The first hyperpolarizability is related to the characteristics of complexants and the excess electron binding states. Subsequently, a series of new strategies for enhancing NLO response and electronic stability of electride and alkalide molecules are exhibited by using various complexants. These strategies include not only the behaviors of pushed and pulled electron, size, shape, and number of coordination sites of complexants but also the number and spin state of excess electrons in these unusual NLO molecules.