Superatom Molecular Orbitals Research Articles

Synthesis of N2-Type Superatomic Molecules.

Exploration of multiple bonds between superatoms remains an uncharted territory. In this study, we present the synthesis and characterization of N2-type superatomic molecules featuring triple bonds between two superatoms. The successful synthesis of M2Au17 (M = Pd, Pt) nanoclusters hinged upon the photoinduced fusion of MAu12 superatoms, achieved through sequential electron transfer and detachment of [AuPR3]+ species. Solid-state structures were confirmed via X-ray crystallography, while their electronic structures were elucidated through density functional theory (DFT) calculations. Analysis of electronic absorption properties, coupled with time-dependent DFT calculations, unveiled a symmetry-dependent electron transition nature between superatomic molecular orbitals, akin to that observed in conventional molecules.

Understanding the Electronic Structure and Chemical Bonding in the 2D Fullerene Monolayer.

Recently synthesized two-dimensional (2D) monolayer quasi-hexagonal-phase fullerene (qHPC60) demonstrates excellent thermodynamic stability. Within this monolayer, each fullerene cluster is surrounded by six adjacent C60 cages along an equatorial plane and is connected by both C-C single bonds and [2 + 2] cycloaddition bonds that serve as bridges. In this study, we investigate the stability mechanism of the 2D qHPC60 monolayer by examining the electronic structure and chemical bonding through state-of-the-art theoretical methodologies. Density functional theory (DFT) studies reveal that 2D qHPC60 possesses a moderate direct electronic band gap of 1.46 eV, close to the experimental value (1.6 eV). It is found that the intermolecular bridge bonds play a crucial role in enhancing the charge flow and redistribution among C60 cages, leading to the formation of dual π-aromaticity within the C60 sphere and stabilizing the 2D framework structure. Furthermore, we identify a series of delocalized superatom molecular orbitals (SAMOs) within the 2D qHPC60 monolayer, exhibiting atomic orbital-like behavior and hybridization to form nearly free-electron (NFE) bands with σ/π bonding and σ*/π* antibonding properties. Our findings provide insights into the design and potential applications of NFE bands derived from SAMOs in 2D qHPC60 monolayers.

High‐Energy Nitrogen Rings Stabilized by Superatomic Properties

AbstractHow to stabilize nitrogen‐rich high‐energy‐density molecules under conventional conditions is particularly important for the energy storage and conversion of such systems and has attracted extensive attention. In this work, the theoretical study shows for the first time that the stabilization mechanism of the nitrogen ring conforms to the superatomic properties at the atomic level. This result occurs because the stabilized anionic nitrogen rings generally exhibit planar high symmetry and the injected electrons occupy the superatomic molecular orbitals (SAMOs) of the nitrogen rings. According to these results, the typical stabilized anionic nitrogen ring structures N64−, N5−, and N42− are identified, which possess delocalized molecular orbitals with the same symmetry as the atomic orbitals and electron arrangement consistent with the electron shell arrangement of the atom. On this basis, a pathway is further designed to stabilize nitrogen rings by introducing metal atoms as electron donors to form neutral ThN6, LiN5, and MgN4 structures, thereby replacing the anionization of systems. This study highlights the importance of developing nitrogen‐rich energetic materials from the perspective of superatoms.

Deceptive orbital confinement at edges and pores of carbon-based 1D and 2D nanoarchitectures.

The electronic structure defines the properties of graphene-based nanomaterials. Scanning tunneling microscopy/spectroscopy (STM/STS) experiments on graphene nanoribbons (GNRs), nanographenes, and nanoporous graphene (NPG) often determine an apparent electronic orbital confinement into the edges and nanopores, leading to dubious interpretations such as image potential states or super-atom molecular orbitals. We show that these measurements are subject to a wave function decay into the vacuum that masks the undisturbed electronic orbital shape. We use Au(111)-supported semiconducting gulf-type GNRs and NPGs as model systems fostering frontier orbitals that appear confined along the edges and nanopores in STS measurements. DFT calculations confirm that these states originate from valence and conduction bands. The deceptive electronic orbital confinement observed is caused by a loss of Fourier components, corresponding to states of high momentum. This effect can be generalized to other 1D and 2D carbon-based nanoarchitectures and is important for their use in catalysis and sensing applications.

Superatom molecular orbital in C80.

The Superatom Molecular Orbitals (SAMO) in fullerene derivatives are of great interests which gives a wide basement for many electronic applications. In this work, the Density Functional Theory reveals the SAMO states of endohedrally doped C80 derivatives with Li, Sc, Mn, Ti, Ca, Fe, and Co atoms in molecular and periodic structures. The choice and position of metal atoms in endohedrally doped C80 derivatives largely affects the orientation of SAMO energies and wavefunction distributions. Among various derivatives, the Co-substituted C80 constitutes the lowest SAMO energy. The charge transfer study infers the influence of metal atoms inside the cage on SAMO energies. At higher energies, pz-, 2s-, and pxy- SAMO bands have been overlapped with higher dispersion bands which depict the increased intermolecular interaction in delocalized bands causing a larger dispersion. These results give new insights for future studies on lowering SAMO energy nearly to the fermi level in higher fullerenes.

On-surface synthesis of nitrogen-doped nanographene with an [18]annulene pore on Ag(111)

On-surface synthesis is of importance to fabricate low dimensional carbon-based nanomaterials with atomic precision. Here, we synthesize nitrogen-doped nanographene with an [18]annulene pore and its dimer through sequential reactions of debromination, aryl–aryl coupling, cyclodehydrogenation and C–N coupling on Ag(111) from 3,12-dibromo-7,8-diaza[5]helicene. The inner structures of the products were characterized with scanning tunneling microscopy with a CO terminated tip at low temperature. Furthermore, the first four unoccupied electronic states of the nanographene were investigated with a combination of scanning tunneling spectroscopy and theoretical calculations. Except for the LUMO + 2 state observed at +1.3 V, the electronic states at 500 mV, 750 mV and 1.9 V were attributed to the superatom molecular orbitals at the [18]annulene pore, which were significantly shifted towards the Fermi level due to the hybridization with the confined surface state.

Splitting of Degenerate Superatomic Molecular Orbitals Determined by Point Group Symmetry

We first confirm an idea obtained from first-principles calculations, which is in line with symmetry theory: Although superatomic molecular orbitals (SAMOs) can be classified according to their angular momentum similar to atomic orbitals, SAMOs with the same angular momentum split due to the point group symmetry of superatoms. Based on this idea, we develop a method to quantitatively modulate the splitting spacing of molecular orbitals in a superatom by changing its structural symmetry or by altering geometric parameters with the same symmetry through expansion and compression processes. Moreover, the modulation of the position crossover is achieved between the lowest unoccupied molecular orbital and the highest occupied molecular orbital originating from the splitting of different angular momenta, leading to an effective reduction in system energy. This phenomenon is in line with the implication of the Jahn–Teller effect. This work provides insights into understanding and regulating the electronic structures of superatoms.

High-pressure-induced electronic and structural transition of superatoms.

High pressure has been recognized as an important tool in molecule and materials research, and thus, it is expected to be used to understand the evolution of electronic states and geometric structures in superatoms. In this work, by studying three characteristic axial compressions on a typical endohedral metallofullerene superatom U@C28 with Td symmetry, we find that the triplet ground electronic state is preserved when the compression moves along the direction that reduces the symmetry to D2d, but the electronic state of the structure compressed along the direction of symmetry reduction to C2v or Cs is transformed into a singlet. The transition is attributed to the distinction in the response of electron spin to different axial compressions, which results in a change in the electron occupation mode of the system. Furthermore, we also confirm the gradual evolution from stereo to near-plane superatoms and the connection between their electron structures. This is reflected in the fact that the electron density distributions of the superatomic molecular orbitals (SAMOs) with extension along the restricted degrees of freedom (Dz2, Fz3 SAMOs) gradually contract, and the delocalization destruction of special orbitals is associated with this freedom. In addition, Raman and ultraviolet-visible spectra show a hyperchromic effect and redshift of characteristic peaks during axial compression, which are expected to be used for fingerprinting the superatomic planarization. Therefore, our work provides new insights based on high pressure for future research toward the discovery of physical properties and applications of superatoms.

Field emission angular distribution from single molecules

We demonstrate that the field emission from a single molecule at the apex of a metallic tip exhibits a well-defined angular distribution pattern (field emission angular distribution, FAD) with a spherical harmonics shape, regardless of the species of the molecule. From carefully controlled simultaneous measurements of the emission pattern, the emission current and emission energy, we deduced the formation mechanism of the FAD pattern in which the electron from the metallic tip resonantly tunnels into vacuum via a molecule-induced state situated at the Fermi level of the tip. The characteristic FAD patterns were consistent with the Fourier transform of the superatom molecular orbitals (SAMOs). Thus, the present method is unique for real-time imaging of the SAMOs, or the low-lying Rydberg states, of single molecules.

Smallest Endohedral Metallofullerenes [Mg@C20]n (n = 4, 2, 0, −2, and −4): Endo-Ionic Interaction in Superatoms

This work reports a series of endohedral metallofullerene superatoms [Mg@C20]n, where n = 4, 2, 0, -2, and -4. It was found that Mg transfers virtually all of its 3s electrons to the C20 shell, resulting in the ionic states of Mg2+@[C20]n-2. Detailed calculations revealed that the superatomic electronic configuration of these clusters is 1S21P61D101F4-n. The first nine superatomic molecular orbitals (SAMOs), 1S21P61D10, housed with 18 electrons, are largely based on [C20]n-2 with small contributions from magnesium, while the outmost SAMOs, 1F4-n, with 4 - n extra electrons, reside solely on [C20]n-2. The interaction between the Mg2+ ion and [C20]n-2 was found to be predominately ionic in character. Furthermore, ultraviolet-visible spectra provide a theoretical basis for fingerprinting these clusters. It is hoped that this work will encourage the synthetic pursuit of these smallest superatomic systems.

Superatomic states under high pressure

The study of superatoms has attracted great interest since they apparently go beyond the traditional understanding of the periodic table of elements. In this work, we clearly show that superatoms can be extended from conventional structures to states under pressure condition. By studying the compression process of the CH4@C60 system formed via embedding methane molecules inside fullerene C60, it is found that the system maintains superatomic properties in both static states, and even dynamic rotation situations influenced by quantum tunneling. Remarkably, the simulations reveal the emergence of new superatomic molecular orbitals by decreasing the confined space to approach the van der Waals boundary between CH4 and C60. Our current results not only establish a complete picture of superatoms from ambient condition to high pressure, but also offer a perspective for the discovery and exploration of new properties in superatom systems under extreme conditions.

Theoretical prediction of superatomic orbitals in spherical trihedral metallo-borospherenes Be3B12+1/+2

Superatomic molecular orbitals (SAMOs) have attracted particular attention due to the efficient overlapping of electronic wave functions between adjacent molecules, a key factor for enhancing charge transfer in molecular materials. Inspired by the recently discovered spherical trihedral metallo-borospherenes D3h Ln3B18– (Ln = La, Tb), of which both the B18-framework and the three Ln atoms are integral parts of surface cages, here we conceive the smallest spherical trihedral metallo-borospherenes with B6 motifs to date, D3h Be3B12+1/+2, which possess two equivalent directly connected B6 triangles, without any linking units at the jointing vertices, forming three equivalent η6-B6 rings and each centered with a Be atom. Interestingly, the hypothesized cage-like structure is proved to be with the lowest energy for each composition, and possesses SAMOs characters, of which the electronic configurations are 1S21P61D101F62S21F82P61Gα11Gβ0 and 1S21P61D101F62S21F82P61G0, corresponding to the systems with one or two positive charges, respectively. Our finding of SAMOs in the frontier orbitals of spherical trihedral metallo-borospherene furnishes a potential molecular unit for charge transport using superatomic channels.

Theoretical prediction of superatom molecular orbitals (SAMOs) in the ice-like cluster (H2O)10 and its impact in the supramolecular chemistry and function of nanostructured water

Superatom molecular orbitals (SAMOs) are atom-like molecular orbitals that have been observed in the buckminsterfullerene C60 molecule. We studied the ice-like water cluster (H2O)10 that is expected to be ubiquitous in nature in nanoscale microenvironments at ambient temperatures due to the nanoconfinement effect. We report for the first time the theoretical prediction of the existence of SAMOs in the adamantanoid nanostructured water cluster (H2O)10. The geometry of the adamantanoid water cluster (H2O)10 was optimized using the Restricted Hartree-Fock method and the ab-initio second-order Møller-Plesset perturbation theory (MP2) method, as well as the aqueous solvent phase polarizable continuum method (PCM). The optimized (H2O)10 structure is a distorted adamantanoid that closely matches the crystal structure of (H2O)10 that was previously reported. The theoretical studies using Hartree-Fock (HF), MP2, or Density Functional Theory (DFT) predict that the adamantanoid water cluster (H2O)10 exhibits SAMOs in the low-lying LUMOs. The orbital energies calculated with these methods are reported. The lowest-lying SAMOs in the water cluster (H2O)10 is the most intriguing SAMO due to its s-orbital character and potential to delocalize electrons in a large area that surrounds the cluster. The electron affinity of the water cluster (H2O)10 was calculated using HF, MP2, and DFT methods and the values obtained from each method were (kJ/mol) +74.71, -770.58, and -48.57, respectively. The existence of SAMOs in low-lying LUMO levels indicate of possible supramolecular chemistry of the water clusters with interfacial implications in nanoconfined spaces. Overall, these theoretical results predict new fundamental properties of water clusters that may have deep implications and impact in biological and nanotechnology systems that involve nanostructured water in confined spaces.

Superatom Molecular Orbitals of Li@C60: Effects of the Li Position and the Substrate

Understanding the character of superatom molecular orbitals (SAMOs) of fullerenes, especially those of the endohedral fullerenes, can potentially facilitate the utility of these molecules in organic electronics beyond conventional limits. However, the detailed nature of SAMOs in molecular films on substrates has yet to be unraveled. Using density functional theory, we investigate the wavefunction distributions and electronic structures of SAMO states of a Li@C60 monolayer in dependence on the position of Li within the cage and the type of substrate species. We find that the characteristics of the SAMOs in terms of shape and energy are quite sensitive to the Li position due to different charge redistributions. The substrate affects the intermolecular distances in the Li@C60 films and modifies the widths and dispersion of the SAMO bands while retaining energetics similar to that of the isolated Li@C60 monolayer. The substrate also affects the SAMO effective masses, making it possible to tune them via substrate-induced interaction. A properly chosen substrate can so be beneficial for Li confinement and SAMO stability, reflecting the molecule–substrate interaction and the charge transfer at the interface. These findings provide insights into the design and engineering of SAMOs of molecular films.

Dft Prediction of Superatom Molecular Orbitals (Samos) in Nanostructured Water (H2o)10 and its Potential Applications

Dft Prediction of Superatom Molecular Orbitals (Samos) in Nanostructured Water (H2o)10 and its Potential Applications

Superatomic Rydberg State Excitation.

Superatomic molecular orbitals (SAMOs) have symmetries (angular quantum numbers) similar to those of atoms, and thus, it is possible to realize Rydberg state excitations (RSEs) in superatomic molecules. In this Letter, the feasibility of superatomic Rydberg state excitation (SRSE) is explored using gold superatoms based on first-principles calculations. The results show that the SRSE exists in the high and low excited states of the gold superatoms and their SAMOs make a major contribution to electronic transitions. The radial distribution function of electronic density shows that the main distribution of electrons in the lowest unoccupied molecular orbitals and other unoccupied superatomic molecular orbitals is extremely far from the geometric center, and thus, they can be unambiguously identified as Rydberg orbitals. We found that due to the two-dimensional ductility of the planar SAMOs, superatoms are superior in the RSE regulation. Our findings may provide a new source of superatom-based RSE and will contribute to the regulation and efficient preparation of Rydberg states.

Direct Visualization of Nearly Free Electron States Formed by Superatom Molecular Orbitals in a Li@C60 Monolayer.

Using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we directly determine the spatial and energetic distributions of superatom molecular orbitals (SAMOs) of an Li@C60 monolayer adsorbed on a Cu(111) surface. Utilizing a weakly bonded [Li+@C60] NTf2- (NTf2-: bis(trifluoromethanesulfonyl)imide) salt makes it possible to produce a Li@C60 monolayer with high concentration of Li@C60 molecules. Because of the very uniform adsorption geometry of Li@C60 on Cu(111), the pz-SAMO, populated above the upper hemisphere of the molecule, exhibits an isotropic and delocalized nature, with an energy that is significantly lower compared to that of C60. The isotropic overlapping of pz-SAMOs in the condensed monolayer of Li@C60 results in a laterally homogeneous STM image contributing to the formation of a free-electron-like states. These findings make an important step toward further basic research and applicative utilization of Li@C60 SAMOs.

Near Fermi Superatom State Stabilized by Surface State Resonances in a Multiporous Molecular Network.

Two-dimensional honeycomb molecular networks confine a substrate’s surface electrons within their pores, providing an ideal playground to investigate the quantum electron scattering phenomena. Besides surface state confinement, laterally protruding organic states can collectively hybridize at the smallest pores into superatom molecular orbitals. Although both types of pore states could be simultaneously hosted within nanocavities, their coexistence and possible interaction are unexplored. Here, we show that these two types of pore states do coexist within the smallest nanocavities of a two-dimensional halogen-bonding multiporous network grown on Ag(111) studied using a combination of scanning tunneling microscopy and spectroscopy, density functional theory calculations, and electron plane wave expansion simulations. We find that superatom molecular orbitals undergo an important stabilization when hybridizing with the confined surface state, following the significant lowering of its free-standing energy. These findings provide further control over the surface electronic structure exerted by two-dimensional nanoporous systems.

Superatomic Rydberg State Excitation

Superatomic molecular orbitals (SAMOs) have symmetries (angular quantum numbers) similar to those of atoms and thus it is possible to realize Rydberg state excitations (RSEs) in superatomic molecules. In this letter, the feasibility of superatomic Rydberg state excitation (SRSE) is explored using gold superatoms based on first-principles calculations. The results show that the SRSE exists in both the high and low excited states of the gold superatoms and their SAMOs make a major contribution to electronic transitions. The radial distribution function (RDF) of electronic density shows that the main distribution of electrons in the lowest unoccupied molecular orbitals and other unoccupied superatomic molecular orbitals is extremely far from the geometric center and thus they can be unambiguously identified as Rydberg orbitals. We found that due to two-dimensional ductility of planar superatoms’ transition orbitals, superatoms have the superiority in RSE regulation. Our findings provide a new source of superatom-based RSE and will contribute to the regulation and efficient preparation of Rydberg states.

Intramolecular bonding properties in actinide embedded nearly planar superatoms

The electronic structure of isoelectronic superatoms formed by actinides embedded gold ring, An@Au6 ([Ac@Au6]− and Th@Au6, [Pa@Au6]+), have been analyzed to investigate the intramolecular interaction using density functional theory. The bonding properties are similar but not identical. The probability densities of 1P and 1D superatomic molecular orbitals are similar, and compared with 6d orbitals, 5f orbitals show localization behaviors, and the change in their contribution of the covalent bonding relate to the decrease in 5f energy levels. Moreover, additional evidence for the bonding being affected by the uncertainty of electronic configuration could be gathered.